Astronomical Facts ‘n Stuff

Mini Encyclopedia of Astronomical terms, abbreviations, objects & facts.


Version ...... 2

Glossary of Astronomical & Astrophotography Terms ...... 3

Astronomy Acronyms & Abbreviations ...... 150

Spectral Classes ...... 159

Stellar Evolutionary Paths ...... 160

The ...... 161

Planetary ...... 163

Asteroids from the Main Belt ...... 164

Asteroids from the ...... 166

Modern ...... 167

The ...... 170

Meteor Showers ...... 171

Significant People in ...... 173

SI Units and Specifics ...... 176

Unofficial Units and Specifics ...... 177

Formulas ...... 178

The Periodic Table ...... 196

The Electromagnetic Spectrum ...... 197

Lens & Characteristics...... 198

4 October 2012 1 © Paul Thomas

Version History

Version Date Changes made 1.0.0 20 Jul 2012 Original document release 1.0.1 12 Aug 2012 Added Lens & Mirror characteristics, Electromagnetic spectrum, Solar System quick info, more terms. 1.1.0 6 Sep 2012 Added more definitions, Added more Information, Added Significant People, Changed page format to landscape. 1.2.0 30 Sep 2012 Separated Asteroids & Kuiper Belt from Solar System, Added Meteor Showers, Added Formulae, Added Planetary Info. 1.2.1 3 Oct 2012 Added more, and tidied up terms.

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Glossary of Astronomical & Astrophotography Terms


Abbe See “Eyepiece”

Abell Cluster A cluster that belongs to the Abell catalogue. This is a listing of more than 4,000 galaxy clusters that meet certain criteria, one of which is having at least 30 within a range. The catalogue is divided into five groups of richness according to how many galaxies the cluster contains. Class 0 clusters contain between 30 and 49 galaxies, and class 5 clusters contain more than 299 galaxies.

Aberration Any optical defect and/or design error which causes any of the processed to deviate from reaching the focal point, therefore reducing the quality of the image.

Aberration of Starlight The apparent displacement of a star's position as a consequence of 's motion through and the finite speed of light.

Ablation The vaporisation of the surface layers of a body entering the as a consequence of the heating that results from the compression of air ahead of it.

Absolute Brightness () A measure of the true brightness of an object. The absolute brightness or magnitude of an object is the apparent brightness or magnitude it would have if it were located exactly 32.6 light- (10 parsecs) away.

Absolute Magnitude A scale for measuring the actual brightness of a celestial object without accounting for the distance of the object. Absolute magnitude measures how bright an object would appear if it were exactly 10 parsecs (about 33 light years) away from Earth.

Absolute Zero This is the coldest theoretically possible (-273.15 degrees Celsius), where the motion of atoms in a material would stop completely, leaving them only with a small amount of quantum mechanically .

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Absorption The process by which light transfers its energy to matter. For example, a gas cloud can absorb starlight that passes through it. After the starlight passes through the cloud, dark lines called lines appear in the star’s continuous spectrum at wavelengths corresponding to the light-absorbing elements.

Absorption Line A dark line in a continuous spectrum caused by absorption of light. Each chemical element emits and absorbs radiated energy at specific wavelengths, making it possible to identify the elements in the atmosphere of a star or other celestial body by analysing which absorption lines are present.

Accretion The accumulation of dust and gas onto larger bodies such as , , and

Accretion Disk A rotating disk of gas surrounding a compact object (such as a neutron star or ), formed by material falling inward.

Achromatic Lens A lens with two or more elements, usually of differing types, designed to produce an image substantially free from false colour and bringing most of the viewed colours to a sharp focus.

Active Galactic Nucleus A very bright, compact region found at the centre of certain galaxies. The brightness of an is thought to come from an around a . The black hole devours matter from the accretion disk, and this in-fall of matter provides the firepower for , the most luminous type of active galactic nucleus.

Active Galaxy A galaxy possessing an active galactic nucleus at its centre.

Actual of View A characteristic of eyepiece/ combinations. The actual field of view is the angle, measured on the sky, from one edge of the eyepiece field of view to the other. It is approximately equal to the apparent field of view divided by the magnification.

Adaptive Optics or Active Optics Guiding on steroids. In most A/O units a lens is moved in real to correct for movements in the guide star utilising refraction. This differs from traditional guiding where corrections are sent to the mount over periods of ~0.5 to 5 . At long focal lengths can be better than normal guiding.

Advanced Camera for Surveys An optical camera aboard the that uses CCD detectors to make images. The camera covers twice the 4 October 2012 4 © Paul Thomas

area, has twice the sharpness, and is up to 10 more efficient than the telescope’s Wide Field and Planetary Camera 2. The ACS wavelength range spans from to near- light. The camera’s sharp eye and broader viewing area allow astronomers to study the cycles of galaxies in the remotest regions of the .

Aerogel An advanced material used by the Stardust spacecraft to capture small particles of cometary dust. It is 99.8 per cent air and was effective at slowing down the cometary particles gently so that they weren’t damaged.

Aerosol Refers to small particles suspended in the air, as opposed to large particles like snowflakes and raindrops. Aerosols make the air look hazy by light.

Afocal Coupling The technique of imaging through a camera lens held up to the eyepiece of a telescope. It is used for cameras with non- removable lenses.

Afterglow The fading fireball of a gamma-ray burst — a sudden burst of gamma rays from deep space — that is observable in less energetic wavelengths, such as X-ray, optical, and radio. After an initial explosion, an expanding gamma-ray burst slows and sweeps up surrounding material, generating the , which is visible for several or . The afterglow is usually extremely faint, making it difficult to locate and study.

Airglow The faint background glow in the sky caused by gas in the . Because of airglow the night sky is never completely dark as seen from Earth's surface.

Airy Disk The Airy disk is the brightest spot formed by a star image as seen through a telescope. It is surrounded by alternating rings of light and dark (these are due to diffraction - any light passing through an aperture is diffracted, and the effect is inversely proportional to the size of the aperture.) An optical system of good quality increases the relative brightness of the central Airy disk compared to the surrounding diffraction rings. (Defocusing a star image will accentuate the diffraction rings and is sometimes useful for assessing a telescope’s optics.).

Albedo The reflective property of a non-luminous object. A perfect mirror would have an of 100% while a black hole would have an albedo of 0% (The albedo of Earth is 0.36, that of the is 0.07).

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Alfven Waves These are transverse waves that travel through electrically conducting fluids or gases in which a magnetic field is present, like the ’s plasma.

All-Sky Camera This has a lens (or element of its design) that enables it to image the majority, if not all, of the sky.

Almanac A book of tables detailing the positions, times and other data about celestial objects, usually produced annually.

Alpha Particle A cluster of two protons and two neutrons; a helium nucleus.

Alpha Process A process by which lighter elements capture helium nuclei (alpha particles) to form heavier elements. For example, when a nucleus captures an alpha particle, a heavier oxygen nucleus is formed.

Alt-Azimuth Mount A type of telescope mount that is simpler to construct than an equatorial mount, but requires simultaneous movement about the vertical (altitude) and horizontal (azimuth) axes to track a celestial object.

Altitude The angular distance from the observer's horizon, usually taken to be that horizon that is unobstructed by natural or artificial features (such as mountains or buildings), measured directly up from the horizon toward the zenith; positive numbers indicate values of altitude above the horizon, and negative numbers indicate below the horizon.

Aluminising The process of coating a telescope mirror with a thin layer of vaporised aluminium.

Amplifier Glow Known as 'amp glow' this is the heat given off by the electronics in the camera showing up on the image due to heating of the sensor.

Analemma Observed every from the same location at the same time, the Sun follows a figure-of-eight path through the sky. Known as an analemma, this pattern is due to the tilt of the Earth’s axis as it the Sun.

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Anastigmatic Describes an optical system which is corrected for spherical aberration, coma, and astigmatism. The point is, that in many common optical designs, spherical aberration and coma tend to be more serious problems than astigmatism, so there isn't much point in correcting astigmatism unless the other two are already dealt with.

Andromeda Galaxy The large located some 700,000 parsecs from the sun; the most distant object visible to the unaided eye.

Angstrom Unit of length convenient for measuring wavelengths of electromagnetic : 1 A = 10-10m.

Angular Diameter The apparent size of an object, usually expressed in degrees, , or seconds of arc.

Angular Distance The apparent distance between two objects on the measured in degrees, minutes or seconds of arc.

Angular Momentum A measure of the , radius, and rotational velocity of a rotating or orbiting body. In the simple case of an object in circular , the angular momentum is equal to the mass of the object times its distance from the centre of the orbit times its .

Angular Resolution The ability of an instrument, such as a telescope, to distinguish objects that are very close to each other. The angular resolution of an instrument is the smallest angular separation at which the instrument can observe two neighbouring objects as two separate objects. The angular resolution of the human eye is about a of arc. As car headlights approach from a far-off point, they appear as a single light until the separation between the increases to a point where they can be resolved as two separate lights.

Annular A where the Moon passes directly in front of the Sun but is too far from Earth to completely cover the solar disc. At the peak of an annular eclipse, the Moon is surrounded by a ring of .

Anomaly The angle at the Sun between a and its perihelion.

Ansae Literally handles. Originally a description of the appearance of 's rings before they were recognised as being a ring

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system. Now used to describe (i) the extension of Saturn's rings outside the disc of the planet, and (ii) extensions from the central star of some planetary nebulae (due to bipolar outflow of material).

Antapex (plural antapexes or antapices) The point opposite the solar apex.

Antarctic Circle The line of latitude on Earth’s surface that is 23.5 degrees north of the South Pole. The Antarctic Circle marks the northernmost point in the southern hemisphere that experiences the sun.

Anthelion (plural anthelia) A faint, white halo rarely seen in the sky opposite the sun on the parhelic circle.

Antimatter A type of matter in which each particle (antiproton, antineutron, etc.) is opposite in charge and certain other properties to a corresponding particle (proton, neutron, etc.) of the same mass of the ordinary type of matter from which the solar system is made. Particles of antimatter are known to exist, but it is not known why matter is dominant in this region of the universe or whether regions exist in which antimatter is common.

Apastron For an orbit around a star, the farthest point from that star.

Aperture The size of the opening through which light passes in an optical instrument such as a camera or telescope. A higher number represents a smaller opening while a lower number represents a larger opening.

Aperture Fever An amateur astronomer who is eager to the point of obsession, to have of ever larger clear aperture, is said to have aperture fever.

Aphelion The point in the orbit of a solar-system object where it is farthest from the sun.

Aplanatic This describes the type of telescope which has been made so that its optical systems show no signs of aberrations from spherical aberration and coma.

Apoapse (also Apocentre) The position in an orbital path that is the greatest distance from the primary body. 4 October 2012 8 © Paul Thomas

Apoapsis The point in an orbit when the two objects are farthest apart. The opposite of periapsis.

Apochromat A type of refractor objective made from usually three lenses of different materials, selected to bring three colours of light to the same focal point, thereby bringing the colours that are in between to very nearly the same focal point.

Apo(chromatic) A lens system that brings all three primary colours to the same point of focus, and is thus considered free of chromatic aberration. (Some consider that a true apochromatic lens ought to eliminate spherical aberration also.)

Apocenter The furthest point of an in an elliptical orbit to its centre of attraction (the principal focus of the ).

Apogee of farthest distance from the Earth of a body in orbit around the Earth.

Apollo An asteroid whose orbit brings it closer than 1 to the sun.

Apparent Field of View The angular size of the field as seen through a particular eyepiece without a telescope. (Varies with different designs, and is often specified on the eyepiece. Roughly; Orthoscopics 40 degrees, Plossls 50 degrees, Superwides 60-70 degrees, Ultrawides 70+ degrees).

Apparent Horizon Where the sky appears to meet the Earth. Because of perspective effects, different observers generally have different apparent horizons. Because of refraction, even the sea horizon usually lies above the geometric horizon.

Apparent Magnitude The apparent brightness of an object in the sky as it appears to an observer on Earth. Bright objects have a low while dim objects will have a higher apparent magnitude.

Apparition The period or time when an object is visible and well placed for observation.

Appears Local time at which the satellite appears visually. The first figure indicates the visual brightness of the object. The smaller the

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number, the brighter and more eye-catching it appears to an observer. The units are astronomical magnitudes [m]. Azimuth is given in degrees counting from geographic north clockwise to the east direction. The three-character direction code is given as well. In case the satellite exits from the Earth shadow and comes into the glare of the Sun, the elevation above horizon is given in degrees for this . If this figure is omitted, the satellite is visible straight from the horizon.

Appulse The apparent close approach of two celestial bodies as seen from Earth, such as a star and a planet, or two planets for example.

Apsides Bodies in an elliptical orbit all reach a point when they are furthest or closest to their parent object, eg. a planet around a star. Apsides is a term for these points. Earth’s apsides are its perihelion and aphelion.

Arc (measurement of) Angles on the celestial sphere, measured in degrees, minutes and seconds. Arc may be an expression of the angular distance between two celestial objects or the angular size of an object.

Arcminute A small unit of angular measurement, spanning one-sixtieth of a ; an arcsecond is one-sixtieth of an arcminute. Astronomers measure the separation between stars in the night sky in terms of degrees.

Arctic Circle The line of latitude on Earth’s surface that is 23.5 degrees south of the North Pole. The Arctic Circle marks the southernmost point in the Northern Hemisphere that experience the .

Arcsecond (arcsec) Measurement of angular separation: a 1-inch stick would subtend an angle of 1 arcsec at a viewer's eye at a distance of about 6.5 miles.

Arecibo Short name for the National Astronomy and Ionospheric Centre (NAIC) at Arecibo, Puerto Rico; often refers only to the NAIC 1000-ft (305-m) zenith antenna, the 's largest radio astronomy collector.

Areography The proper name for the geography of .

Argument of the Pericentre The angle, measured around the orbital , between the ascending node and the pericentre.

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Armillary Sphere An instrument consisting of graduated metal circles used to represent the motions of celestial bodies around the earth.

Array An orderly arrangement or impressive display. For radio telescopes, an array is a group of individual radio dishes that work together. The VLA (Very Large Array) has 27 telescope dishes arranged in a “Y” pattern.

Artificial Star This is simply an artificial point of light used to test the optics of a telescope in the absence of, or instead of, a suitable real star for collimation and other adjustments.

Ascending Node The position in the orbit of a planet (or the Moon) where it crosses the plane of the , moving northward.

Aspheric Not spherical.

Aspides The points in the Moon's orbit where it is either closest to Earth (perigee) or farthest from Earth (apogee).

Asterism A pattern formed by stars that aren’t necessarily in the same constellation, for example the Plough and the Summer Triangle.

Asteroid A small planetary body in orbit around the Sun, larger than a but smaller than a planet. Most asteroids can be found in a belt between the orbits of Mars and . The orbits of some asteroids take them close to the Sun, which also takes them across the paths of the planets. Beyond the orbit of a gigantic supply of such bodies seem to exist.

Astigmatism The distortion of an image resulting from a failure of light rays from different parts of the same zone to focus in the same plane.

Asteroseismology The study of the internal structure of stars by analysis of the way they pulsate.

Astrobiology The study of life throughout the Universe, its origin, , ecology and destiny.

Astrograph A photographic telescope. An was used by Clyde Tombaugh to discover in 1930. 4 October 2012 11 © Paul Thomas

Astrolabe An astronomical and navigational instrument for gauging the altitude of the Sun and stars.

Astrological Sign One of twelve sections of the that are 30 degrees long and that correspond to the positions of the constellations as they were about 2,600 years ago when the astrological system was established. Do not confuse an astrological sign with the astronomical constellation of the same name, as they only partially overlap.

Astrology A non-scientific system based on superstition, that purports to explain or predict human actions by study of celestial positions.

Astrometry Branch of astronomy that focuses on measurements, especially those relating to positions and movements.

Astronomical Horizon The intersection of a horizontal plane through the eye with the celestial sphere. Because the celestial sphere has an infinite radius, two observers at different heights above sea level, but placed on the same vertical line, have the same astronomical horizon. Also, because of dip, the astronomical horizon always lies above the sea horizon, but it usually is hidden by trees, hills, and buildings on land. These objects then determine the observer's apparent horizon.

Astronomical The times are the moments of beginning/end of the astronomical twilight, i.e., the moments the Sun reaches a depression of 18° below the horizon. If the Sun is below this angle, no brightening of the sky can be observed.

Astronomical Unit Approximately equal to the mean earth-sun distance, which is about 150,000,000 km or 93,000,000 miles. Formally, the AU is actually slightly less than the earth's mean distance from the sun (semi-major axis) because it is the radius of a circular orbit of negligible mass (and unperturbed by other planets) that revolves about the sun in a specific period of time.

Astronomy Astronomy, from the Greek words astron (ἄστρον), "star", and -nomy from nomos (νόμος), "law" or "culture", is the scientific study of the universe and the celestial bodies that reside in it, including their composition, history, location, and motion.

Astrophotography A specialized kind of photography, comprised of concepts and techniques that pertain to the production of photographic images of astronomical objects.

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Astrophysics The branch of astronomy that deals principally with the of the universe, including luminosity, , temperature, and the chemical composition of stars, galaxies, and the .

Atmosphere A layer of gases surrounding a planet, moon, or star. The Earth's atmosphere is 120 miles thick and is composed mainly of , oxygen, carbon dioxide, and a few other trace gases.

Atmospheric Distortion The blurring of an image due to the layer of gases surrounding the surface of Earth. As starlight travels through the atmosphere, pockets of air act like little lenses and bend the light in unpredictable ways. This distortion causes stars to appear to twinkle.

Atom The smallest possible unit of a chemical element. When an atom is subdivided the parts no longer have properties of any chemical element. An atom consists of a nucleus (made of protons and neutrons) with orbiting electrons.

Atom Fourth epoch in the history of the Universe, lasting from about 100 sec to 106 years, in which matter came to dominate radiation as the principal constituent of the Universe.

Atomic Mass Unit Convenient unit for measuring the mass of an atom or molecule: 1 atomic mass unit is defined as I /12 the atomic mass of the most abundant carbon isotope, 12C. A=12 for 12C.

Atomic Nucleus Concentrated, positively charged matter at the centre of an atom; composed of protons and neutrons.

Atomic Number The number of protons in the nucleus of an element. It is the atomic number that defines the identity of an element. Z=6 for 12C.

Atomic Theory A model that offers a logical explanation for the law of multiple proportions and the law of constant composition by stating that all elements are composed of atoms, all atoms of a given element are identical, but the atoms of one element differ from the atoms of any other element; that atoms of different elements can combine to form compounds and a chemical reaction involves a change not in the atoms themselves, but in the way atoms are combined to form compounds.

Atomic Weight The number of protons and neutrons in an atom, averaged over the abundances of the different isotopes. 4 October 2012 13 © Paul Thomas

Attitude The orientation of a spacecraft or satellite with respect to its direction of motion.

Aureole Another term for a Corona.

Aurora A phenomenon produced when the solar wind (made up of energized electrons and protons) disturbs the atoms and molecules in a planet’s upper atmosphere. Some of the energy produced by these disturbances is converted into colourful visible light, which shimmers and dances. Auroras have been seen on several planets in our solar system. On Earth, auroras are also known as the “Northern Lights” (aurora borealis) or “Southern Lights” (aurora australis), depending on in which polar region they appear.

Aurora Australis Also known as the southern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the southern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field.

Aurora Borealis Also known as the northern lights, this is an atmospheric phenomenon that displays a diffuse glow in the sky in the northern hemisphere. It is caused by charged particles from the Sun as they interact with the Earth's magnetic field.

Auroral Corona An auroral corona usually appears during energetic auroral displays. They are seen as rays of auroral emission coming straight at you, where the perspective often makes it look as if they are emanating from a single point in the sky.

Auto-Guider Or Autoguider. A CCD/CMOS device that is optically attached to a guidescope or off-axis guider and electronically attached to the control of the telescope mount. It monitors the position of a guide object on the CCD array and adjusts the telescope's drives so as to keep the object in the same position, thus correcting for any errors in the drive or in polar alignment. It enables long- exposure photography or imaging through the main OTA without the astronomer having to make manual corrections to the drive in response to what he sees in a guidescope.

Autumnal The moment when the Sun crosses the celestial travelling in a southward direction, on or about September 22. In the Northern Hemisphere, it marks the first day of Autumn. The term is also applied to the Sun’s position in the sky at that moment. It is one of two points where the ecliptic and intersect, the other being the vernal equinox.

Averted Vision A technique for observing faint objects through a telescope by viewing slightly to the side, allowing light from the object to fall on an area of the eye more sensitive to faint light. 4 October 2012 14 © Paul Thomas

Axis Also known as the poles, this is an imaginary line through the centre of rotation of an object.

Azimuth Azimuth direction of the object is given in degrees counting from geographic north (0°) clockwise (to the east) direction. East is 90°, south 180°, and west 270°. The three-character direction code is given as well. For example, NNW stands for north-north- west.


Back Focal Length Classically, the distance from the last optical element of a system encountered by the light passing through it, to the focal plane. Latterly, from the point where the beam emerges from within the assembly of optical parts, to the focal plane.

Bahtinov Mask A patterned 'mask' that is put in front of the scope to assist in achieving perfect focus (removed when focused), named after inventor Pavel Bahtinov.

Bailly's Beads Bright points of light seen along the edge of the Moon just before and just after a total eclipse of the Sun. They are caused by sunlight shining though valleys (or between peaks) at the Moon's limb.

Barlow Lens An intermediate concave lens (or lenses) placed in front of the eyepiece of a telescope to increase the effective focal length of a telescope. Sometimes called negative lenses. A 2x Barlow, will double the power of an eyepiece (ie. a 20mm eyepiece becomes a 10mm).

Barred This is an irregular galaxy which exhibits a bar structure.

Barred Spiral Galaxy A galaxy with a “bar” of stars and interstellar matter, such as dust and gas, slicing across its centre. The is thought to be a .

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Barrel Distortion An aberration of optical systems, in which magnification decreases with distance away from the optical axis. With such a system, squares are imaged with their sides bulged out, looking sort of like barrels.

Barycentre The centre of of the Earth-Moon system. The Earth is 81 times the mass of the Moon and the barycentre of the Earth- Moon system actually lies inside the Earth.

Baryon Particles Any of the subatomic particles that interact through the strong nuclear force. Most commonly, these are protons and neutrons. Their presence in the universe is determined through their gravitational and electromagnetic interactions.

Basin Basins or impact basins are the results of massive impacts on a planetary surface. They appear as vast craters. Some lunar basins have been covered or partially filled by lava, which has later erupted from beneath the surface.

Bayer Designation This is given to stars in a constellation to indicate their order of brightness using the Greek alphabet; where Alpha (a) denotes the brightest star in a constellations and Beta (b) denotes the brightest and so on.

Bayer Mask Coloured filters in front of camera pixels to give coloured image.

Beam Splitter This is an optical component of a bino viewer that splits the light coming into the eyepiece holder into two beams for viewing.

BeppoSAX A space-based X-ray observatory built and operated by the Italian Space Agency and the Netherlands Agency for Aerospace Programs. BeppoSAX has been instrumental in identifying and locating gamma-ray bursts.

Best Seen Between / hmax This is the best visibility time interval of the object. The calculation takes into account the magnitude of the object (required elevation above horizon), and the elevation of the Sun. The time is given in local (LCT), i.e., the and definitions as selected by you. hmax is the maximum altitude over the horizon, that the object reaches during this time period.

Bi-Polar Outflow When stars are born they begin to form jets at their poles known as bi-polar outflows. These jets are thought to originate from material that collapsed onto the star as it was forming. They may, in fact, be crucial to the process.

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Bias Frames A bias frame (may also be known as 'offset') is an image taken with no actual exposure time. These images are used to adjust the light frames (the actual pictures) to mitigate the noise inherent in the chip itself. If a camera won't take a 0-length image (shot with the shutter closed) then a 'pseudo bias' can be taken by leaving the lens cap on, setting the camera for its fastest shutter speed, standing in a dark closet and pressing the shutter. Take a few of them. Not to be confused with a dark frame which uses an exposure of the same length as the light (picture) frames.

Big Bang The initial singularity that started the space and time of our Universe, now thought to have occurred 13 to 15 billion years ago.

Big Bang Theory A cosmological model, in which the universe was once compressed to infinite density and has been expanding ever since. Originally a term used by unbelieving scoffers, now as widely accepted as the model itself.

Billion One-thousand million, 109, in the USA only; elsewhere a million million, or 1012.

Binary (star system) A system of two stars that revolve around a common centre of gravity.

Binding Energy Energy derived from the conversion of mass to energy when neutrons and protons are combined to form nuclei.

Binning The technique of grouping together pixels on a CCD camera chip to make larger pixels. Reduces noise, but decreases resolution.

Binocular Two telescopes aligned side by side, so you can look at something with both eyes at once. They are traditionally labelled with two numbers, separated by "x". So a binocular labelled "25x90" is called a "Twenty Five by Ninety; a 25 times magnification, with an objective lens of 90mm in diameter.

Bino-Viewer An optical gadget that splits the single beam of light coming out of a telescope into two parts, so you can observe with both eyes at once. Bino-Viewers generally do not work with all telescopes or all eyepieces.

Biosphere The part of the Earth, or other planet, and its atmosphere capable of supporting life

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Bit Depth A CCD camera's analog has to be converted to digital values in order for a computer to work with them. This is done by an Analog-Digital converter. The bit depth determines the number of values that can be assigned to a pixel: 256 for 8-bit and 65536 for 16-bit.

Blackbody Body capable of absorbing energy of all wavelengths falling on it; it is also capable of radiating all in a particular ratio to its absorbing properties. The value of the ratio depends only on the temperature of the body.

Black Dwarf A non-radiating ball of gas that results either when a white dwarf radiates all its energy or when gas contracts gravitationally but contains too little mass to begin .

Black Hole The collapsed core of a massive star. Stars that are very massive will collapse under their own gravity when their fuel is exhausted. The collapse continues until all matter is crushed out of existence into what is known as a singularity. The gravitational pull is so strong that not even light can escape.

Black Hole Stars Astronomers know that in the early Universe, black holes with many times that of the Sun roamed space. But they’re not sure where these monsters came from. Enter a new theory – black hole stars. Black hole stars were supermassive stars in the early Universe. At their centres were massive black holes, formed by crushing gravity. As a supermassive star became more massive and grew, the black hole at its centre would have gorged on the star’s gas and would also have grown. Eventually the star would begin to die as it cooled down and, in doing so, would expose the massive black hole at its .

Blazar These are powerful jets of radiation coming from the heart of a (see ‘Quasar’) that we are viewing ‘head on’.

Blue Moon Refers to either: The third in a with four full moons. The second full moon of a . The moon tinted towards blue as it appears in the sky, caused by dust or smoke in the atmosphere

Blueshift The shortening of a light wave from an object moving toward an observer. For example, when a star is traveling toward Earth, its light appears bluer.

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BL Lacertæ objects Objects which are strong emitters of infra-red radiation. They are very luminous and remote and are thought to have similar properties to quasars.

Bode's law The physical laws describing the properties of a numerical scheme that roughly gives the radii of the orbits of the seven innermost planets and the radius of the . More numerology than .

Bohr atom Nils Bohr's model of the hydrogen atom, in which the energy levels are depicted as concentric circles of radii that increase as (level number) 2.

Bok Globule A Bok Globule is a dense cloud of dust and gas in space thought to be important in the role of , named after the astronomer Bart Bok.

Bolide An asteroid or that explodes, crashes onto the Earth or another body in the Solar System, generating a huge fire-ball.

Bolometer An instrument used to measure heat radiation.

Bolometric Magnitude The magnitude of a celestial object corrected to take account of the radiation in parts of the spectrum other than the visible, therefore it is the total sum of all emissions received from an object from the whole electromagnetic spectrum.

Boundary Layer A relatively thin layer of fluid next to a boundary (such as a solid surface). For example: The layer of air next to the Earth's surface (the “planetary boundary layer”). Ordinarily, only the boundary layer is appreciably affected by the properties of the surface. The rest of the fluid is unaffected, so the part of the atmosphere above the boundary layer (which is typically a few hundred meters thick) is called the “free atmosphere.”

Bracketing Bracketing is an imaging method where you take several exposures (of the same object) with slightly different settings either side of the planned settings, to see what works best.

Brandon See “Eyepiece”

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Brown Dwarf An object substantially (~13 x) larger than Jupiter but with a mass no more than 40 per cent that of the Sun. These objects are not big enough for to heat them to the point that nuclear reactions can be triggered. Brown dwarfs may be very common in the universe and could even have planets in a habitable zone.

Bulb (or B setting) This is a setting usually seen on old SLR cameras (and now seen in a different guise in the manual mode on some DSLRs), which allows the shutter of the camera to be left open for an extended period of time. This is especially crucial for long-exposure work where the shutter may have to be open for several minutes to capture enough light from a celestial object.

Burster A sporadic source of intense X rays, probably consisting of a neutron star onto which new matter falls at irregular intervals.


Calcium-K Calcium-K is a specific wavelength (roughly 393.3 nanometres) of ultraviolet light, emitted by calcium atoms that are missing one electron. Specific solar telescopes filter all of the Sun’s light apart from this particular wavelength.

Caldwell Catalogue A listing of 109 bright star clusters, nebulae and galaxies that weren’t included in the Messier catalogue. The catalogue was compiled by Sir Patrick Caldwell-Moore.

Captured rotation Rotation of an object that spins at the same rate as that object takes to orbit another object. Sometimes referred to as synchronous rotation. The Moon is a good example of an object that has captured rotation.

Carbon-14 (14C) A radioactive isotope of carbon produced in the upper atmosphere and present in living plants and animals that can be used in carbon-14 dating because it decays to nitrogen (14N) and a beta ray with a half-life of about 5,730 years.

Carbonaceous A containing controls, with a high abundance of carbon and other volatile elements.

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Carbon Cycle A chain of nuclear reactions, involving carbon at its intermediate stages, that transforms four hydrogen atoms into one helium atom with a resulting release in energy. The carbon cycle is only important in stars hotter than the sun.

Carbon stars Red stars of spectral type 'R' and 'N' which have carbon-rich .

Carey Mask A patterned 'mask' that is put in front of the scope to assist in achieving perfect focus (removed when focused).

Cassegrain (telescope or focus) Cassegrain telescope is a reflecting telescope that uses two curved of particular shapes to form the image: The primary mirror is a concave paraboloid, just as in a Newtonian. The secondary mirror is a much smaller convex hyperboloid, positioned a little way toward the primary from the primary's focal plane. The secondary reflects the converging beam of light back toward the primary, which typically has a hole in the centre, so that the focal plane of the combined mirrors is easily accessible behind the primary.

Catadioptric A telescope that uses a system incorporating both lenses and mirrors to form an image. Common types: Schmidt-Cassegrain (SCT) and Maksutov-Cassegrain (MCT, mak) telescopes.

Celestial Co-Ordinates A system by which the position of a body on the celestial sphere is plotted with reference to a reference plane and a reference direction. The four systems in use are Ecliptic Co-ordinates, Equatorial Co-ordinates, Galactic Co-ordinates, and Horizon Co- ordinates.

Celestial Equator An imaginary line that divides the celestial sphere into a northern and southern hemisphere. Our (the ) equator, infinitely projected into space.

Celestial Meridian The line of zero in the equatorial coordinate system.

Celestial Object A natural object which is located outside of Earth's atmosphere, such as the Moon, the Sun, an asteroid, planet, or star.

Celestial Poles The North and South poles of the celestial sphere.

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Celestial Sphere An imaginary sphere of great (or infinite) radius that is centred on the earth and is used for practical purposes in astronomical observing. Since stars are very distant from us, they make up a background that is essentially unchanging from to year; of course, over a period of years, the closer stars will move very slightly and factors such as precession cause a change in the appearance of the stars in our skies over many years. But we create a map grid and some of these map grids include equatorial coordinates (right ascension and ), ecliptic coordinates (ecliptic and latitude), and galactic coordinates (galactic longitude and latitude) - Which refer to the earth's rotation, the earth's revolution about the sun, and the Milky Way galaxy's plane, respectively.

Cell The mechanical support that grips a lens or mirror, and which is in turn fastened to the telescope tube, or to some other piece of telescope structure.

Centaurs These are an unstable orbital class of minor planets that behave with characteristics of both asteroids and . Centaurs have transient orbits that cross or have crossed the orbits of one or more of the giant planets, and have dynamic lifetimes of a few million years. It has been estimated that there are around 44,000 centaurs in the Solar System with diameters larger than 1 km. Any centaur that is perturbed close enough to the Sun is expected to become a comet.

Central Meridian The imaginary line through the poles of a planet that bisects the planetary disc.

Cepheid Variable This is a whose light pulsates in a regular cycle. The period of fluctuation is linked to the brightness of the star. Brighter Cepheid’s will have a longer period.

Chandra X-Ray Observatory A space-based X-ray observatory; also known as the Advanced X-ray Facility (AXAF). Chandra is designed to observe X-rays from high-energy regions of the universe, such as hot gas in the remnants of exploded stars. The satellite was launched and deployed in July 1999.

Chandrasekhar Limit The mass beyond which a white dwarf must inevitably collapse into a neutron star, about 1.4 solar masses.

Chaos A distinctive area of broken terrain. The unordered state of matter in classical accounts of cosmogony. The hypothetical first epoch in the history of the Universe, lasting 10" sec: a period about which we cannot yet even speculate.

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Charge-Coupled Device An electronic detector that records visible light from stars and galaxies to make photographs. These detectors are very sensitive to the extremely faint light of distant galaxies. They can see objects that are 1,000 million times fainter than the eye can see. CCDs are electronic circuits composed of light-sensitive picture elements (pixels), tiny cells that, placed together, resemble mesh on a screen door. The same CCD technology is used in digital cameras.

Charles' Law For a given mass of gas at constant pressure, the volume varies directly with the temperature (on the absolute scale).

Charm An arbitrary name that corresponds to a property that distinguishes certain elementary particles, including types of quarks, from each other.

Chemical Compound A pure substance consisting of atoms or ions of two or more different elements. The elements are in definite proportions. A chemical compound usually possesses properties unlike those of its constituent elements. For example, table salt (the common name for sodium chloride) is a chemical compound made up of the elements chlorine and sodium.

Chemical Evolution The chemical (i.e., pre-biological) changes that transformed simple atoms and molecules into the more complex chemicals needed for the origin of life. For example, hydrogen atoms in the cores of stars combine through nuclear fusion to form the heavier element helium.

Chemically Peculiar Stars Stars manifesting anomalies in the relative abundances of elements, which may arise from mechanical rather than nuclear effects; so-called manganese stars, for example, show a great overabundance of manganese and gallium, usually accompanied by excess .

Chroma (or Chrominance Noise) This is the blotchy variation in colour sometimes seen in DSLR images.

Chromatic Aberration The introduction of false colour into an image, caused by a lens bending different wavelengths of light unevenly, which focuses the colours in different places.

Chromosphere The chromosphere is the layer of the Sun’s atmosphere above the visible ‘surface’ of the photosphere and beneath the outer tenuous corona. Meaning ‘sphere of colour’, it is the place where dynamic prominences and other similar events occur.

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Chondrite A type of stony meteorite that contains numerous small spherules of silicate (silica, silicon dioxide) minerals. A subset of this type of , the carbonaceous , contains several per cent organic carbon.

Circumpolar Star A star or constellation that doesn’t set (i.e. doesn’t disappear below the horizon) over the course of the night, due to its proximity to one of the poles. The constellation of Cassiopeia is circumpolar.

Civil Twilight The times are the moments of beginning/end of the civil twilight, i.e., the moments the Sun reaches a depression of 6° below the horizon. On clear weather, no significant dim-out can be distinguished compared to the time of /.

Clear Aperture The diameter of the beam of light that goes into a telescope. In most cases, that is the unobstructed diameter of the front lens, or the corrector, or the primary mirror, depending on telescope type.

Close to Moon/Sun The satellite is closer than 1.5 degrees from the center of the Moon or the Sun, but the satellite does not cross in front of the Moon/Sun. The direction and distance to the center line on Earth is given. For the Sun, move to the indicated center line position and observer with proper equipment.

Close to The Moon or main object appears close to the listed star or planet. These events may be useful for reasons of 'near miss' or to make it easier to find the fainter object in the sky. Usually, such constellations give a nice view.

Closed Universe A possible state of the universe. In this state, the expansion of the universe will eventually be reversed; it is characterized by positive curvature, being finite in extent but having no boundaries. Recent observations indicate that this is unlikely to be the true state of our universe.

Clock-Face Direction In a simple -face coordinate system with the clock face superimposed on the satellite itself, with 12:00 o’clock being at the top and 9:00 o’clock being at the left, the satellite will seem to move toward the given direction. This number is helpful when observing with .

Cluster (Astronomical) Group of stars numbering from a few to hundreds of thousands of stars. Galactic clusters, sometimes called open clusters, contain up to a few hundred members and occur rather close to the plane of the Galaxy. Globular clusters contain tens of

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thousands of stars distributed about their centre in a spherical manner and are found far from the plane of the Galaxy as well as in it toward the centre of the Galaxy.

CNO Cycle A nuclear-fusion-reaction sequence in which hydrogen nuclei are combined to form helium nuclei, and in which other nuclei, such as isotopes of carbon, oxygen, and nitrogen, appear as catalysts or by-products. The CNO cycle is dominant in the cores of stars on the upper . Same as carbon cycle.

Coated This is most commonly used to describe a lens or prism in which the surfaces where the beam of light goes from air into glass, or from glass into air, have been coated with something that reduces the amount of light that gets reflected, and hence, wasted – or worse, reflected into the focal path and disturbing the image. Beware: this is not the same as multi coated or fully coated.

Coelostat A system of two moveable mirrors used in solar telescopes. The mirrors follow the Sun and keep its image in the same location as Earth rotates.

Coherent Radiation Radiation in which the phases of waves at different locations in a cross-section of radiation have a definite relation to each other; in non-coherent radiation, the phases are random. Only coherent radiation shows interference.

Cold Dark Matter This is the favoured model to describe dark matter (a mysterious matter that doesn’t emit any light). Its constituent particles move slowly (which is why it’s called ‘cold’), and therefore it was easy for dark matter to clump together in the early Universe.

Collecting Area The area of a telescope’s primary light-collecting mirror. A telescope’s light-gathering power rises with an increase in its collecting area.

Colliding Galaxies A galactic “car wreck” in which two galaxies pass close enough to gravitationally disrupt each other’s shape. The collision rips streamers of stars from the galaxies, fuels an explosion of star birth, and can ultimately result in both galaxies merging into one.

Collimate The term used for adjusting a telescope to gain maximum optical performance by means of alignment of the optical parts.

Collisional Process An event involving a collision of objects; for example, the excitation of a hydrogen atom when it is hit by an electron.

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Colour ( [USA]) .. Of an object, a visual property that depends on wavelength. The perception of colour stems from the varying spectral sensitivity of different types of cone cells in the retina, to different parts of the spectrum, colours may be defined and quantified by the degree to which they stimulate these cells. The colour of an object is a complex result of its surface properties, its transmission properties, and its emission properties, all of which factors contribute to the mix of wavelengths in the light leaving the surface of the object. An arbitrary name assigned to a property that distinguishes three kinds of quarks.

Colour Fringing Another name for chromatic aberration.

Colour Index The difference B-V between the blue (B) and visual (V) magnitudes of a star. If B is less than V (that is, the star is brighter in blue than in visual light), then the star has a negative colour index, and is a relatively hot star. If B is greater than V, the colour index is positive, and the star is relatively cool.

Colour-Magnitude Diagram A Hertzsprung-Russell diagram in which the temperature on the horizontal axis is expressed in terms of colour index.

Colure A great circle that passes through the celestial poles and either the (equinoctial colure) or (solstitial colure).

Coelostat A device, usually consisting of two mirrors, that is designed so as to reflect the light from a celestial object into a fixed instrument, where it forms a non-rotating image.

Coma A comet's atmosphere (composed of dust and/or various gases) surrounding its nucleus. The coma is rather tenuous, and stars can be occasionally easily seen through it, shining from behind, and yet, the coma is usually thick enough that it masks our view of the true nucleus of the comet, as seen from the earth. As a comet's nucleus is usually quite small, it is not able to retain its coma for long periods of time, and the coma material gradually drifts away into space. Much coma material is thrown back into what we see as the comet's tail. But all coma material originates in the comet's nucleus, and solar sublimation due to heating causes gases to move outward, often in jets, taking dust material with them to form the coma and tail.

Comatic Aberration (Coma) An optical defect in a lens, which means light rays that enter the edge of the lens at an angle converge so as not to be brought to a sharp focus. The result is a smearing of detail towards the edge of the field of view. Coma is an optical aberration, where stars at the edge of a field of view appear to broaden out into triangle or fan shapes, caused by an imperfection in the lens or mirror. 4 October 2012 26 © Paul Thomas

Comet A gigantic ball of ice and rock that orbits the Sun in a highly eccentric orbit. Some comets have an orbit that brings them close to the Sun where they form a long tail of gas and dust as they are heated by the Sun's rays.

Comet Tail A tail is made up of dust and gas from a comet’s coma. A tail forms when the solar wind separates dust and gas from the coma, pushing it outward and away from the Sun in either a slightly curved path (for dust) or a straight path (for gas).

Commensurability This is the property of two objects orbiting the same body whose periods are in a rational proportion. For example, the of Saturn around the Sun is very nearly 5/2 the orbital period of Jupiter.

Comparison Spectrum A spectrum of known elements on earth usually photographed on the same photographic plate as a stellar spectrum in order to provide a known set of wavelengths or zero Doppler shift.

Concave In lenses, it is the side that goes in and causes light rays to spread apart (diverge). In mirrors, it is also the side that goes in, but it causes light rays to meet (converge).

Conjunction There are at least three definitions of . Bodies are said to be in conjunction when they have the same ecliptic longitude (this is the strict definition) or when they have the same Right Ascension or when they are at their closest. Planets are said to be "at conjunction" when they are in conjunction with the Sun. For extended bodies (e.g. Sun, Moon, planets), the body's position is taken to be its centre.

Conjunction, Inferior A conjunction when one of the inferior planets (Mercury or ) appears to lie very close to the Sun, or in line with the Sun, as seen from Earth, but with the planet between Earth and the Sun. Obviously, a superior planet (Mars, Jupiter, Saturn, , Neptune or Pluto) cannot be at inferior conjunction!

Conjunction, Superior A conjunction when one of the inferior planets (Mercury or Venus) appears to lie very close to the Sun, or in line with the Sun, as seen from Earth, but with the planet on the far side of the Sun.

Conservation Law A statement that the total amount of some property (angular momentum, energy, etc.) of a body or set of bodies does not change.

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Conservation of Energy and Mass A fundamental law of physics, which states that the total amount of mass and energy in the universe remains unchanged. However, mass can be converted to energy, and vice versa.

Constellation A grouping of stars that make an imaginary picture of mythological figures in the sky. The International Astronomical Union designated 88 constellations in 1922. The majority of constellations visible from the northern hemisphere are named after the original 48 constellations in the , an astronomy work written by the ancient Greek astronomer sometime in the 2nd , A.D.

Contact Binary Stars Two gravitationally bound stars (a system) that have both filled their ‘Roche lobes’. These are teardrop-shaped regions around each star in the binary system, within which matter is gravitationally bound to each star. Any matter beyond a star’s Roche lobe will fall onto the other star in the binary system.

Contact Eclipsing Binary Eclipsing binary stars each other during their orbit around each other and their combined brightness varies as they do so. In some cases the two stars’ outer atmospheres fill the Roche lobe making contact with each other to make a ‘contact eclipsing binary’.

Continuously Habitable Zone Region around a star in which a planet can maintain appropriate conditions for the existence of life (including the retention of a significant amount of liquid ) for a period sufficient to allow the emergence of life.

Continuous Spectrum A spectrum with radiation at all wavelengths but with neither absorption nor emission lines.

Continuum The continuous spectrum that we would measure from a body if no spectral lines were present.

Contrast This is the difference in brightness of an object against its background, or the difference in brightness across its surface. Since most objects are inherently low in contrast anyway, any other light interference will further reduce contrast and obscure detail.

Convection The transfer of heat through a liquid or gas caused by the physical upwelling of hot matter. The heat transfer results in the circulation of currents from lower, hotter regions to higher, cooler regions. An everyday example of this process is boiling water. Convection occurs in the Sun and other stars.

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Convection Cells These are huge ‘bubbles’ (some the size of Jupiter) under the surface of the Sun, caused by the rising and sinking of super-hot gas in the Sun’s interior. Smaller convection cells cause ‘granules’ on the surface. These can be observed with a specially filtered H-alpha telescope.

Convection Zone The region below a star's surface where energy flows outward by the rising of hot gas known as convection.

Convex In lenses, it is the side that bulges out and causes light rays to meet (converge). In mirrors, it is also the side that bulges, but it causes light rays to spread apart (diverge).

Corona The outer part of the Sun's atmosphere. The corona is visible from Earth during a total solar eclipse. It is the bright glow seen in most solar eclipse photos.

Coronagraph A type of telescope with which the corona can be seen at times other than that of a total solar eclipse.

Coronal Mass Ejection A vast cloud of hot plasma, mainly composed of electrons and protons, that is ejected from the surface of the Sun.

Corrector Plate This is a lens plate that fits on the front of Schmidt-Cassegrain and Maksutov-Cassegrain telescopes to correct optical aberrations.

Cosmic Abundances The relative proportions of chemical elements in the Sun, the solar system, and the local region of the Milky Way galaxy. These proportions are determined by studies of the spectral lines in astronomical objects and are averaged for many stars in our cosmic neighbourhood. For example, for every million hydrogen atoms in an average star like our Sun, there are 98,000 helium atoms, 360 carbon atoms, 110 nitrogen atoms, 850 oxygen atoms, and so on.

Cosmic Background Radiation The primordial radiation field that fills the universe. It was created in the form of gamma rays at the time of the big bang, but has since cooled so that today its temperature is 3 K and its peak wavelength is near 1.1 millimetres (in the portion of the spectrum). Also known as the 3-degree background radiation. Also called cosmic microwave background radiation, CMBR.

Cosmic Ray Atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere with extremely high amounts of energy. 4 October 2012 29 © Paul Thomas

These are energetic particles that originate outside Earth’s atmosphere. The most energetic cosmic rays are of an unknown, extragalactic origin and travel at nearly the speed of light.

Cosmic String A tube-like configuration of energy that is believed to have existed in the early universe. A cosmic string would have a thickness smaller than a trillionth of an inch but its length would extend from one end of the visible universe to the other.

Cosmogony The study of celestial systems, including the solar system, stars, galaxies, and galactic clusters.

Cosmological Constant A term added to the field equations by Einstein in order to allow solutions in which the universe was static; that is, neither expanding nor contracting. Although the need for the term disappeared when it was discovered that the universe is expanding, the cosmological constant is retained in the field equations by modern cosmologists, but is usually assigned the value zero.

Cosmological Distance A distance far beyond the boundaries of our galaxy. When viewing objects at cosmological distances, the curved nature of becomes apparent. Possible cosmological effects include and .

Cosmological Principle This principle states that the distribution of matter across very large distances is the same everywhere in the universe and that the universe looks the same in all directions. According to this principle, our view of the universe is like the view from a boat on an ocean, which is essentially the same for any other person on any other boat on any other ocean. Measurements of matter and energy in the universe on the largest observable scales support the cosmological principle.

Cosmological Redshift A Doppler shift toward longer wavelengths that is caused by a galaxy's motion of recession, which in turn is caused by the expansion of the universe.

Cosmology A branch of science that deals with studying the origin, structure, and nature of the universe.

Cosmos The Universe.

Counterglow The English name for the skyglow opposite the Sun, very difficult to observe, caused by very thinly spread interplanetary material. Usually known by its German name, the Gegenschein.

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Crater A bowl-shaped depression formed by the impact of an asteroid or meteoroid. Also the depression around the opening of a volcano.

Craterlet A small ranging from a few millimetres across to a few metres.

Crayford A type of telescope focuser in which the focus tube is moved by a roller, and also slides on rollers, as well, and makes for precise adjustments.

Crepuscular Rays When the lower atmosphere is hazy, light passing through gaps between broken clouds can produce bright “beams” that are made visible by scattering by aerosol particles. These are usually seen best when the Sun is low, or at twilight

Crescent The phase of a body that is less than one-half illuminated.

Critical Mass The mass of an isotope above which a self-sustaining chain reaction can occur.

Crown Glass Originally the main material used to make flat planes of glass for windows, it is composed of soda-lime glass. It can be used to make lenses and prisms. Crown glass bends and disperses, or spreads out, light less than flint glass.

Cubewano Is a low-eccentricity Kuiper belt object that orbits beyond Neptune and is not controlled by an orbital with Neptune. Cubewanos have orbits with semi-major axes in the 40–50 AU range and, unlike Pluto, do not cross Neptune’s orbit. That is, they have low-eccentricity and sometimes low-inclination orbits like the classical planets.

Culmination An object culminates when it reaches greatest and least altitudes ( upper culmination and their lower culmination respectively). For non-circumpolar objects, the lower culmination is below the horizon. Most objects (the Moon sometimes being a notable exception) culminate when they reach the observer's meridian.

Curvature of Field An aberration of optical systems, in which the surface on which images in sharp focus are formed is not flat, but curved. Also called Field Curvature.

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Cytherean An adjective used to describe things related to the planet Venus. The correct form is really ‘Venereal’, but this is rarely used. Astronomers use ‘Cytherean’ because the island of Cytherea crops up in the mythology of Aphrodite (also called Venus).


D Lines A pair of lines from sodium that appear in the yellow part of the spectrum.

Danjon Scale A method of measuring brightness, ranging from 0-4, darkest to brightest respectively. The Danjon scale is represented by the letter L.

Dark Adaptation Your eye does not reach its full power of detecting low light levels (its full dark adaptation) until you have been in a dark environment for quite a long time, perhaps several . To obtain full dark adaptation, the darkness of the surroundings must be light free.

Dark Dust Cloud A region of interstellar space that contains a rich concentration of gas and dust. Such a cloud is often irregular in shape but sometimes has a well-defined edge. Visible light cannot pass through these clouds, so they obscure the light from stars beyond them.

Dark Energy A mysterious force that seems to work opposite to that of gravity and makes the universe expand at a faster pace.

Dark Frames These are images taken before, after or during your capture of light frames and are taken with the dust cover on the scope so no light gets in. These are used to adjust the light frames to correct for things like amplifier glow, hot pixels and gradients. They should be taken at the same temperature that the light frames were taken at and have exactly the same exposure settings (ISO, length etc.).

Dark Matter A term used to describe matter in the universe that cannot be seen, but can be detected by its gravitational effects on other bodies.

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Dark Matter Halo A giant halo of dark matter found around a galaxy. These have been detected by the effect their gravity has on the rotation of spiral galaxies.

Dark This is an of gas and dust which is silhouetted against a brighter background such as a field of stars or a glowing nebula.

Daughter Isotopes Isotopes formed by the radioactive decay of another isotope.

Dawes Limit The Dawes limit has to do with the physiology of human vision as well as the optical quality of telescopes. It is an empirically determined standard of how well an excellent small telescope can be expected to perform, in excellent conditions, when observing double stars that are not too faint, in which the two components are of the same brightness, or very nearly so.

Day, Sidereal The interval between two successive meridian passages (or culminations) of the same star. A sidereal day is equal to 23 hours, 56 minutes and 4.091 seconds. This is the true rotational period of the Earth.

Day, Solar The mean interval between successive meridian passages of the Sun. It is equal to 24 hours, 3 minutes and 56.55 seconds. The solar day is slightly longer than the sidereal day because the Sun seems to moves eastward against the stars, on average at roughly one degree per day. dB A unit of power ratio; the gain or loss in power in dB is equal to 10 times the logarithm of the power ratio. 1dB is approximately the smallest change in volume of which a normal ear can detect.

Decay Constant For an atom that undergoes radioactive decay, the decay constant is the proportionality factor between the time rate of decay and the total number of atoms present; it is the inverse of the mean lifetime of an atom.

Deceleration Parameter (q0) A particular measure of the rate at which the expansion of the universe is slowing down.

Declination One element of the astronomical coordinate system on the sky that is used by astronomers. Declination, which can be thought of as latitude on the earth projected onto the sky, is usually denoted by the lower-case Greek letter delta and is measured north 4 October 2012 33 © Paul Thomas

(+) and south (-) of the celestial equator in degrees, minutes, and seconds of arc. The celestial equator is defined as being at declination zero (0) degrees; the north and south celestial poles are defined as being at +90 and -90 degrees, respectively.

Deconvolution A process used in image processing to help reverse the effects of optical distortion.

Decretion Disk This is a circumstellar disk formed from gas ejected from the central star that now follows a near Keplerian orbit around it. This type of disk can be found around many Be stars.

Deep-Sky Objects This is the common name of a range of faint objects including galaxies, nebulae and star clusters, in other words objects which lie beyond the solar system. These objects are also often fondly referred to as ‘faint fuzzies’ by amateur astronomers.

Degenerate Gas A gas in which either free electrons or free neutrons are as densely spaced as allowed by laws of quantum mechanics. Such a gas has extraordinarily high density, and its pressure is not dependent on temperature, as it is in an ordinary gas. Degenerate electron gas provides the pressure that supports white dwarfs against collapse, and degenerate neutron gas similarly supports neutron stars.

Degree A unit used in the measurement of angles, heavily used particularly in astronomy. Due to ancient Babylonian mathematics, we still divide a circle into 360 even units of arc and call each of these units one degree. The entire sky, therefore, spans 360 degrees. Up to about 180 degrees of sky is visible from any given point on earth with an unobstructed horizon (as measured from, say, east to west, or north to south). The degree is used to make measurements of distance, or position (as with declination) in astronomy. In turn, a degree is composed of 60 minutes of arc, and also of 3600 seconds of arc.

Degree of Arc One degree of arc is 1/360 of a full circle. The apparent sizes of objects as seen from Earth can be measured in degrees of arc. The of the full moon or the Sun as seen from Earth is one-half of a degree.

Delta Distance of the celestial body from Earth in Astronomical Units (AU). For the Moon, Delta is the topocentric distance of the Moons mass centre from the observer in Earth radii (ER). It is also the fourth letter in Greek alphabet.

Density The amount of matter contained within a given volume. Density is measured in grams per cubic centimetre (or per litre). The density of water is 1.0, iron is 7.9, and lead is 11.3.

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Density-Wave Theory The explanation of spiral structure of galaxies as the effect of a wave of compression that rotates around the center of the galaxy and causes the formation of stars in the compressed region.

Decending Node The position in the orbit of a planet (or the Moon) where it crosses the plane of the ecliptic, moving southward.

Detatched Object Detached objects are a dynamical class of bodies in the outer Solar System beyond the orbit of Neptune. These objects have orbits whose points of closest approach to the Sun are sufficiently distant from the gravitational influence of Neptune, that they are essentially unaffected by Neptune and the other planets: this makes them appear to be "detached" from the Solar System. In this way, they differ substantially from the majority of the known Trans-Neptunian Objects.

Detection In electromagnetics, an operation converting the vector electromagnetic wave to a scalar time series proportional to either the amplitude or the power of the wave, with or without an accompanying angular time series. A crucial aspect of detection is: the signal-to-noise ratio after detection is the square of the signal-to-noise ratio before detection.

Determinism The doctrine according to which like causes always produce like effects and, conversely, events are entirely explainable by their antecedent causes.

Deuterium An isotope of hydrogen with a proton and a neutron in the nucleus (mass of 2 amu).

Dew Cap (Shield) A cylindrical extension of the telescope tube beyond the upper end, whose purpose is to retard the formation of dew on optical elements at or near that end.

Diagonal Mirror (i) The small flat mirror used near the upper end of a Newtonian telescope, to direct the converging beam of light over to the side of the tube. (ii) Star diagonals, that use mirrors and not prisms.

Dichotomy / Half-Phase An inner planet or moon is half lit from the Sun, i.e., its phase is 50%. This does not correspond to the event of greatest , due to different eccentricities of the planet and Earth.

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Differential Flexure The result of one part of a telescope system being rigid and another moving slightly in relation to it, very noticeable when applied to guiding during imaging.

Differential Gravitational Force A gravitational force acting on an extended object, such that the portions of the object closer to the source of gravitation feel a stronger force than the portions farther away. Such a force, also known as a tidal force, acts to deform or disrupt the object, and is responsible for many phenomena, ranging from synchronous rotation of moons or double stars to planetary ring systems to the disruption of galaxies in clusters.

Differential Rotation The rotation of a body such as a gaseous planet or the Sun so that different parts are rotating at different speeds. For example, a star or planet which rotates faster at its equator than it does at its poles.

Differentiation The sinking of relatively heavy elements into the core of a planet or other body. Differentiation can occur only in fluid bodies, so any planet that has undergone this process must once have been at least partially molten. Also, a process whereby a stem cell acquires the characteristic features of a given cell type.

Diffraction Grating A very closely ruled series of lines that, through their diffraction of light, provide a spectrum of radiation that falls on it.

Diffraction Limited A term given to mean the level of performance that would be apparent if there was no atmospheric disturbance to distort the view. Unless you are beyond the Earth’s atmosphere, there is always some atmospheric disturbance present. To say that a telescope is diffraction limited, is to say that no conceivable improvement in its optical quality can make its images noticeably sharper. It is so good, that when used in excellent conditions, then only the wave nature of light itself prevents the telescope from delivering perfectly sharp images.

Diffraction-Limited Optics The theoretical limit of the resolving power of a telescope’s optics. This limitation arises because the diffraction, or bending, of light as it passes through the aperture of a telescope causes a star to be observed as a bright disc (the Airy disc) with bright rings around it, rather than a point of light.

Digital Image A visible image that is recorded by an electronic detector and subdivided into small picture elements (pixels). Each element is assigned a number that corresponds to the brightness recorded at its physical location on the detector. Computer software converts the numerical information into a visual image. The Hubble Space Telescope records digital images.

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Dioptre A measurement of the optical power of lenses and curved mirrors in telescopes and binoculars. The dioptre is the reciprocal of the lens or mirror’s focal length in metres.

Dioptre Adjustment This is the ability to adjust the power of the eyepieces on a pair of binoculars to suit your eyes.

Dip The dip of the apparent (or sea) horizon is its angular distance below the astronomical horizon.

Direct Motion Another term for prograde motion.

Disk A flattened, circular region of gas, dust, and/or stars. It may refer to material surrounding a newly-formed star; material accreting onto a black hole or neutron star; or the large region of a spiral galaxy containing the spiral arms. Also, the apparent circular shape of the Sun, a planet, or the moon when seen in the sky or through a telescope

Dispersion Visible light is actually made up of different colours. Each colour bends by a different amount when refracted by glass. That’s why visible light is split, or dispersed, into different colours when it passes through a lens or prism. Shorter wavelengths, like purple and blue light, bend the most. Longer wavelengths, like red and orange light, bend the least.

Distance Modulus The difference m-M between the apparent and absolute magnitudes for a given star. This difference, which must be corrected for the effects of interstellar , is a direct measure of the distance to the star.

Distortion An aberration of optical systems, in which magnification changes systematically with distance away from the optical axis, causing distortions of images.

Diurnal Another name for daily.

Dob(sonian) A type of reflector. A simple alt/az mounted scope that is easy to use and offers very good value for money.

Doppler Effect Named after C. J. Doppler, the apparent change in wavelength of sound or light caused by the motion of the source, observer or 4 October 2012 37 © Paul Thomas

both. Light waves emitted by a moving object as received by an observer will be blueshifted (compressed) if approaching, redshifted (elongated) if receding. A similar effect occurs with sound waves. How much the changes depends on how fast the object is moving toward or away from the receiver.

Doppler Imaging A technique that uses high-resolution spectrographic data of a star to produce a map of the star’s surface.

Doppler Shift A change in frequency resulting from relative motion along the line between the transmitter and the receiver. If the source and the receiver are approaching each other, the frequency received is higher than the frequency transmitted by a factor, depending on the actual relative velocity. Knowledge of this shift is used to determine the relative velocity.

Double Bond Two shared pairs of electrons.

Double Star A grouping of two stars. This grouping can be apparent, where the stars seem close together, or physical, such as a binary system.

Doublet Two simple lenses used in combination, placed close together or in contact. If they are cemented together, they constitute a "cemented doublet". If they are merely closely adjacent, they are a "separated doublet".

Drake Equation An approach to estimating some of the factors in guessing at the number of communicating civilizations in the galaxy.

Drawtube The moving part of a telescope's focuser.

Drifting (Signal) Refers to a signal with an apparent time rate of change in its typical frequency. All drift to some extent. In a SETI system, the dominant drift should be largely the result of only the time rate of change in the Doppler shift.

Duct If the ray curvature within a thermal inversion is stronger than the curvature of the Earth's surface, rays can be continuously guided along the surface of the Earth without ever escaping to space. This region in which rays are trapped is called a duct. An observer within the duct sees a superior mirage of distant objects within the duct. Ducting occurs when the temperature gradient within the inversion is steeper than about 10 K in 100 meters (1 degree in 10 m).

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Dust Not the dust one finds around the house—which is typically fine bits of fabric, dirt, and dead skin cells—but rather the irregularly shaped grains of carbon and silicates measuring a fraction of a micron across that are found between the stars. Dust is most evident by its absorption, causing large dark patches in regions of our Milky Way Galaxy and dark bands across other galaxies.

Dwarf A member of a class of small spheroidal galaxies, similar to standard elliptical galaxies except for their small size and low luminosity. Dwarf galaxies are probably the most common in the universe, but cannot be detected at distances beyond the of galaxies.

Dwarf Nova A close binary-star system containing a white dwarf; material from the companion star falls onto the other at sporadic intervals, creating brief nuclear outbursts.

Dwarf Planet This classification was introduced in 2006 for celestial objects that orbit the Sun, are nearly spherical in shape, but are not satellites and have not cleared their neighbourhood of other celestial bodies. This new class includes Pluto (formerly a planet), (the largest object in the Main Belt) and (a trans-Neptunian object that was found to be larger than Pluto), the planet that started the debate.

Dwarf Spheroidal A that contains few stars and hardly any gas. Most of the Milky Way’s satellites are dwarf spheroidals.

Dwarf Star Most of the stars in the Universe are dwarf stars. Their masses are between a tenth and one hundred times the mass of the Sun.

Dynamo A device that generates electricity through the effect of motion in the presence of a magnetic field. The solar dynamo explains and the solar activity cycle.


Early-Type Star Is a legacy term that refers to hotter and more massive stars, in contrast to late-type stars that are cooler and less massive. The term originated from historical stellar models that assumed stars began their early life at a high temperature then gradually

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cooled off as they aged. It can be used to refer to the higher temperature members of any particular population or category of stars, rather than just all stars in general.

Earth The third planet from the Sun (sol) and one of four terrestrial planets in the inner solar system. Earth, the only planet where water exists in large quantities, has an atmosphere capable of supporting myriad life forms. The planet is 150 million kilometres (93 million miles) away from the Sun. Earth has one satellite - the Moon.

Earthshine Sunlight illuminating the moon after having been reflected by the earth.

Eccentricity A measure of the flatness of an ellipse, defined as half the distance between the foci divided by the semi-major axis. Elliptical orbits have an eccentricity >0 and <1, parabolic paths have an eccentricity =1, and hyperbolic paths have an eccentricity >1

Eclipse The total or partial blocking of one celestial body by another.

Eclipse Mapping A technique used to indirectly map the surface of an eclipsing binary star, to look for star spots and make other studies by using Doppler imaging.

Eclipsing Binary A star with a companion that periodically passes in front of it, and it as seen from Earth.

Ecliptic The apparent path of the sun against the sky background (celestial sphere); formally, the mean plane of the earth's orbit about the sun. This is the only place in the sky where solar and lunar eclipses occur.

Ecliptic Co-Ordinate System The system of specifying positions in the sky that uses the ecliptic—the Sun’s apparent annual path through the fixed stars—as the fundamental reference plane. Ecliptic co-ordinates are useful when specifying positions in the solar system and especially positions relative to the Sun.

Ecliptic Latitude The angular distance of an object relative to the ecliptic, expressed in degrees. Distances north of the ecliptic are positive; distances south are negative. The ecliptic latitude of the Sun is always zero.

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Ecliptic Longitude The angular distance of an object eastward from the vernal equinox, measured in degrees along the ecliptic. The ecliptic longitude of the Sun is zero when the Sun is on the vernal equinox and increases through the year by very nearly one degree per day.

Ecliptic Plane Or plane of the ecliptic, is the plane defined by the Earth's orbit around the Sun. Hence, the as viewed from Earth defines the intersection of this plane with the celestial sphere. The ecliptic plane is used as a reference plane for describing the position of other Solar System bodies. It differs from the celestial equator because of the of the Earth.

Ecosphere The portion of the atmosphere from sea-level to about 4000 meters in which it is possible to breathe without technological assistance.

Effective Temperature Of a star or planet is the temperature of an ideal black body that would emit the same total amount of electromagnetic radiation.

Egress When a celestial body emerges from a shadow or from a transit.

Einstein Ring This is a rare form of gravitational lens that occurs when a distant galaxy’s light is magnified by another galaxy directly in front of it, as seen from Earth. The result is a distorted image of a background galaxy in the shape of a perfect ring.

Ejecta Material that is ejected. Used mostly to describe the content of a massive star that is propelled outward in a explosion. Also used to describe the material that is blown radially outward in a meteor impact on the surface of a planet or moon.

Electric Field A force field set up by an electric charge.

Electromagnetic Force The force created by the interaction of electric and magnetic fields. The electromagnetic force can be either attractive or repulsive, and is important in countless situations in astrophysics.

Electromagnetic Radiation Another term for light. Light waves created by fluctuations of electric and magnetic fields in space.

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Electromagnetic Spectrum The full range of frequencies, from radio waves to gamma waves, that characterizes light.

Electromagnetism The science dealing with the physical relationship between electricity and magnetism. The principle of an electromagnet, a magnet generated by electrical current flow, is based on this phenomenon.

Electron A tiny (1/1830 the mass of a proton), negatively charged particle that orbits the nucleus of an atom. The charge is equal and opposite to that of a proton in the nucleus, and in a normal atom the number of electrons and protons is equal, so that the overall electrical charge is zero. It is the electrons that emit and absorb electromagnetic radiation, by making transitions between fixed energy levels.

Electron Carrier In a chain of chemical reactions, molecules that accept electrons from an electron donor and pass these to an electron acceptor.

Electron Configuration The arrangement of an atom's electrons in space.

Electron Degeneracy Occurs when electrons are compressed into a very tiny volume. Electron degeneracy is the force that supports a white dwarf against its own gravity, preventing it from collapsing. A neutron star is supported by neutron degeneracy. If a star is massive enough, not even neutron degeneracy can support its weight. The result is a black hole.

Electron Volt (eV) A unit of energy that is equal to the energy that an electron gains as it moves through a potential difference of one volt. This very small amount of energy is equal to 1.602 x 10–19 joules. Because an electron volt is so small, engineers and scientists sometimes use the terms MeV (mega-[million]) and GeV (giga-[billion]) electron volts.

Element (i) A substance composed of a particular kind of atom. All atoms with the same number of protons (atomic numbers) in the nucleus are examples of the same element and have identical chemical properties. For example, gold (with 79 protons) and iron (with 26 protons) are both elements, but table salt is not because it is made from two different elements: sodium and chlorine. The atoms of a particular element have the same number of protons in the nucleus and exhibit a unique set of chemical properties. There are about 90 naturally occurring elements on Earth. (ii) Any single part of an optical train such as a mirror or lens. In the case of lenses or eyepieces they may comprise several pieces of glass, each one known as an element. The combination of elements is used to correct the faults that would often be present in a single element lens or eyepiece.

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Elementary Particle Any of a number of sub-atomic particles.

Ellerman Bombs Ellerman bombs are bright explosions seen in the Sun’s chromosphere around newly formed active regions. They are named after the solar astronomer Ellerman who discovered them in September 1915.

Ellipse An ellipse is an oval shape. Johannes Kepler discovered that the orbits of the planets were elliptical in shape rather than circular.

Elliptical Galaxy A galaxy whose structure shaped like an ellipse and is smooth and lacks complex structures such as spiral arms.

Elongation The angular distance of a planetary body from the Sun as seen from Earth. A planet at greatest eastern elongation is seen in the sky and a planet at greatest western elongation will be seen in the sky.

Emersion A term used to describe when an object re-emerges after an or eclipse.

Emission Line A bright line in a spectrum caused by emission of light. Each chemical element emits and absorbs radiated energy at specific wavelengths. The collection of emission lines in a spectrum corresponds to the chemical elements contained in a celestial object.

Emission Nebula A cloud of interstellar gas that glows by the light of emission lines. The source of excitation that causes the gas to emit may be radiation from a nearby star, or heating by any of a variety of mechanisms.

Energy The ability to do work. Energy can be in either kinetic form, when it is a measure of the motion of an object, or potential form, when it is stored but capable of being released into kinetic form.

Energy Levels The specific, quantized energy levels that an electron may have in an atom.

Enrichment (Isotope) The process by which the proportion of one isotope of an element is increased relative to the others.

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Entropy Tendency of systems to become more disordered (and thus more uniform) over time; also a measure of disorder; in thermodynamics, a measure of the amount of heat energy in a closed system that is not available to do work.

Ephemeris A table of data arranged by date. Ephemeris tables are typically to list the positions of the Sun, Moon, planets and other solar system objects.

Epicycle A small circle whose centre is on the circumference of a larger circle; in Ptolemaic astronomy it was seen as the basis of revolution of the "seven planets", given a fixed central Earth.

Epoch (i) An epoch is a division of a geologic period; it is the smallest division of geologic time, lasting several million years. (ii) The date at which the co-ordinates on a will be correct with respect to precession.

Equant (i) The centre of a planetary epicycle. (ii) Having comparable measurements in all directions.

Equation of Time The correction which must be applied to in order to obtain mean solar time.

Equatorial Co-Ordinates The astronomical coordinate system in which positions are measured with respect to the celestial equator (in the north-south direction) and with respect to a fixed direction (in the east-west direction). The coordinates used are declination (north- south, in units of angle) and right ascension (east-west, in units of time).

Equatorial Mount A telescope mount with one axis parallel to the earth’s polar axis. This provides easy tracking of celestial objects, and is preferred by many for astrophotography.

Equinox Literally meaning "equal night (as day)”. An intersection of the ecliptic and the celestial equator. The two points (vernal, autumnal) at which the Sun crosses the celestial equator in its yearly path in the sky. The equinoxes occur on or near March 21 and September 22. The equinoxes signal the start of the Spring and Autumn .

Eras (Geologic) All of Earth's history since the appearance of the first life forms is divided roughly into four eras: Precambrian, from 3.5 billion to 4 October 2012 44 © Paul Thomas

570 million years ago; Palaeozoic, from 570 to 225 million; Mesozoic, from 225 to 65 million; and Cainozoic, from 65 million to the present. The last two eras are broken down into the following periods: the Mesozoic into Triassic) Jurassic, and Cretaceous; the Cainozoic into Tertiary and Quaternary.

Erfle See “Eyepiece”

Erg A unit of energy in the metric system, corresponding to the work done by a force of one dyne (the force that is required to accelerate one gram by one cm/sec2) producing a displacement of one centimetre.

Ergosphere A region surrounding a rotating black hole (or other system satisfying Kerr's solution) from which work can be extracted.

Escape Velocity The velocity required for an object to escape the gravitational field of a body such as a planet .In a more technical sense, the is the velocity at which the kinetic energy of the object equals its gravitational potential energy; if the object moves any faster, its kinetic energy exceeds its potential energy, and the object can escape the gravitational field.

Etalon An etalon is an optical device used to filter specific wavelengths of radiation by reflecting it many times between two parallel glass panels.

Ether (i) A compound containing an oxygen atom bonded to two hydrocarbon groups. (ii) The sky or heavens; the upper air. (iii) This was the fifth element in addition to air, earth, fire and water. (iv) A classical physical element, considered as prevalent in the heavens and inaccessible to humans. In some versions of alchemy. (v) A substance (aether) once thought to fill all space that allowed electromagnetic waves to pass through it and interact with matter, without exerting any resistance to matter or energy (disproved by Einstein in his ).

Euclidean Space A space with zero curvature; a space where the sum of the angles of a triangle is 180°.

European Space Agency A fifteen-member consortium of European countries for the design, development, and deployment of satellites. The Space Telescope - European Coordinating Facility (ST-ECF) supports the European astronomical community in exploiting the research opportunities provided by the Earth-orbiting Hubble Space Telescope. The ESA members are Austria, Belgium, Denmark, Finland, 4 October 2012 45 © Paul Thomas

France, Germany, Ireland, Italy, the Netherlands, Norway, Portugal, Spain, Sweden, Switzerland, and the United Kingdom, with Canada as a cooperating state.

Event Horizon The invisible boundary around a black hole which nothing can escape the gravitational pull - not even light.

Evolutionary Track is a curve on the Hertzsprung–Russell diagram that a solitary star, of a particular mass and composition, is expected to follow during the course of its evolution. This curve predicts the combination of temperature and luminosity that a star will have during part or all of its lifetime.

Evolved Star A star that is near the end of its life cycle where most of its fuel has been used up. At this point the star begins to loose mass in the form of stellar wind.

Exit Pupil (Eye-Ring) The position of the image of the objective lens or primary mirror formed by the eyepiece. It is the smallest disc through which all the collected light passes and is therefore the best position for the eye's pupil.

Exobiology The study of life as it might occur elsewhere than on earth. Also, the study of the origin of life, at any location.

Exoplanet An abbreviation of ‘extra solar planet’, meaning a planet not in the Solar System but orbiting another star.

Exosphere The upper part of an atmosphere where gas is very tenuous and rarefied. Earth’s exosphere lies about 500km above its surface. Mercury has no substantial atmosphere, but does have a very thin layer of gas extending around it that is essentially an exosphere.

Extinction The apparent dimming of star or planet when low on the horizon due to absorption by the Earth's atmosphere.

Extra-Low Dispersion ED glass is a type of glass that reduces the dispersion of light into its different wavelengths, allowing for better chromatic aberration correction in refractor.

Extragalactic A term that means outside of or beyond our own galaxy. 4 October 2012 46 © Paul Thomas

Extra-Terrestrial A term used to describe anything that does not originate on Earth.

Eyepiece The lens at the viewing end of a telescope. The eyepiece is responsible for enlarging the image captured by the instrument. Eyepieces are available in different types and powers, yielding differing amounts of magnification and field of view.

Telescope eyepiece types:

Brandon Four lenses; medium to high power use with any telescope at focal ratios down to f/4; very sharp at the centre (somewhat less so at the edges) for lunar, planetary, , and binary star observing; excellent colour correction and contrast; very low astigmatism; the sharpest design currently available for eyepiece projection photography; very minor field curvature and ghosting. Erfle An early 20th-century, 5-element eyepiece design that performs reasonably well at low-power focal lengths. Although Panoptics and Naglers are more highly regarded, an Erfle can provide a decent wide-field view at a much lower price. Huygens The two-element Huygenian eyepiece was invented by Christiaan Huygens (pronounced “HOY-kens”) in the 1600s. This design is inferior to more recent designs, so it is now obsolete, except that some Huygenian (“H”) eyepieces are still supplied with cheap imported telescopes. Eye relief is extremely short and it has a narrow apparent field of view. Will work reasonably well with focal ratios of perhaps f/15 or more. Kellner A 3-element eyepiece design that can perform reasonably well at low-power (wide-view) focal lengths. Although this is a very old eyepiece design it is often sold at reasonable prices, under the “modernized” name Super Modified Achromat (SMA). In essence, the Kellner is an achromatized Ramsden. Koenig The König eyepiece has a concave-convex positive doublet and a convex~flat positive singlet. The strongly convex surfaces of the doublet and singlet face and (nearly) touch each other. The doublet has its concave surface facing the light source and the singlet has its almost flat (slightly convex) surface facing the eye. It was designed in 1915 by German optician Albert König as a simplified Abbe. The design allows for high magnification with remarkably high eye relief. The field of view is typically 55° - 70° Lanthanum A specific eyepiece design. Lanthanum LV is a "house brand" of the large Japanese optical manufacturer, Vixen. The Lanthanum LV eyepiece series is particularly noted for having a long eye relief of 20 mm, even for the shortest focal lengths. At least some of the Lanthanum LV eyepieces are composed of seven simple lenses, cemented together into three doublets and a singlet. The front most doublet consists of a built-in Barlow lens. The rest of the eyepiece comprises two doublets separated by a singlet. Masayuma A 5-element eyepiece design that provides a wider FOV than orthoscopic eyepieces, without sacrificing very much contrast. The highly regarded Takahashi and Celestron Ultima eyepieces use this design. Monocentric A Monocentric is an achromatic triplet lens with two pieces of crown glass cemented on both sides of a flint glass element. The elements are thick, strongly curved, and their surfaces have a common centre giving it the name "monocentric". This design, is free from ghost reflections and gives a bright contrasty image, a desirable

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feature when it was invented (before anti-reflective coatings). It has a narrow field of view of around 25° Nagler Naglers have seven lenses, Nagler 2 and Ultra Wide have eight; up to three times the field area of a Plössl, but sharper edge to edge; up to ten times sharper at the edges than older wide angle designs such as Erfles; field is so wide you have to move your head from side to side to see all of it; more like looking out a window into space rather than looking through an eyepiece; superb colour correction, with little ghosting or field curvature; some loss of contrast on planets due to many lens elements; short focal lengths use a built-in Barlow to achieve high power without reducing eye relief; medium to high power use down to f/4; for star clusters, , and galaxies with any scope, but particularly good with fast focal ratio reflectors. Ortho A 4-element eyepiece design, sometimes referred to as an Abbe design after its inventor, the great German Abbe optical designer Ernst Abbe who created the design in 1880 (and subsequently became a partner in the Carl Zeiss Optical Works). In high-power focal lengths an ortho provides an excellent view for planetary observing, at a reasonable price. Panoptic Six lenses, with an ED (extra low dispersion glass) lens for exceptional contrast and a very wide field; up to five times the field edge correction of older wide field designs; for low to medium powers down to f/4 with any telescope, although particularly good with fast focal ratio reflectors when combined with a Paracorr coma corrector. Pentax A manufacturer of very high quality (and expensive) eyepieces whose design works particularly well for medium-power to medium-high-power views. In that range, personally I absolutely love these eyepieces, but note that eyepiece preference is a personal thing that varies depending on the idiosyncrasies of any particular person’s eyesight. Plössl A 4-element eyepiece design invented by the Austrian optician Simon Plössl in the 19th century, that provides a somewhat wider FOV than does an ortho. Unlike orthos, the design of Plössls has been modified and improved by manufacturers to the point that it now provides excellent views at both long and short focal lengths. The design provides good value for its price and has thus become the most popular general-eyepiece design among folks new to observing, or observers on a more limited budget. Nevertheless at any specific short, medium, or long focal length there are better specific eyepiece designs available (albeit at generally higher prices) so in the long run experienced observers usually end up not using many Plössls. Ramsden The Ramsden eyepiece comprises two plano convex lenses of the same glass and similar focal lengths, placed less than one eye-lens focal length apart. It is suitable for use with instruments operating using near monochromatic light sources, e.g. polarimeters. RKE An RKE eyepiece has an achromatic field lens and double convex eye lens, a reversed adaptation of the Kellner eyepiece. This design provides slightly wider field of view than classic Kellner design and makes it design similar to a widely spaced version of the König. RKE stands for Rank, Kaspereit, Erfle, the three designs from which the eyepiece was derived. SMA Super Modified Achromat - a fancy term for an eyepiece using the Kellner optical design. Speers-Waler A specific eyepiece design, noted for a very wide apparent field of view, almost 80 degrees, and for excellent correction at fast focal ratios. "Waler" is an acronym for "Wide Angle, Long Eye Relief". Takahashi Proprietary fully-multicoated five lens design with long eye relief; for low to very high power use to f/4; two 4 October 2012 48 © Paul Thomas

shortest focal lengths use one ED (extra low dispersion glass) lens for exceptional contrast and colour correction; usable with all scope types; suitable for all types of observing; very good sharpness and contrast; very little ghosting.

Eye Relief The distance, within which, the eye can be placed from the eyepiece, to obtain sharp vision.


F-Number (Focal Ratio) The ratio of the focal length divided by the aperture (in the same units).

Faculae Faculae are brighter, more prominent patches of the solar photosphere (the ‘surface’ of the Sun). They are sometimes an indication of a forming and show up particularly well through white-light solar filters.

Faint Object Camera An instrument aboard the Hubble Space Telescope that recorded high-resolution images of faint celestial objects in deep space. Built by the European Space Agency, the camera collected ultraviolet and visible light from celestial objects. The camera served as Hubble’s “telephoto lens” - recording the most detailed images over a small field of view. The FOC’s resolution allowed Hubble to single out individual stars in distant star clusters. The instrument was replaced in March 2002 during Servicing Mission 3B.

Faint Object Spectrograph An instrument aboard the Hubble Space Telescope that acted like a prism to separate light from the into its component colours, providing a wavelength “fingerprint” of the object being observed. Such information yields clues about an object’s temperature, chemical composition, density, and motion. Spectrographic observations also reveal changes in celestial objects as the universe evolves. The instrument was replaced in February 1997 during the Second Servicing Mission.

Far-Infrared Spectrum The region of the infrared spectrum that exhibits the longest wavelengths and the lowest frequencies and .

Fault A geological term that refers to a fracture or a break in a hard surface like the Earth’s crust. This area is a zone of weakness and may be the site of or volcanoes. All planets or moons with a hard crust are candidates for faults or breaks on their surfaces.

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Feed (Antenna Feed, Line Feed) In a reflecting antenna system, the device that converts a guided (by wire, cable, or wave guide) electromagnetic wave into an electromagnetic radiation field, and vice versa, when reciprocity theorem holds as it so often does. Commonly, feeds are some form of horn antenna, but they may be dipole arrays.

Field Curvature An aberration of optical systems, in which the surface on which images in sharp focus are formed is not flat, but curved.

Field Flattener Field curvature can be corrected by a simple lens placed at or close to the focal "plane" of the instrument in question. Such a lens is called a field flattener. If the field curvature is concave toward the incoming beam of light, the field flattener must be a negative lens. If the field curvature is convex toward the incoming beam of light, the field flattener must be a positive lens. The size of the field flattener must be equal to or slightly greater than the size of the desired flat field.

Field Galaxy This is a galaxy that does not belong to a larger cluster of galaxies, but is gravitationally alone.

Field An instrument rather resembling a binocular, but which uses a special eyepiece design, instead of prisms, to achieve an upright image. Field glasses are thereby generally lighter and less expensive than binoculars, but they have an extremely narrow apparent field of view.

Field of View The amount of sky seen through a telescope’s eyepiece or optical device (the angular separation between opposite edges of the visual field).

Field Rotation If a mount is not well polar aligned, even though it may track an object very well, there will be some residual rotation over time due to the error in alignment. This sometimes manifests itself in images as elongated stars in the corners of the image.

Field Star This is a randomly situated star that lies along the line of sight to a group of physically associated stars under study, such as a star cluster. These field stars can contaminate the results for a study and so they need to be identified.

Field Stop The field stop is the physical size of aperture available to look through on any given eyepiece.

Filament A strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun. 4 October 2012 50 © Paul Thomas

Filter A type of window that absorbs certain colours of light while allowing others to pass through. Astronomers use filters to observe how celestial objects appear in certain colours of light or to reduce the light of exceptionally bright objects. For example, a pair of sunglasses acts as a type of filter, reducing the amount of incoming light while still allowing some light to pass through to the eyes.

Filter Wheels Rotating wheels in a telescope instrument that allow specific colours of light from a celestial object to pass through and form an image on the detector. The Wide Field and Planetary Camera 2 aboard the Hubble Space Telescope has 12 filter wheels, each of which holds four filters.

Filtergram A photograph taken through a filter that passes only a very narrow band of wavelengths; usually applied to solar photographs.

Finder (Finderscope) A small, wide-field telescope attached to a larger telescope. The finder is used to help point the larger telescope to the desired viewing location.

Fine Guidance Sensors The Fine Guidance Sensors are cameras that help keep the Hubble Space Telescope pointed precisely in the right direction, and lock onto “guide stars” and measure their positions relative to the object being viewed. Adjustments based on these precise readings keep Hubble pointed in the right direction. The sensors also are used to perform celestial measurements.

Fireball An extremely bright meteor. Also known as bolides, fireballs can be several times brighter than the full Moon. Some can even be accompanied by a sonic boom.

First Law of Thermodynamics Energy is neither created nor destroyed.

First Point of Aries The Vernal Equinox point, i.e. that where the centre of the Sun, moving northwards, crosses the equator. It is the reference direction for the equatorial system of co-ordinates.

Fission, Nuclear The splitting of an atomic nucleus.

Flare A rapid eruption of material from the surface of the sun or other star. 4 October 2012 51 © Paul Thomas

Flat Frames Flat frames are images taken with a constant, 'flat' light source such as that from a light box or the sky. They show up things like dust motes on the sensor, Vignetting or uneven illumination in the optical system. Expose them to about 1/3 histogram.

Flat Universe A geometric model of the universe in which the laws of geometry are like those that would apply on a flat surface such as a table top.

Flint Glass The lead glass that was produced in the United States and the United Kingdom prior to the 1860s. This glass is used to make telescope lenses and prisms. Flint glass bends and disperses, or spreads out, light more than crown glass.

Flocculus (plural flocculi) A marking on the surface of the sun associated with a solar prominence.

Fluorite A special kind of optical material, of particular value in making achromats and apochromats.

Fluorite Doublet An achromat in which one lens is fluorite. Fluorite doublets are better than achromats made with conventional glass, by a factor of about eight, at bringing all colours of visible light to the same focal position.

Flux The flow of fluid, particles, or energy through a given area within a certain time. In astronomy, this term is often used to describe the rate at which light flows. For example, the amount of light () striking a single square centimetre of a detector in one second is its .

Focal Length The distance from a lens or mirror to the point at which the rays from an object at infinity, come to a focus.

Focal Plane The plane (usually this is actually the surface of a sphere of large radius) where the image is formed by the main optics of the telescope. The eyepiece examines this image.

Focal Point The focal point of a lens or mirror is the point in space where parallel light rays meet after passing through the lens or bouncing off the mirror. A “perfect” lens or mirror would send all light rays through one focal point, which would result in the clearest image. 4 October 2012 52 © Paul Thomas

Focal Ratio (f/ number) - The Focal Length of a scope divided by its Aperture (in the same units). Low is Fast; good for faint objects. High is Slow; good for brighter, planetary objects.

Focuser The focuser is the small piece of mechanical equipment that holds the eyepiece. It is moveable to vary the distance between the eyepiece and the lens or mirrors to achieve focus. There are many different types of focuser on the market from simple rack- and-pinion devices to more elaborate double speed, feather-touch focusers.

Force In physics, something that can or does cause a change of momentum, measured by the rate of change of momentum with time.

Force Field A way of describing phenomena that result from action at a distance, that is, even though objects are not touching.

Fork Mount A mount where the telescope swings in declination or in altitude between two arms. It is suited only to short telescope tubes, such as Cassegrains, and variations thereof. It requires a wedge to be used equatorially.

Fossil Group These are the remnants of galaxy clusters that have seen most of their galaxies merge together. They can be identified by their vast haloes of X-rays. Massive elliptical galaxies will form in these groups, which contain the dark matter of entire galaxy clusters.

Foucault Pendulum A pendulum that varies in the direction of its swing as the Earth rotates. Used to demonstrate that it is Earth that rotates and not the sky.

Fraunhofer Lines The absorption lines of a solar or other stellar spectrum.

Frequency The rate (in units of Hertz, or cycles per second) at which electromagnetic waves pass a fixed point. The frequency, usually designated ƒ, is related to the wavelength λ and the speed of light c by ƒ = c/λ.

Full Well Capacity This is the number of electrons that a pixel can hold before saturating and possibly causing blooming.

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Fully Coated A term for an optical system in which every optical surface where light passes from glass into air, or from air into glass, is coated with low- coatings, probably magnesium fluoride.

Fully Multicoated A term for an optical system in which every optical surface where light passes from glass into air, or from air into glass, is coated with high-tech, low-reflection coatings, which transmit more light than magnesium fluoride.

Fusion The process by which atomic nuclei collide so fast that they stick together, form new atoms, and emit a large amount of energy. In the centre of most stars, hydrogen fuses into helium. The energy emitted by fusion prevents the star’s enormous mass from collapsing in on itself and causes the star to glow.


Gain When a CCD is read the pixel charge may be in micro-volts and an amplifier is needed to bring the signal into the range where it is usable. The amount that the signal is increased is the gain. An amplifier not only reads the signal, but reads any noise inherent in the chip which has to be accounted for.

Gain Stability crudely defined by AG/G = g(t)', the smaller this quantity over the relevant time interval, the less the gain instability or the greater the gain stability. Gain (amplification) of analog signalling systems always varies somewhat with time; g(t) contains a variety of "noise" terms of zero mean and various secular terms; the latter dominate unless proper precautions are in force.

Galactic Cluster An asymmetric type of collection of stars that shared a common origin.

Galactic Co-Ordinates The system of celestial co-ordinates in which the as the reference plane and the galactic centre as the reference direction. The positions are given in galactic latitude and galactic longitude.

Galactic Disk A flattened disk of gas and young stars in a galaxy. Some galactic disks have material concentrated in spiral arms (as in a spiral galaxy) or bars (as in barred spirals).

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Galactic Halo Spherical regions around spiral galaxies that contain dim stars and globular clusters. The radius of the halo surrounding the Milky Way extends some 50,000 light-years from the galactic centre.

Galactic Nucleus The central concentration of matter (stars, gas, dust, and perhaps a black hole) in a galaxy, typically spanning no more than a few light-years in diameter.

Galactic Plane The imaginary projection of the Milky Way’s disk on the sky. Most of the galaxy’s stars and interstellar matter reside in this disk. Objects in the galaxy are often referred to as being above, below, or in the galactic plane.

Galaxy A large grouping of stars in a formation. Galaxies are found in a variety of sizes and shapes. Our own Milky Way galaxy is spiral in shape and contains several billion stars. Some galaxies are so distant the their light takes millions of years to reach the Earth.

Galaxy Cluster A collection of dozens to thousands of galaxies bound together by gravity.

Galaxy Epoch fifth epoch in the history of the Universe, lasting on the order of 1010 yr., during which matter largely coagulated into galactic masses.

Galaxy Evolution The study of the birth of galaxies and how they change and develop over time.

Galaxy A vast collection of galaxy clusters that may contain tens of thousands of galaxies spanning over a hundred million light-years of space. Galaxy are the largest structures in the universe.

Galilean Moons (Satellites) The name given to Jupiter's four largest moons, , , Calisto & . They were discovered independently by Galileo Galilei and Simon Marius.

Galilean Telescope The kind of telescope built by Galileo featured a singlet objective and a singlet eyepiece. The eyepiece consisted of a so-called "negative" lens, which is what most people think of as the opposite of a magnifying glass. This kind of lens gives a very narrow apparent field of view, but it gives an image that is upright and has the left and right sides correctly positioned, whereas most astronomical telescopes give an image that is upside down, and/or has left and right reversed. 4 October 2012 55 © Paul Thomas

Gamma-Ray The form of electromagnetic radiation with the highest energy and the shortest wavelength. Any having an energy greater than about 100,000 electronVolts (eV). In comparison, visible light has an energy of 1.65 to 3.1 eV, and x-rays have an energy of 124 eV and upward.

Gamma-Ray Burst Flashes of high-energy electromagnetic radiation from random locations in space. They may be caused by the merger of two black holes or neutron stars, or the explosion of a supernova. They are the brightest and most energetic explosions known.

Ganymede One of Jupiter’s largest moons. Ganymede, the largest satellite in our solar system, is about 5300 kilometres (3300 miles) wide and larger than the planet Mercury.

Gas The state of matter in which the substance maintains neither shape nor volume.

Gas Giant A large planet with a small, rocky core and a deep atmosphere composed mostly of hydrogen and helium. Our solar system contains four gas giants: Jupiter, Saturn, Uranus, and Neptune. This group is also known as Jovian planets.

Gaseous Nebula A glowing cloud of gas in interstellar space. The cloud of gas may be either an emission nebula, which absorbs ultraviolet light from nearby stars and re-radiates visible light, or a reflection nebula, which reflects light off of its dust particles.

Gauss A unit of measurement of a magnetic field. Earth has a surface magnetic field strength of between 0.3 and 0.6 Gauss.

Gegenschein The diffuse glowing spot, seen on the ecliptic opposite the sun's direction, created by sunlight reflected off of interplanetary dust.

General Relativity This is Einstein’s theory of gravity, which states that mass and energy curve space-time – the fabric of the Universe. Gravity is the result of the curvature of space-time, which causes light to bend around an object and planets to orbit stars.

Geocentric An adjective meaning “cantered on the Earth.” Most early civilizations had a geocentric view of the universe.

Geodesy Is the study of the size and shape of the Earth. The corresponding adjective is geodetic. 4 October 2012 56 © Paul Thomas

Geoid Is the equipotential surface corresponding to mean sea level.

Geometric Albedo Is the ratio of the brightness of an astronomical body at a phase angle of zero to an idealized flat, fully reflecting, diffusively scattering (Lambertian) disk with the same cross-section. It is a measure of how much of the incoming illumination is being scattered back toward an observer and has a value between zero and one.

Geometric Horizon Where the apparent sea horizon would be if there were no refraction; equivalently, where the cone with vertex at the observer's eye and tangent to the sea surface would meet the celestial sphere.

Geostationary Orbit The orbit of a satellite which is both geosynchronous and in the equatorial plane. The satellite will appear to remain in a fixed position in relation to the observer.

Geosynchronous Orbit An orbit in which a satellite's orbital velocity is matched to the rotational velocity of the planet. A spacecraft in geosynchronous orbit appears to hang motionless above one position of a planet's surface.

Geothermal Energy Energy derived from the heat of Earth's interior.

German Equatorial Mount A common type of equatorial mount. As opposed to an English equatorial mount, here the OTA is offset from the centre of the mount and is balanced by means of counterweights on the other side and an interconnecting shaft. The shaft rotates at 90 degrees to the mount’s polar axis (i.e. the shaft points constantly at 0 degrees declination but can move through all hours of right ascension), and the OTA can rotate about the shaft’s axis so as to point at any angle of declination.

Ghost Crater These are craters who have been in filled, buried or submerged by geological processes but are sometimes still just visible on the surface of a planet or moon.

Ghost Images Ghost images are spurious images caused by unwanted reflections in an optical system. They are sometimes in focus, or nearly so. Several common eyepiece designs are plagued by ghosts.

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Giant Massive clouds of gas in interstellar space composed primarily of hydrogen molecules. These clouds have enough mass to produce thousands of stars and are frequently the sites of new star formation.

Giant Planets Jupiter, Saturn, Uranus, and Neptune.

Giant Star A type of star brighter than main sequence stars of the same spectral type.

Gibbous A either side of the full moon when the moon's disc as seen from the Earth is larger than a semicircle (or of a planet).

Giga 109 (as in gigahertz, GHz); one billion (U.S.A.).

Globular Cluster A tight, spherical grouping of hundreds of thousands of stars. Globular clusters are composed of older stars, and are usually found around the central regions of a galaxy.

Gnomon The raised area of a that casts a shadow on a dial so that the time of day can be read.

Golden Handle When the Moon is about 2 days past first quarter, the mountains Jura Montes on the Moon, start to get sunlight, while the plain Sinus Iridium is still in deep night. The bright mountains look like a handle stretching from the bright side of the Moon to the dark side.

Goto (Go-To) Shorthand for the kind of command interface to a computer-controlled telescope, in which the observer tells the telescope what object to "go to" next.

Gradient An image showing a gradient has dark sky at one edge lighter than the dark sky at another edge. This is not wanted. Some gradients such as those caused by the optical system can be handled by flat frames, others need a bit of ingenuity.

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Gram A unit of mass equal to the quantity' of mass contained in one cubic centimetre of water.

Grand Unified Theory A theory stating that that strong and weak nuclear forces and electromagnetic forces are varying aspects of the same fundamental force.

Granulation The "grains of rice" appearance of the Sun's surface, which results from convection cells within the Sun.

Graticule A system of parallel lines or crossed lines at the telescope's focal plane, used in micrometres.

Grating A surface ruled with closely spaced lines that, through diffraction, breaks up light into its spectrum.

Gravitation One of the four fundamental forces of nature, the force by which two masses attract each other.

Gravitational Clustering The process by which a large-scale structure grows as its gravity attracts smaller building blocks. Astronomers believe that all the large-scale structures (such as galaxies, galaxy clusters, and galaxy superclusters) that we see in the universe today formed through gravitational clustering.

Gravitational Collapse Occurs when a massive body collapses under its own weight. For example, interstellar clouds collapse to become stars until the onset of nuclear fusion stops the collapse.

Gravitational Constant A value used in the calculation of the gravitational force between objects. In the equation describing the force of gravity, “G” represents the gravitational constant and is equal to 6.672x10–11 Nm2/kg2.

Gravitational Instability A condition that occurs when an object’s inward-pulling gravitational forces exceed the outward-pushing pressure forces, thus causing the object to collapse on itself. For example, when the pressure forces within an interstellar gas cloud cannot resist the gravitational forces that act to compress the cloud, then the cloud collapses upon itself to form a star.

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Gravitational Lens A massive celestial object, like a galaxy, that curves space-time around it to such an extent that it bends the path of light from distant bright objects like quasars. This phenomenon was predicted by Albert Einstein in his general theory of relativity.

Gravitational Redshift The reddening of light from a very massive object caused by photons escaping and traveling away from the object’s strong gravitational field. An example of gravitational redshift is light escaping from the surface of a neutron star.

Gravitational Wave Predicted in Einstein’s general theory of relativity, these waves are believed to be ripples in the fabric of space-time caused by bodies such as binary systems (binary white dwarfs, neutron stars or black holes). The Laser Interferometer Space Antenna (LISA) is one of several missions that will hopefully detect gravitational waves.

Gravitationally Bound Objects held in orbit about each other by their gravitational attraction. For example, satellites in orbit around the Earth are gravitationally bound to this planet since they can’t escape its gravity. By contrast, the Voyager spacecraft, which explored the outer solar system, was launched with enough energy to escape Earth’s gravity altogether, and hence it is not gravitationally bound.

Gravity (Gravitational Force) The attractive force between all masses in the universe. All objects that have mass possess a gravitational force that attracts all other masses. The more massive the object, the stronger the gravitational force. The closer objects are to each other, the stronger the gravitational attraction.

Gravity Assist An effect through which an orbiting object, such as a spacecraft or a comet, gains or loses speed by virtue of the gravitational might of a planet or other celestial object that it passes. For example, the Cassini spacecraft in its journey to Saturn used a gravity assist from Earth to increase its velocity by about 36,000 kilometres per (22,300 miles per hour).

Gravity Waves These are waves produced by the gravitational effects of one body on another. The Moon causes rocks on Earth to physically bulge up – not as much as water, but measurably. Gravity waves shouldn’t be mistaken for gravitational waves predicted by general relativity.

Great Circle The intersection of a plane that passes through the centre of a sphere with the surface of that sphere; the largest possible circle that can be drawn on the surface of a sphere.

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Great Red Spot A circulating storm located in Jupiter’s upper atmosphere. The storm, which rotates around the planet in six days, is the width of two to three Earths. Galileo first observed the spot in the 17th century.

Great Rift the Great Rift (sometimes called the Dark Side, Dark Rift, or, Dark River) is a series of overlapping, non-luminous, molecular dust clouds that are located between the Solar System and the Sagittarius Arm of the Milky Way Galaxy at a distance of about 100 parsecs (300 ly or 3×1015 km) from Earth. The clouds are estimated to contain about 1 million solar masses of plasma and dust. To the , the Great Rift appears as a dark lane that divides the bright band of the Milky Way lengthwise, through about one-third of its extent, and is flanked by lanes of numerous stars.

Greatest Eastern Elongation The greatest angular distance to the east of the Sun reached by Mercury or Venus. When a planet is at its eastern elongation, it sets after the Sun and is at its best visibility in the evening sky.

Greenhouse Effect A warming of the Earth's surface and lower layers of the atmosphere caused by interaction of solar radiation with atmospheric gases (mainly carbon dioxide, , and water vapour) and its conversion to heat because it is transparent to incoming visible radiation but opaque to the infrared radiation that is emitted by the surface of the planet.

Greenhouse Gas Gases such as carbon dioxide, methane, and water vapour that produce a greenhouse effect.

Gregorian Calendar The calendar in current use, with normal years that are 365 days long, with leap years every fourth year except for years that are divisible by 100 but not by 400.

Gregorian Telescope A Gregorian telescope is a reflecting telescope that uses two specific shapes of concave mirrors to form the image. The primary mirror is a concave paraboloid, just as in a Newtonian. The secondary mirror is a much smaller concave ellipsoid, positioned a little way beyond the primary's focal plane. The secondary reflects the beam of light diverging from the focus, back toward the primary, which typically has a hole in the centre, so that the focal plane of the combined mirrors is easily accessible behind the primary.

Ground State The state of an atom in which all electrons are in the lowest possible energy levels.

Group A vertical column of the periodic table; a family of elements; also, A characteristic part of a molecule. 4 October 2012 61 © Paul Thomas

Great Red Spot A large high pressure storm system on Jupiter akin to a cyclone on earth. It is almost three times the diameter of the earth, produces winds of up to 400km/h and has been raging for at least 300 years. It transits the face of Jupiter every 8 hours.

Guan Sheng Optical Maker of telescopes in Taiwan, many of which are re-branded as "Bintel", "Southern Cross", "SkyWatcher", etc., particularly the Dobsonians.

Guide Star A star that a telescope’s guidance system locks onto to ensure that a celestial object is followed and observed as the telescope moves, owing either to the Earth’s rotation or the telescope’s orbital trajectory. The Hubble Space Telescope uses two of its three Fine Guidance Sensors to detect and lock onto guide stars. The telescope’s science operations centre has more than 15 million guide stars in its database - the Guide Star Catalogue.

Guiding Used in astroimaging. Guiding uses a second imaging device to "lock on" to a stars centroid and send correcting signals to the mount as this star moves. With good Polar Alignment, a stable mount and good guiding, long exposures are possible with very little translation of the objects being imaged. Guiding becomes more and more difficult as the scope focal length and FOV reduces.

Guidescope A (usually) smaller telescope operated in conjunction with a CCD camera or modified webcam, mounted on top of a main imaging scope. It’s used to guide the movement of the mount to iron out any imperfections in the tracking.

Gyrochronology Scientists have found a more accurate way to calculate the ages of stars. Gyrochronology uses the and colour of a star to determine its , with the age of the Sun being used to calibrate the measurements. Unlike another technique for calculating stellar ages, gyrochronology works well for main sequence stars (stars that are still burning hydrogen) and lone stars that don’t belong to a cluster.

Gyroscope A gyroscope is a spinning wheel mounted on a movable frame that assists in stabilizing and pointing a space-based observatory. Gyroscopes are important because they measure the rate of motion as the observatory moves and help ensure the telescope retains correct pointing during observations. The gyroscopes provide the general pointing of the telescope while the fine guidance sensors provide the “fine tuning.” Gyroscopes are used in navigational instruments for aircraft, satellites, and ships. The Hubble Space Telescope has six gyroscopes for navigation and sighting purposes.

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H-I Region An interstellar region of neutral hydrogen.

H-II Star Forming Region A large nebulous region of hydrogen gas that is being excited and ionised by the strong ultraviolet radiation from hot newly formed stars.

Ha Filter A particular type of filter used for observing the sun, or DSO Hydrogen line emissions.

Hb Filter Used to observe very faint objects which wouldn't normally be seen without one e.g. Horsehead Nebula.

H-Alpha Light Atoms emit radiation at certain wavelengths based on how the electrons move between certain fixed energies. If an electron jumps to a specific energy level within a hydrogen atom it will emit radiation of a specific wavelength.

Habitable Zone A region around a star where planets with liquid water may be present. A planet on the near edge of the habitable zone would have a surface temperature slightly lower than the boiling point of water. A planet on the distant edge of the habitable zone would have a surface temperature slightly higher than the freezing point of water.

Hadron A class of subatomic particle that includes mesons and baryons. A hadron is defined as a particle that interacts via the strong force.

Hadron Epoch Second epoch in the history of the Universe, lasting on the order of a second; named for the heavy elementary particles (protons, neutrons, mesons) that were the most abundant form of matter at the time.

Half-Life The period of time needed for half of the radioactive isotopes in a sample to decay to daughter atoms.

Halo (a) The extended outer portions far above and below the plane of a galaxy such as the Milky Way. The halo is thought to contain

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a large fraction of the total mass of the galaxy, mostly in the form of dim stars and interstellar gas. (b) the extensive cloud of gas surrounding the head of a comet.

Hartmann Mask An alternative focusing tool, like a Bahtinov Mask.

HDF-N Hubble Deep Field North (HDF-N) is a tiny region of the northern sky near the toward which the Hubble Space Telescope was pointed for ten straight days in 1995. Because this observation was designed to detect very faint light from the most distant galaxies Hubble can observe, the field contains few bright celestial objects. Seemingly devoid of light, this small area provided a “keyhole” view of the universe’s past, reaching across space and time to see infant galaxies. By probing these remote regions of space, astronomers are gaining more information on galaxy development.

HDF-S Hubble Deep Field South (HDF-S) is a tiny region of the southern sky near the Southern Cross toward which the Hubble Space Telescope was pointed for ten straight days in 1998. Because this observation was designed to detect very faint light from the most distant galaxies Hubble can observe, the field contains few bright celestial objects. Seemingly devoid of light, this small area provided a “keyhole” view of the universe’s past, reaching across space and time to see infant galaxies. By probing these remote regions of space, astronomers are gaining more information on galaxy development.

Head Of a comet, the nucleus and coma together.

Heat A measure of a quantity of energy; of how much energy a sample contains.

Heat Capacity (of a substance) The amount of heat needed to change the temperature of the substance by 1°C.

Heat of Vaporization (of a substance) The amount of heat involved in the evaporation or condensation of 1g of the substance.

Height Linear distance (usually measured in meters) above sea level; as contrasted with altitude, which is angular distance above the astronomical horizon. Usually called “elevation” by geographers.

Heliacal Rising The date when a star first becomes visible in the sky, rising just before the Sun.

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Helical Focuser A telescope focuser in which adjustment of focus is made by screwing in and out either the eyepiece itself, or a focus tube which contains it. Many binoculars have a helical focuser for one or both eyepieces.

Heliocentric Sun-centred; using the sun rather than the earth as the point to which we refer. A heliocentric measurement, for example, omits the effect of the Doppler shift caused by the earth's orbital motion.

Heliocentric Model This literally means ‘Sun-centred’ and is the accepted model for the Solar System that puts the Sun at the centre, with the planets orbiting around it.

Heliopause The point where the pressure of the local interstellar medium overwhelms that of the solar wind, stopping it from expanding any further out into space.

Helioseismology The study of the internal structure of the Sun, which involves examining how pressure waves propagate within it.

Heliosheath The heliosheath is the region of the beyond the termination shock. Here the wind is slowed, compressed and made turbulent by its interaction with the interstellar medium. Its distance from the Sun is approximately 80 to 100 astronomical units (AU) at its closest point.

Heliosphere A ‘bubble’ that surrounds our Solar System created by the influence of the Sun’s solar wind and magnetic field interacting with interstellar space.

Helium Atom consisting of two protons and two electrons.

Helium Flash A rapid burst of nuclear reactions in the degenerate core of a moderate-mass star in the hydrogen shell-burning phase. The flash occurs when the core temperature reaches a sufficiently high temperature to trigger the triple-alpha reaction.

Hemisphere Half of a spherical or roughly spherical body; for example, the northern and southern halves of the Earth, above and below the equator.

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Hertz (Hz) Measure of frequency with units of sec-1, formerly called cycles per second; an oscillating system that completes a cycle a second has a frequency of 1Hz.

Hertzsprung- Russell Diagram A diagram on which stars are represented according to their absolute magnitudes (on the vertical axis) and spectral types (on the horizontal axis).Because the physical properties of stars are interrelated, stars do not fall randomly on such a diagram, but instead lie in well-defined regions according to their state of evolution. Very similar diagrams can be constructed that show luminosity instead of absolute magnitude, and temperature or colour index in place of spectral type.

Hertzsprung Gap A region above the main sequence in a Hertzsprung-Russell diagram through which stars evolve rapidly and thus in which few stars are found.

Heterocyclic Compound A cyclic compound in which one or more atoms in the ring is (are) not carbon.

Heterotrophic Hypothesis The concept introduced by A.I. Oparin and J.B.S. Bernal that the earliest forms of life were heterotrophs that used non- biologically produced organic matter as their carbon source.

Heterotrophy Literally, other-feeding; the condition of an organism that is not able to obtain nutrients by synthesizing non organic materials from the environment, and that therefore must consume other life forms to obtain the organic products necessary for life e.g., animals, fungi, most bacteria.

High-Velocity Star A star whose velocity relative to the solar system is large. As a rule, high-velocity stars are Population II objects following orbital paths that are highly inclined to the plane of the galactic disk.

Histogram A histogram is a graph of how many pixels of each intensity level occur in an image. They are X/Y graphs and show pixel intensity along the X axis and number of pixels on the Y axis. An un-stretched histogram may look like a flat line with a large spike at the far left side.

Histogram Stretching Turning the histogram with a 'flat line with a bump' into a wide-based mountain. Most software allows this to be done linearly (levels) or non-linearly (curves).

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Homeobox A highly conserved sequence of 180 nucleotides common to many regulatory genes and coding for the DNA-binding part of the corresponding regulatory proteins.

Homeostasis The ability of living organisms to keep constant certain of their physical or chemical properties by self-regulation.

Homeotic Gene A regulatory gene containing a homeobox sequence.

Homogeneous Having the quality of being uniform in properties throughout. In astronomy, this term is often applied to the universe as a whole, which is postulated to be homogeneous.

Homologous Series A series of compounds in which adjacent members of the series differ by a fixed unit of structure.

Horizon The horizontal line that appears to separate the Earth from the sky. A confusion astronomically, as the term is used in many ways: apparent horizon, astronomical horizon, and geometric horizon. To an astronomer, one of these horizons is a collection of directions in space, and in fact a circle on the celestial sphere.

Horizon Co-Ordinates The system of celestial co-ordinates in which the observer's horizon is the reference plane and the north point is the reference direction. The positions are given in altitude and azimuth.

Horizontal Gene Transfer The transfer of genes from one organism to another, as opposed to vertical gene transfer, from parent to offspring.

Horizontal Branch A sequence of stars in the H-R diagram of a globular cluster, extending horizontally across the diagram to the left from the red- giant region. These are probably stars undergoing helium burning in their cores, by the triple-alpha reaction.

Horizontal Refraction The astronomical refraction at the astronomical horizon. Not refraction parallel to the horizon, as non-astronomers might suppose.

Hormone connection from the cells that secrete it to the cells on which it acts. 4 October 2012 67 © Paul Thomas

Host Galaxy A galaxy in which a cosmic phenomenon, such as a supernova explosion or a gamma-ray burst, has occurred.

Hot Pixels These are pixels that have 'turned on' on your sensor for whatever reason. They show up in the image as static red, green or blue 'dots' that are fairly bright. Dark frames usually get rid of them but sometimes one has to go through a final image removing them with the clone stamp tool.

Hour Angle Of a celestial object as seen from a particular location, the difference between the local and the right ascension (H.A. = L.S.T. - R.A.).

Hour Circle (i) A great circle passing through an object and the celestial poles. (ii) The setting circle on the polar axis of an equatorial mount.

Hubble A diagram formulated by astronomer Edwin Hubble that is used to classify galaxies. It is the shape of a tuning fork, beginning at one end with elliptical galaxies and then splitting into spirals and barred spirals.

Hubble Constant The numerical factor, usually denoted H, that describes the rate of expansion of the universe. It is the proportionality constant in the Hubble law v ~Hd, which relates the speed of recession of a galaxy (v) to its distance (d) . The present value of H has recently become fairly well known; estimates range between 47 and 63 km/sec Y Mpc, giving an around 14 billion years.

Hubble's Law Mathematically expresses the idea that the recessional velocities of faraway galaxies are directly proportional to their distance from us. Hubble’s Law describes the relationship of velocity and distance by the equation V=Hoxd, where V is the object’s recessional velocity, d is the distance to the object, and Ho is the Hubble constant. Essentially, the more distant two galaxies are from each other, the faster they are traveling away from each other. American astronomer Edwin Hubble discovered this relationship in 1929 when he observed that galaxies and clusters of galaxies were generally moving away from each other.

Hubble Colours Artificially coloured image using Ha and O-III Filters instead of RGB.

Hubble Space Telescope An orbiting telescope that collects light from celestial objects in visible, near-ultraviolet, and near-infrared wavelengths. The telescope was launched April 24, 1990 aboard the NASA Space Shuttle Discovery. The 12.5-ton (11,110-kg), tube-shaped 4 October 2012 68 © Paul Thomas

telescope is 13.1 m (43 ft) long and 4.3 m (14 ft) wide. It orbits the Earth every 96 minutes and is mainly powered by the sunlight collected by its two solar arrays. The telescope’s primary mirror is 2.4 m (8 ft) wide. The telescope is operated jointly by the National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). HST is one of the many NASA Origins Missions, which include current satellites such as the Far Ultraviolet Space Explorer (FUSE) and space observatories such as the James Webb Space Telescope (JWST).

Human Genome Project An international protect to map the entire genome of Homo sapiens.

Humectant A moistening agent.

Huygens See “Eyepiece”

Hydrocarbon Any of a diverse group of organic compounds composed of hydrogen and carbon.

Hydrogen Simplest atom, consisting only of one proton and one electron; the most abundant element in the Universe.

Hydrogen-Alpha (Sun) A spectral line which has a wavelength of 656.3nm. It is this wavelength of light, from the Sun, which is observed with certain specialist solar telescopes.

Hydrogen-Alpha Filter A type of that’s used for observing the Sun. It blocks all wavelengths of light except for a very narrow portion at 656.3nm.

Hydrogen-Beta Filter This filters out all light except that of the hydrogen-beta line that has a wavelength of 486.5nm. It is used to observe very faint objects, such as the Horse Head Nebula and California Nebula, which wouldn’t normally be seen without one.

Hypergiant Unstable massive stars who are constantly loosing mass and who have tremendous luminosities of between 100,000 and a million times that of our Sun.

Hypervelocity Stars These stars are thought to be the result of a violent close encounter of a binary star with a supermassive black hole. As one of 4 October 2012 69 © Paul Thomas

the stars is consumed by the black hole, the other is violently thrown away. It’s sent travelling through the galaxy at phenomenal speed.

Hypotheses Guesses that can be tested by experiment.


Ice A term used to describe water or a number of gases such as methane or ammonia when in a solid state.

Ideal Gas Law The volume of a gas is proportional to the amount of gas and its temperature and inversely proportional to its pressure.

Illuminance The amount of visible light per unit area. This is the visual quantity analogous to the radiometric quantity “irradiance”.

Image Depth Generally how well faint elements are clearly resolved. A good measure of an images depth is to consider the faintest star clearly visible.

Image Processing We are trying to capture details of objects that are generally not visible to the naked eye. Images straight from a CCD or DSLR tend to be very dark with all the information within a very narrow part of the histogram. They may also not be well colour balanced and be lacking in sharpness and contrast. Image Processing is akin to developing a negative in that all the detail present is coaxed back. Common image processing software include Photoshop, GIMP and PixInsight. Image Processing is a huge topic in its own right but is an important aspect of AstroImaging.

Image Scale How many arc seconds each pixel in the image represents. Calculated via the formula [pixel size (in µm) x 206 / Scope Focal Length].

Image Tube An electronic device that receives incident radiation and intensifies it or converts it to a wavelength at which photographic plates are sensitive.

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Imbrian Period The period in the Moon’s history when the large basins Imbrium, Crisium, Tranquilitatis, Serenitatis, Fecunditatis and Procellarum were formed, and filled with basalts. The Imbrian period occurred between 3.8 and 3.2 billion years ago.

Immersion The entry of an object into shadow during an eclipse, or the covering of an object during an occultation.

Impact Crater A crater formed on the surface of a or a satellite by the impact of a meteoroid or .

Impact Event A collision between two solar system bodies that releases exceptionally large amounts of energy. Some examples are the 1908 Siberian Tunguska impact by a comet or an asteroid and the asteroid that struck Earth 65 million years ago, which may have led to the extinction of the dinosaurs and other species of the Cretaceous-Tertiary .

Impactor The part of the Deep Impact spacecraft that crashed into comet 9P/. When launched, the impactor and the flyby spacecraft were attached to each other. The spacecraft launched the impactor a day before the crash. As the impactor punched through the comet’s crust, the flyby craft recorded the event from a safe distance away.

Inclination A measure of the tilt of a planet's orbital plane in relation to that of the Earth.

Index of Refraction Ratio of the speed of light in a vacuum to its speed in a given medium.

Inertia The tendency of an object to remain in its state of rest or uniform motion; this tendency is directly related to the mass of the object.

Inferior Conjunction A conjunction of an inferior planet that occurs when the planet is lined up directly between the Earth and the Sun.

Inferior Planet A planet that orbits between the Earth and the Sun. Mercury and Venus are the only two inferior planets in our solar system.

Inflation The theorised rapid expansion () of the Universe right after the Big Bang.

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Infrared Electromagnetic radiation at wavelengths that are longer than those the red end of the visible-light spectrum and shorter than (roughly between 1 and 100 microns). Very little infrared light reaches the surface of the Earth, although some can be observed by high-altitude aircraft (such as the Kuiper Observatory) or telescopes on high mountaintops (such as the peak of Mauna Kea in Hawaii).

Ingress The period during a transit or an eclipse when the smaller celestial body begins to cross the disc of the larger object. Egress marks the exit of the transiting or eclipsing body.

Integrated Magnitude The integrated magnitude is the magnitude of a diffuse object if it were gathered up into a point like source.

Intensity The amount, degree, or quantity of energy passing through a point per unit time. For example, the intensity of light that Earth receives from the Sun is far greater than that from any other star because the Sun is the closest star to us.

Interstellar Medium The matter that lies between the stars of a galaxy. It is composed of 99 per cent gas and 1 per cent dust. Hydrogen makes up 90 per cent of the gas and the remainder is helium and other heavier elements. The dust is mainly made up of carbon and silicates.

Interference The property of radiation, explainable by the wave theory, in which waves that are in phase can add (constructive interference) and waves that are out of phase can subtract (destructive interference); for light, this gives alternate light and dark bands.

Interferometer A device that uses the property of interference to measure such properties of objects as their positions or structure.

Interferometry An astronomical technique in which the images from two or more telescopes are superimposed. Interferometry has many uses, and one of the most intriguing is its ability to cancel out the light from a point source such as a star by superimposing the images in such a way that the oscillations in light intensity from one image are the reverse of the oscillations from the other image. With this method, planets orbiting the star can be directly observed, and the composition of their atmospheres can be investigated using spectral analyses.

Intergalactic Occurring between galaxies.

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International The manned ISS is according to NASA the biggest and most complex scientific project in history. During twilight passed, the space station is easily seen by everyone as a strikingly bright and silently running star. It crosses the sky in a few minutes basically from west to east.

Interplanetary Existing or occurring between planets.

Interplanetary Dust Particle Small, often microscopic rocky particles of interplanetary debris.

Interplanetary Medium Gas and dust between the planets.

Interstellar Cloud A region of relatively high density in the inter-stellar medium. Interstellar clouds have ranging between 1 and 1011 particles per cubic centimetre, and in aggregate, contain most of the mass in interstellar space.

Interstellar Extinction The obscuration of starlight by interstellar dust. Light is scattered off of dust grains, so that a distant star appears dimmer than it otherwise would. The scattering process is most effective at short (blue) wavelengths, so that stars seen through inter- stellar dust appear reddened and dimmed.

Interstellar Medium The sparse gas and dust located between the stars of a galaxy.

Interstellar Reddening This is an effect produced by the incremental absorption and scattering of electromagnetic energy from interstellar matter, an effect known as extinction. This effect causes the more distant objects such as stars to appear redder and dimmer than expected. It is not to be confused with the separate phenomenon of red shift.

Interstellar Space The dark regions of space located between the stars.

Intervalometer A camera accessory that measures time intervals, often used to create time-lapse sequences.

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Inverse A method of converting photons from lower to higher energy through interaction with electrons that are moving with velocities close to the speed of light.

Inverse-Square Law Any law describing a force or other phenomenon that decreases in strength as the square of the distance from some central reference point. The term inverse-square law is often used by itself to mean the law staling that the intensity of light emitted by a source such as a star diminishes as the square of the distance from the source.

Inversion This refers to something being upside down.

Ion An atom that has gained or lost one or more electrons, giving it an overall negative or positive electrical charge respectively.

Ionic Gas Also known as ionized gas. Gas whose atoms have lost or gained electrons, causing them to be electrically charged. In astronomy, this term is most often used to describe the gas around hot stars where the high temperature causes atoms to lose electrons.

Ionization This is when an atom is stripped of or receives electrons. It often happens in astronomy where a gaseous nebula is ionized by stars within it.

Ionizing Radiation Radiation that produces ions as it passes through matter.

Ionosphere The zone of the earth's upper atmosphere, between 80- and 500-km altitude, where charged subatomic particles (chiefly protons and electrons) are trapped by the earth's magnetic field.

Iridium An element with an atomic number of 77 and is possibly the densest of all elements (Osmium being the other contender). It is considered a low toxicity substance, but in powder form is a known irritant and a fire hazard. Pure iridium is very brittle and is nearly impossible to machine, and is the most corrosive resistant metal known. A thin, worldwide layer of iridium exists in a layer of sediment that was put down at the end of the Cretaceous period (approx 65 million years ago). Since meteors and asteroids contain a higher percentage of iridium than the earth's crust, this iridium enriched layer is seen as evidence that the earth was struck by a large meteor or asteroid at that time. Dust from the impact

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would have spread around the globe, depositing the iridium. The dust also would have blocked the sun for a time, resulting in the extinction of many plant and animal species.

Iridium Flare A bright flash in the night sky caused by sunlight glinting off solar panels of Iridium communications satellites. See for details of when an Iridium flare will be visible from your location.

Irradiance The amount of power per unit area in a beam of light. Sometimes also called flux. Units: W m-2

Irregular Galaxy A galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually filamentary or very clumpy in shape.

ISO Number This is the number that indicates the sensitivity (or ‘speed’) of a film or a given DSLR exposure.

Isochron A parameter used in isotopic dating of geologic materials, experimentally determined from comparison of the isotopic compositions of two or more components (usually minerals) that share a common age.

Isomerization The conversion of a compound into one or more of its isomers.

Isomers Compounds that have the same molecular formula but different structural formulas and properties.

Isotopes different forms of an element that have the same number of protons in their nuclei, and thus the same atomic number, but that have different numbers of neutrons and thus different atomic masses. There are two kinds of isotopes, stable and unstable. Isotopes that are unstable are called radioactive and disintegrate at a constant decay rate. Examples of stable isotopes include carbon-12 and carbon-13. Carbon-14, and uranium-238 and -235, are examples of unstable isotopes.

Isotopic This term describes anything relating to isotopes. Isotopes are atoms of the same element that have the same number of protons, but different numbers of neutrons

Isotopic Date Age of a rock (or organic substance less than 60,000 years old) determined by measurement of the ratio of a parent isotope to one of the products of its radioactive decay. 4 October 2012 75 © Paul Thomas

Isotopic Fractionation Separation of isotopes of an element, in organisms often mediated by an enzyme.

Isotropic Having the property of appearing the same in all directions. In astronomy, this term is often postulated to apply to the universe as a whole.


J2000, Precession, Nutation The plains of ecliptic and equator shift with time by perturbations from the Sun, Moon and planets. The long-term shift is called precession; the short periodic variations are called nutation. The given celestial coordinates are referred to the true direction of the vernal equinox and the true obliquity of the ecliptic to the standard reference time 1 January 2000. For this date many star charts and coordinate tables are printed.

Jansky (Jy) Convenient unit of incident spectral flux density used in radio astronomy; 1 Jy = 10-26 W/m2 Hz (named for Karl G. Jansky, initial discoverer of extraterrestrial radio ).

Jeans Instability Is a physical state in which an interstellar cloud of gas will begin to undergo collapse and form stars. A cloud can become unstable against collapse when it cools sufficiently or has perturbations of density, allowing gravity to overcome the gas pressure.

Jets Narrow, high-energy streams of gas and other particles generally ejected in two opposite directions from some central source. Jets appear to originate in the vicinity of an extremely dense object, such as a black hole, pulsar, or protostar, with a surrounding accretion disk. These jets are thought to be perpendicular to the plane of the accretion disk.

Joule (J ) Unit for work in the MKS system of units: 1 J = 107 ergs = 0.239 cal.

Jovian Moon A belonging to the Jupiter.

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Jovian Planet Same as ; in our solar system: Jupiter (Jove), Saturn, Uranus and Neptune.

Julian Date The interval of time in days (and fraction of a day) since Greenwich on Jan. 1, 4713 BC. The JD is always half a day off from , because the current definition of JD was introduced when the astronomical day was defined to start at noon (prior to 1925) instead of midnight. Thus, 1995 Oct. 10.0 UT = JD 2450000.5.


Kardashev Cultures N. X. Kardashev has distinguished three types of technological societies according to the amount of power they can harness: Type I can engage the power available on a planet; Type II. the power output of a star; and Type III, the power output of a galaxy.

Kellner See “Eyepiece”

Kelvin A temperature scale used in such as astronomy to measure extremely cold . The Kelvin temperature scale is just like the Celsius scale except that the freezing point of water, zero degrees Celsius, is equal to 273 degrees Kelvin. Absolute zero, the coldest known temperature, is reached at 0 degrees Kelvin or -273.16 degrees Celsius.

Kepler Elements Another term for .

Kepler's First Law A planet orbits the Sun in an ellipse with the Sun at one focus.

Kepler's Second Law A ray directed from the Sun to a planet sweeps out equal areas in equal times.

Kepler's Third Law The square of the period of a planet's orbit is proportional to the cube of that planet's semi major axis; the constant of proportionality is the same for all planets.

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Kilohertz, 1000 Hertz.

Kilocalorie A unit of energy, the amount of heat required to raise the temperature of 1kg of water 1°C, 1000 calories.

Kilogram The SI unit of mass, a quantity about equal to 2.2 pounds.

Kiloparsec A unit of distance equal to 1,000 parsecs.

Kinetic Energy The energy of motion. The kinetic energy of a moving object is equal to one-half times its mass times the square of its velocity.

Kinetic Isotopic Fractionation Separation of isotopes of an element as a result of their speeds of movement, in organisms mediated by an enzyme that interacts more readily with one of two or more isotopes of an element.

Kinetic-Molecular Theory A model that uses the motion of molecules to explain the behaviour of the three states of matter.

Kirkwood's Gaps Narrow gaps in the asteroid belt created by with Jupiter

Koenig See “Eyepiece”

Kuiper Belt A large ring of icy, primitive objects beyond the orbit of Neptune between 4.5 and 7.5 billion km from the Sun. Kuiper Belt objects are believed to be remnants of the original material that formed the solar system. Some astronomers believe Pluto and are Kuiper Belt objects.


L Wave A type of seismic wave that travels only over the surface of the earth. 4 October 2012 78 © Paul Thomas

Lagrange Point The points in the vicinity of two massive bodies (such as the Earth and the Moon) where each others’ respective balance. There are five, labelled L1 through L5. The first three lie along the centreline between the centres of mass between the two masses: L1 is on the inward side of the secondary, L2 is on the outward side of the secondary, and L3 is on the outward side of the primary. L4 and L5, the so-called points, lie along the orbit of the secondary around the primary, sixty degrees ahead and behind of the secondary. L1 through L3 are points of unstable equilibrium; any disturbance will move a test particle there out of the Lagrange point. L4 and L5 are points of stable equilibrium, provided that the mass of the secondary is less than about 1/25.96 the mass of the primary. These points are stable because centrifugal pseudo-forces work against gravity to cancel it out.

Laminar Air Flow This is where air flows smoothly in layers, without any interference to disrupt the flow. Such conditions enable better quality seeing. Turbulent air flow reduces the observing quality.

Lanthanum See “Eyepiece”

Lapse Rate The rate at which temperature decreases with height in the atmosphere.

Large Magellanic Cloud An irregular galaxy, the largest of two satellites of the Milky Way.

Lateral Mirage A much mis-used term. Refraction in the horizontal direction is appreciable only when a boundary layer is stabilized by a wall or other near-vertical surface, in such cases, the term “mural mirage” is more descriptive.

Latitude The angular distance of an object north or south of Earth’s equator expressed in degrees. The latitude of the Equator, for example, is 0°; Chicago is at 42° North; the North Pole is at 90° North; and Lima, Peru, is at 12° South. Latitudes south of the equator are expressed in degrees South or as negative degrees.

Launch Window The time opportunity for launching a spacecraft to another body in the Solar System involving minimal energy or the shortest flight time.

Law of Conservation of Energy The amount of energy within the universe is constant; energy cannot be created or destroyed, only transformed.

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Lens A carefully ground or moulded piece of glass, plastic, or other transparent material that causes light to bend and either come together or spread apart to form an image.

Lens Doublet A set of two lenses, one concave and one convex, made from different types of glass. Together the lenses correct both spherical and chromatic aberrations. A single lens alone cannot correct these aberrations.

Lenticular Galaxy A disk-shaped galaxy that contains no conspicuous structure within the disk. Lenticular galaxies tend to look more like elliptical galaxies than spiral galaxies.

Lepton Epoch Lighter elementary particles such as electrons, neutrinos, and muons were the dominant form of matter.

Libration The Moon rotates in one month in a constant motion around its own axis. In the same interval, it revolves on an elliptical motion around the Earth. This combination leads to an inconstant exposure of the lunar surface to an observer on Earth: around 59% of the surface is visible during one month.

Libration Zone Regions of the Moon occasionally visible due to the Moon’s apparent periodic wobble.

Life Era Era in the history of the Universe when life emerges as the dominant element.

Light The common term for electromagnetic radiation, usually referring to that portion of the electromagnetic spectrum that is visible to the human eye. Other bands of the EM spectrum, however, are also often referred to as different forms of light.

Light Bucket Slang term for a telescope of large aperture.

Light Curve A is a graph of the brightness of an astronomical object. It’s usually applied to variable stars, novae or stars that are transited by extra-solar planets.

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Light Echo This is the reflection of light usually seen around the gaseous ejections from massive stars or sometimes supernova. Perhaps the most prominent of all recently observed light echoes was the one seen around the exploding star .

Light Frames These are the actual pictures. The shutter is open, the telescope is tracking and the sensor is being bathed in ancient photons.

Light-Gathering Power The ability of a telescope to collect light from an astronomical source. The light-gathering power is directly related to the area of the primary mirror or lens.

Light-Harvesting Pigment Organic compounds, such as chlorophyll and bacteriochlorophyll, that absorb light energy in photosynthesis.

Light Pollution This is the pervasive orange glow which appears throughout much of the night sky in urban and indeed some rural areas. It is caused by light reflected of bright surfaces and poorly positioned (or badly designed) lighting.

Light-Pollution (Reduction) Filter A filter specially prepared to reflect wavelengths which are prominent in light pollution, and transmit other wavelengths. Such a filter improves the contrast of deep-sky objects, when light pollution is present.

Light Speed Also Lightspeed. The speed of light; In a perfect vacuum, is exactly 299,792,458 metres per second.

Light Year An astronomical unit of measure equal to the distance light travels in a year, 9.4645661 x1012km. (5. 8786125 x1012 miles).

Limb The outer edge or border of a planet or other celestial body.

Limb Darkening The dark region around the edge of the visible disk of the sun or a planet, caused by a decrease in temperature with height in the atmosphere.

Limestone A kind of consisting mainly of calcium carbonate minerals.

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Limiting Magnitude The magnitude of the dimmest object that can be seen through any optical instrument (including the eye). The of an instrument varies with light conditions.

Line of Nodes The imaginary line passing through the ascending and descending nodes of an orbit. It is the line of intersection of the orbital plane with the reference plane.

Line Spectrum The pattern of coloured lines emitted by an element.

Liquid A state of matter in which the substance assumes the shape of its container, flows readily, and maintains a fairly constant volume.

Liquid Metallic Hydrogen Hydrogen in a state of semi rigidity that can exist only under conditions of extremely high pressure, as in the interiors of Jupiter and Saturn.

Litre Unit of volume equal to a cube with sides 10 cm.

Lithosphere The layer in the earth, moon, and terrestrial planets that includes the crust and the outer part of the mantle.

Local Group (Local Supercluster) A small group of 36 nearby galaxies of which our own Milky Way galaxy is a member.

Locked Rotation The condition in which a moon has the same period of rotation as its period of revolution around its parent body. This means that the moon always shows the same face to its parent. Our Moon is in locked rotation around Earth.

Longitude The angular distance of an object east or west of the —the line of zero longitude that runs through Greenwich, England—expressed in degrees. The longitude of Chicago, for example, is 88° West.

Longitude of the Ascending Node The angular distance, measured around the reference plane, between the FPA and the ascending node of an orbit.

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Longitudinal Chromatic Aberration An aberration of optical systems, in which the focal length of a system is different for different colours of light.

Looming The appearance above the horizon of a distant object that would normally be hidden below it. This effect is caused by unusually large terrestrial refraction, usually due to a thermal inversion. Not Mirages.

Losmandy United States manufacturer of well-machined, high-quality, German equatorial mountings, accessories, and attachments.

Low-Reflection Coating Any of several coating materials whose purpose is to reduce unwanted reflections from optical elements which are supposed to transmit light.

Luminance (i) This is the name often given to the ‘light frames’ or exposures gathered from a monochrome CCD i.e. without colour. (ii) The amount of visible light per unit area and per unit solid angle. Often called “surface brightness”.

Luminosity The total energy emitted by an object per second; that is, the power of the object. For stars, the luminosity is usually measured in units of ergs per second.

Luminosity Class One of several classes to which a star can be assigned on the basis of certain luminosity indicators in its spectrum. The classes range from I for supergiants to V for main-sequence stars (also known as dwarfs).

Lunar Eclipse A phenomenon that occurs when the Moon passes into the shadow of the Earth. A partial lunar eclipse occurs when the Moon passes into the penumbra, or partial shadow. In a total lunar eclipse, the Moon passes into the Earth's umbra, or total shadow.

Lunar Limb The extreme edge of the disc of the Moon.

Lunar Month The average time between successive new or full moons. A lunar month is equal to 29 days 12 hours 44 minutes. Also called a synodic month.

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Lunation The interval of a complete lunar cycle, between one and the next. A lunation is equal to 29 days, 12 hours, and 44 minutes.

Lyman Alpha Blob These are vast glowing gas clouds that emit a characteristic wavelength of light known as the Lyman-alpha spectral line in Hydrogen’s spectral signature.

Lyman Limit A specific wavelength (91.2 nm) that corresponds to the energy needed to ionize a hydrogen atom (13.6 eV). Galactic space is opaque at wavelengths shorter than the Lyman limit. Subsequently, light from cosmic objects at wavelengths less than the Lyman limit is exceedingly difficult to detect.


Ma Mega Anna, one million (1,000,000 or 1 x 106) years.

Magellanic Clouds Two small, irregular galaxies found just outside our own Milky Way galaxy. The Magellanic clouds are visible in the skies of the southern hemisphere.

Magnetars These are large Neutron Stars with super powerful magnetic fields, upwards of many trillions of times stronger than our Earths. They are at least 30 times denser than our sun, but 45,000 times smaller.

Magnetic Braking The slowing of the spin of a young star (such as the early sun) by magnetic forces exerted on the surrounding ionized gas.

Magnetic Dynamo A rotating internal zone inside the sun or a planet, thought to carry the electrical currents that create the solar or planetary magnetic field.

Magnetic Field A condition found in the region around a magnet or an electric current, characterized by the existence of a detectable magnetic force at every point in the region and by the existence of magnetic poles.

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Magnetic Pole Either of two limited regions in a magnet at which the magnet's field is most intense.

Magnetosphere A region, surrounding a star or planet, that is permeated by the magnetic field of that body.

Magnification A measurable increase in the apparent size of an object. Magnification = (focal length of a telescope) / (focal length of the eyepiece).

Magnitude The units used to describe brightness of astronomical objects. The smaller the numerical value, the brighter the object. The human eye can detect stars to 6th or 7th magnitude on a dark, clear night far from city lights; in suburbs or cities, stars may only be visible to mag 2 or 3 or 4, due to light pollution. The brightest star, , shines at visual magnitude -1.5. Jupiter can get about as bright as visual magnitude -3 and Venus as bright as -4. The full moon is near magnitude -13, and the sun near mag -26. The magnitude scale is logarithmic, with a difference of one magnitude corresponding to a change of about 2.5 times in brightness; a change of 5 magnitudes is defined as a change of exactly 100 times in brightness.

Main Belt A collection of thousands of rocky and metallic bodies revolving more or less together between the orbits of Mars and Jupiter, roughly 2.0 to 3.5 AU from the Sun. Most asteroids are part of the Main Belt.

Main Group Elements The elements in the A groups of the periodic table that is customary in the United States and in Groups 1, 2, and 13 to 18 in the periodic table recommended by IUPAC.

Main Sequence principal sequence of stars on the graph of luminosity versus effective temperature (H-R diagram), encompassing more than 90% of observable stars. These stars are converting hydrogen to helium by nuclear reactions in their cores The lower mass limit for the Main Sequence is 0.085 Mo and the upper limit is about 60 Mo.

Main-Sequence Fitting A distance-determination technique in which an H-R diagram for a cluster of stars is compared with a standard H-R diagram to establish the absolute magnitude scale for the cluster H-R diagram.

Main Sequence Star A main sequence star is one that is in a stable part of its life converting hydrogen and helium through nuclear fusion. Most stars we see in the night sky are in the main sequence stage and our Sun is currently also a main sequence star.

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Main-Sequence Turn-Off In an H-R diagram for a cluster of stars, the point where the main sequence turns off toward the upper right. The main- sequence turn-off, showing which stars in the cluster have evolved to become red giants, is an indicator of the age of the cluster.

Major Axis The longest diameter of an ellipse; the line from one side of an ellipse to the other that passes through the foci. Also, the length of that line.

Major Planet A name used to describe any planet that is considerably larger and more massive than the Earth, and contains large quantities of hydrogen and helium. Jupiter and Neptune are examples of major planets.

Maksutov, Maksutov-Cassegrain, Maksutov-Newtonian These are forms of catadioptric telescope.

Maksutov Corrector A particular kind of lens used in Maksutov optical systems. Maksutov correctors are thick, and have strongly curved, nearly concentric, surfaces. They resemble large, thick -glasses.

Mantle The area in-between the Earth’s crust and its core (roughly 3,000km thick) that consists of hot solid rock and magma (molten rock).

Mare (plural - maria) A large plain on the lunar surface. Formed by cooling lava, they are generally much smoother than surrounding regions and appear dark compared with the lunar highlands, and are characterised by large basalt plains enriched with titanium and iron. Mare is Latin for ‘sea’.

Masayuma See "Eyepiece"

Mascon A concentration of mass under the surface of the moon, discovered from its gravitational effect on spacecraft orbiting the moon.

Maser An acronym for "microwave amplification by stimulated emission of radiation," a device by which certain energy levels are more populated than normal, resulting in an especially dense emission of radio radiation at a certain frequency when the system drops to a lower energy level. A laser operating in the microwave region of the electromagnetic spectrum.

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Mass A measure of the total amount of material in a body, defined either by the inertial properties of the body or by its gravitational influence on other bodies.

Mass-Energy Equation Einstein's equation E = mc2, in which E is energy, m is mass, and c is the speed of light.

Mass-Luminosity Relation A well-defined relation between the mass and luminosity for main sequence stars.

Mass Number A Nucleon number, the sum of the numbers of protons and of neutrons in the nucleus of an atom.

Mass Spectrometry Instrumental method of identifying the chemical constituents of a substance by means of the separation of gaseous ions according to their differing mass and charge.

Massing The close alignment of three or more planets—or two or more planets and the Moon—as seen from Earth. This occurs when all the bodies involved in the massing have similar ecliptic .

Matter A word used to describe anything that contains mass. (As opposed to pure energy)

Matter Era collective name for the most recent three epochs in the history of the Universe (atom, galaxy, stellar), covering all of time after the Radiation Era.

Maunder Minimum Virtual disappearance of sunspots in the period 1645 to 1715.

Mean Anomaly The anomaly which would exist if a planet orbited at a uniform speed in a circular orbit.

Mean Solar Time Time based on an imaginary "mean Sun". See the tutorial on Time.

Mega 106 (as in megahertz, MHz); one million.

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Megahertz One million Hertz; a million cycles per second.

Megaparsec Equals one million parsecs (3.26 million light-years) and is the unit of distance commonly used to measure the distance between galaxies.

Melting Point The temperature at which a substance changes from the solid to the liquid state.

Meridian An imaginary line that circles the Earth from north to south that marks the point at which the Sun is at its highest. Anti- and post-meridian (am and pm), mark the times before and after the Sun crosses the Meridian at midday, respectively.

Meridian Flip A defined point for the limits of east or west side viewing where in order to prevent damage to the equipment caused by hitting the pier or tripod the mount is stopped and its direction is reversed, effectively the mount moves so the telescope and weights are now opposite side to where they were.

Messier Catalogue The famous catalogue of deep sky objects such as nebulae, galaxies and clusters compiled by Frenchman Charles Messier and his colleague Pierre Méchain in the 18th Century.

Metagalaxy Any very large-scale organized collection of galaxies.

Metal (a) For stellar abundances, any element higher in atomic number than 2, that is, heavier than helium or hydrogen. (b) For a planet or solid, matter that is a good conductor of heat and electricity.

Metalicity Elements in a star heavier than helium.

Meteor This refers to the ionization trail produced by a small particle of rock or dust (Meteroroid) that burns away in the Earth's atmosphere. Meteors are also referred to as shooting stars.

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Meteor Shower An event where a large number of meteors enter the Earth's atmosphere from the same direction in space at nearly the same time. Most meteor showers take place when the Earth passes through the debris left behind by a comet.

Meteor Trail Sometimes a meteor leaves a trail of material as it passes through the Earth’s atmosphere. This ionised material glows for a brief moment before fading and is known as a meteor trail, or train, or more recently, just Meteor.

Meteorite An object, usually a chunk or metal or rock, that survives entry through the atmosphere to reach the Earth's surface.

Meteoroid A rocky piece of debris in space that is smaller than an asteroid. If a meteoroid enters Earth’s atmosphere it is called a meteor. Most meteors burn up in the atmosphere, but those that don’t and survive an impact on the ground are called Meteorites.

Meter The SI unit of length, slightly longer than 39 inches

Methane The colourless gaseous hydrocarbon CH4. In the cold vacuum of space, methane is a white solid but, when hit by sunlight, it can become a gas.

Metonic Cycle The period of 6 939.6 days, or 19 calendar years, after which the Moon's phases recur on the same day of the year. This period is also equal to 253 lunations.

Micrometeorite A particle from space that is small enough to be slowed down when it reaches the Earth's atmosphere without being burnt up. Approximately 50 per square meter hit the top of Earth’s atmosphere each day.

Micrometer (i) A unit of length, one-millionth (10-6) of a meter µm. (ii) A device, of which various types exist, that is used in a telescope for measuring small angular distances between objects.

Micron A micrometer

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Microwave Electromagnetic wave roughly in the range 0.01-1 m in wavelength (ordinary broadcasting utilizes waves in the 200-600 m range; the "short waves" used in long-distance communications are rarely shorter than 10m).

Milky Way The band of light across the sky from the stars and gas in the plane of the Milky Way Galaxy.

Miller-Urey Experiment An experiment conducted by Stanley Miller and Harold Urey at the University of Chicago in 1953 that simulated the conditions on the early Earth. It was designed to test whether organic molecules could be created from inorganic molecules.

Minor Axis The shortest diameter of an ellipse; the line from one side of an ellipse to the other that passes midway between the foci and is perpendicular to the major axis.

Minor Planet Another name used to describe a large asteroid. ie. an object in direct orbit around the Sun that is neither a dominant planet nor originally classified as a comet. A moon cannot be a , because it is orbiting another body.

Mira Variable A red giant star that decreases and increases in brightness over a period of at least 100 days. It’s been shown that variables can change brightness by as much as eight magnitudes.

Mirage An inverted image caused by .

Mo The Solar mass.

Mock Sun An effect caused by ice crystals in Earth's atmosphere which refract sunlight and cause the appearance of two diffuse patches of light 22° either side of the Sun. These patches are termed Parhelia or Sundogs and often appear on the rim of a halo surrounding the Sun. The patches often have coloured fringes, red on one side, blue on the other.

Molar Mass The formula weight expressed in grams.

Molar Volume The volume occupied by 1 mole of a substance under specified conditions. 4 October 2012 90 © Paul Thomas

Mole The formula weight in grams of an element or compound; or a quantity of chemical substance that contains 6.02 x 1023 formula units of the substance.

Molecular Cloud A cold, dense, interstellar cloud in which molecules, mostly molecular hydrogen (H2), form.

Molecule A collection of atoms bound together that is the smallest collection that exhibits a certain set of chemical properties.

Momentum A measure of the tendency that a moving body has to keep moving. The momentum in a given direction (the "linear momentum") is equal to the mass of the body times its component of velocity in that direction.

Mono Black and White Image, or Camera.

Monocentric See "Eyepiece"

Monochromator An optical device, consisting of one or more slits, that selects a narrow band of wavelengths from a broader spectrum.

Monocular A spotting telescope, which resembles half a binocular. Monoculars are often hand-held, and are used in much the same way, and for the same purposes, as binoculars.

Month, Anomalistic The interval between two successive perigee passages of the Moon, equal to 27.55 days.

Month, Nodical or Draconic The interval between successive passages of the Moon through one of its nodes, equal to 27.21 days.

Month, Sidereal The revolution period of the Moon relative to the stars, equal to 27.32 days.

Month, Synodic The interval between two successive New Moon's (a lunation), equal to 29.53 days.

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Month, Tropical The time taken for the Moon to return to the same celestial longitude (7 seconds shorter than the sidereal month).

Moon Any natural satellite of a planet, that shines by the sun's reflected light.

Moon, Harvest The Full Moon that occurs nearest the time of the autumnal equinox. So-called because this will occur around the time of harvest.

Moon, Hunter's The Full Moon that occurs after Harvest Moon. So-called because this will occur around the start of the hunting season.

Moon Illusion An optical illusion in which the Moon appears larger when it is near to the horizon than when it is high in the sky.

Moonlet A satellite of a small celestial object, such as a or an asteroid.

Mount The mechanical element of the imaging system that permits the telescope to be pointed at a target object. Mounts come in two types: Alt/Az - where the movement axes are vertical and horizontal German Equitorial - where the movements are in Declination and Right Ascension.

Mounting Rings Circular clamps which fasten around an optical tube assembly and are threaded, machined, or otherwise prepared to fasten to a mounting. Also called tube rings.

Moving Cluster A physical grouping of stars moving through space that usually shares a common origin. The most famous moving cluster is the Moving Group.

Moving Group or This is a loose grouping of stars that are traveling together through space. Although the members were formed together in the same molecular cloud, they have since moved too far apart to be gravitationally bound as a cluster.

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Multicoated Lens This describes a lens or prism in which the surfaces where the beam of light goes from air into glass, or from glass into air, has been coated with a high-tech, many-layered coating that decreases the amount of light that gets reflected.

Multichannel Spectrum Analyser Short for MCSA/SD, a digital, energy-efficient, real-time, multichannel spectrum analyzer and signal detector. A hardware device that continuously accepts a significant portion of the electromagnetic spectrum and divides the spectral energy into a set of many contiguous frequency bins or output channels. Operational functions included are frequency analysis and signal detection, averaging, threshold testing, etc..

Muon Collider A type of particle accelerator capable of smashing particles called muons and anti-muons together to study high-energy collisions.


Nadir The point on the celestial sphere directly below the observer. Opposite of zenith.

Nagler See “Eyepiece”

Nanometer A unit of length, one billionth (10-9) of a meter nm.

Narrow Band The use of filters to restrict wavebands of light reaching the imaging device. Usually designed to capture the emission line of common elements Ha (Hydrogen), OIII (Oxygen) and SII (Sulphur). When processing and imaging using Narrow Band data, the colours are typically "mapped" to the RGB channels where Ha = Red, OIII = Blue and SII = Green. For the Hubble Palette, R=Sulphur (wavelength is more "red"), Blue=OIII & Green=Ha, but results in a very "green" tint as Ha is the dominant. Aesthetically, the "Green" is then translated to a "gold" colour & the rest towards a blue/cyan/teal colour.

National Aeronautics And Space Administration A Federal agency created on July 29, 1958 after President Dwight Eisenhower signed the National Aeronautics and Space Act of 1958. NASA coordinates efforts as well as traditional aeronautical research functions.

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Natural Philosophical speculation about nature.

Nautical Twilight When the centre of the Sun is between 6° and 12° below the horizon; the marine horizon becomes invisible.

Near-Earth Asteroid An asteroid whose orbit brings it close to Earth's orbit. The criterion is when one is plotted to come closer than 20 times the distance from Earth to the Moon AND is more than 150metrs wide. NASA, will then designate this asteroid as a "Potentially Hazardous Object".

Near-Earth Object A comet or asteroid whose orbit brings it close to Earth's orbit.

Near-Infrared The region of the infrared spectrum that is closest to visible light. Near-infrared light has slightly longer wavelengths and slightly lower frequencies and energies than visible light.

Nebula An interstellar gas cloud. Those that emit visible light are called diffuse nebulae, and there are two types: emission nebulae are self-luminous; reflection nebulae are illuminated by radiation from nearby stars

Nebular Hypothesis A general theory that describes how stars and their associated solar systems are formed from the condensation of clouds of dust and gas in space

Neutrino Spinning, tiny, elementary particle, always travels at the speed of light, with essentially no mass and no electric charge, that hardly interacts with matter and can pass straight through it. These particles are created in nuclear reactions in the Sun and supernovae. Very high-energy neutrinos may also be created in active galactic nuclei or gamma-ray bursts.

Neutron A massive, neutral elementary particle, one of the fundamental constituents of an atom. Neutrons and protons are similar in mass, about 1830 times more massive than electrons.

Neutron Star A compressed core of an exploded star made up almost entirely of neutrons. Neutron stars have a strong gravitational field and some emit pulses of energy along their axis. These are known as pulsars.

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New Moon The phase of the moon when the side of the moon facing the earth is the side that is not illuminated by sunlight.

Newtonian Telescope A type of reflecting telescope where the beam reflected by the primary mirror is reflected by a flat secondary mirror so that the focus falls to the side of the telescope tube.

Newton’s Law of Universal Gravitation Two bodies attract each other with equal and opposite forces. The magnitude of this force is proportional to the product of the two masses and is also proportional to the inverse square of the distance between the centres of mass of the two bodies.

Newton’s First Law of Motion A body continues in its state of constant velocity (which may be zero) unless it is acted upon by an external force.

Newton’s Second Law of Motion For an unbalanced force acting on a body, the acceleration produced is proportional to the force impressed. The constant of proportionality is the inertial mass of the body.

Newton’s Third Law of Motion In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.

New General Catalogue The (NGC) was originally compiled and published by J. L. E. Dreyer in 1888. Two additional supplements—called the Index Catalogue (IC)—were published in 1895 (IC I) and 1908 (IC II). These catalogues were an early attempt to create a single list containing all the non-stellar objects known at the time. The original NGC/IC catalogue contains 13,226 entries.

Night Vision The increased ability to see dim object, such as faint stars, due to a sensitization of the eye's 'rod' receptors. Exposure to bright light desensitizes the rods and therefore reduces night-vision.

Noctilucent Clouds These are thin clouds of ice and dust particles that are so high in the Earth’s atmosphere that they are bright during twilight, in the early summer months (dusk, and just before dawn).

Noble Gas One of a group of rare but extremely stable gases with low reaction rates (helium, neon, argon, krypton, xenon, radon).

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Node A point of intersection between two great circles. Eclipses of the Sun and Moon occur when these bodies are simultaneously near the nodes of their paths in the sky.

Noise In astronomy this refers to the unwanted signal in CCD images. There are two main causes of noise in CCD cameras used for astronomical imaging: the intrinsic noise in the image itself and thermal noise. Having the CCD cooled via fans or Peltier cooling systems can reduce noise.

Non-Metals The group of elements to the right of the heavy, stepped, diagonal line in the periodic table.

Nonthermal Radiation Radiation that cannot be characterized by a single number (the temperature). Normally, we derive this number from Planck's law, so that radiation that does not follow Planck's law is called non-thermal.

North Celestial Pole A direction determined by the projection of the Earth’s North Pole onto the celestial sphere. It corresponds to a declination of +90 degrees. The North Star, , sits roughly at the NCP.

Northern Lights The aurora of the northern hemisphere. Also called the Aurora Borealis.

Nova A star that flares up to several times its original brightness for some time before returning to its original state.

Nuclear Fission The splitting of an atomic nucleus into two large fragments.

Nuclear Fusion A nuclear process whereby several small nuclei are combined to make a larger one whose mass is slightly smaller than the sum of the parts. The difference in mass is converted to energy according to Einstein’s famous equation, E=mc2 where he discovered that the energy contained in such a reaction (E) is equivalent to the difference in mass (m) times the square of the speed of light (c) which is a very, very large number. This is the source of the Sun’s energy.

Nuclear Reactor A plant that produces energy by nuclear fission.

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Nuclear Transformation The process by which an atomic nucleus is transformed into another type of atomic nucleus. For example, by removing an alpha particle from the nucleus, the element radium is transformed into the element radon.

Nuclear Winter A period of dark, cold weather that may be caused by the dust and smoke entering the atmosphere from the explosion of nuclear bombs.

Nucleon Number The total number of protons and neutrons in an atom; the mass number A.

Nucleons Protons and neutrons in an atomic nucleus.

Nucleosynthesis The binding of atoms to produce heavier elements. It takes place in stars (see nuclear fusion) and occurred in the first three minutes and 45 seconds after the Big Bang when it’s referred to as Big Bang nucleosynthesis.

Nucleus (i) Of an atom, the core of an atom, which has a positive charge, contains most of the mass, and takes up only a small part of the volume. (ii) of a comet, the chunks of matter, taking up a volume no more than a few kilometres across, at the centre of the head of a comet. (iii) of a galaxy, the innermost regions of a spiral galaxy; it does not show spiral structure and is visible from the sky as a bulge in the otherwise flat disk of the galaxy.

Number Density Is the quantity of some specified particle or object class per unit volume. For atoms, molecules or subatomic particles, the volume is typically in cm–3 or m–3. With stars, cubic parsecs (pc–3) are often used.

Nutation A bobbing motion that accompanies the precession of a spinning rigid body Any of several irregularities in the precession of the equinoxes, caused by varying torque applied to the Earth, by the Sun and the Moon


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O-III filter This is a special filter designed to let through a very specific wavelength of light emitted by oxygen in nebulae. It can help filter out light pollution and improves the view of objects such as planetary nebulae.

O and B Association A group of O and B giant Young stars close together in space. The members of an O and B association were formed at roughly the same time.

Objective The principal lens or mirror of an optical system.

Oblate Having an equatorial diameter greater than the polar diameter.

Obliquity The angle by which the spin axis of a planet to the plane of its ecliptic differs from 90°.

Obliquity of the Ecliptic The angle between the plane of the ecliptic and that of the celestial equator.

Observable Universe The portion of the entire universe that can be seen from Earth.

Observation In science, an observation is a fact or occurrence that is noted and recorded. The Hubble Space Telescope is a tool astronomers use to make observations of celestial objects.

Observatory A structure designed and equipped for making astronomical observations. Observatories are located on Earth and in space.

Obstruction Ratio In telescopes which have a diagonal mirror or a secondary mirror so positioned as to obstruct part of the incoming beam of light, the presence of that obstruction reduces the contrast of the image somewhat. The amount of contrast degradation depends on the relative size of the obstruction, compared to the clear aperture of the telescope.

Occultation An event that occurs when one celestial body conceals or obscures another. For example, a solar eclipse is an occultation of the Sun by the Moon.

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Ocular Adjustment Increasing or decreasing the distance between the two eyepieces (which can also be called the ocular lenses) in a pair of binoculars.

Off-Axis A telescope whose optical elements are not mechanically symmetric about a single optical axis. The common types are reflectors, in which the primary mirror is tilted, so as to deflect the reflected beam off to the side, so that other optical components, which might otherwise partially obscure the incoming beam, will not do so.

Okta In meteorology, an okta is a unit of measurement used to describe cloud cover from 0 = clear to 8 = overcast.

Olbers's Paradox Time observation that the sky is dark at night contrasted to a simple argument that shows that the sky should be uniformly bright, due to there being a star in every line of sight in an infinite and uniform universe. The paradox is resolved by the redshift of the expanding universe.

Oort Cloud A theoretical shell of comets that is believed to exist at the outermost regions of our solar system. A vast collection of bodies made up of rock and ice that orbit the Sun at a distance starting in the region beyond the orbit of Pluto and extending out to nearly 1.5 light-years or 50,000 A.U. Unlike the bodies of the Solar System, which orbit the Sun in roughly the same plane, the objects form a vast sphere around the Sun. It is estimated that billions of objects exist in this region, and there is evidence that this is the place where most comets originate. The Oort cloud was named after the Dutch astronomer who first proposed it.

Opacity The degree to which light is prevented from passing through an object or a substance. Opacity is the opposite of transparency. As an object’s opacity increases, the amount of light passing through it decreases. Glass, for example, is transparent and most clouds are opaque.

Open Cluster A collection of young stars that formed together. They may or may not be still bound by gravity. Some of the youngest open clusters are still embedded in the gas and dust from which they formed.

Open Universe The version of big bang cosmology in which the universe will expand forever. An open universe has infinite volume.

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Opposition The position of a planet when it is exactly opposite the Sun as seen from Earth. A planet at is at its closest approach to the Earth and is best suitable for observing.

Optical Activity The property of some crystals, gases, liquids, and solutions to rotate plane-polarized light to the left or right. It occurs because the molecules that make up the substance through which the light beam is shone are asymmetric, i.e., they have no plane of symmetry. Asymmetric molecules are mirror images of each other that cannot be superimposed. This asymmetric property is also referred to as handedness. Examples are the L- and D-forms of amino acids.

Optical Brightener A compound that absorbs the invisible ultraviolet component of sunlight and re-emits it as visible light at the blue end of the spectrum.

Optical Depth The number of factors of the transcendental number (2.71828. . . ) that radiation is dimmed in passage through a gas.

Optical Double A pair of stars that appear extremely close together in the sky even though they are at different distances from us and are not physically linked.

Optical Tube Assembly The tube that holds your mirrors and/or lenses.

Optics The science that deals with the properties of light; in this case specifically dealing with the way light changes directions when it is either refracted and dispersed by a lens or reflected from a mirror.

Orbit The path of a celestial body as it moves through space.

Orbital A region of space in an atom occupied by one or two electrons.

Orbital Elements The six numerical values that completely define the orbit of one body about another of known mass. They are the semi-major axis (a), the eccentricity (e), the inclination to the reference plane (i), the (M), the argument of the pericentre (ω), and the longitude of the ascending node (Ω). The elements vary with time as a consequence of perturbations of other

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bodies, so their epoch is important. For comets and asteroids, the perihelion conditions are often of interest, so the date of perihelion (T) and perihelion distance (q) are usually used instead of M and a. (At T, M=0; q = a(1-e) )

Orographic Clouds These are clouds that form as air is forced to rise over high obstructions on the surface of a planet. This could be a mountain on Earth or a volcano on Mars, for example.

Orrery A dynamic model of the Solar System that shows the relative positions and motion of the planets and moons.

Orthoscopic See “Eyepiece”

Osculating Orbit The orbit that a body would follow if the only gravitational force acting on it was that of the primary body, i.e. if its motion was not perturbed by the presence of other bodies.

Outer Space Any region of space beyond limits determined with reference to boundaries of a celestial system or body, especially the region of space immediately beyond Earth's atmosphere.

Outgassing The slow release of gas from a solid or liquid; especially the release of gases into the atmosphere of a planet.

Ozone O3 a gaseous molecule, made of three oxygen atoms, which forms a layer in the upper atmosphere that shields the Earth against excessive ultraviolet radiation.

Ozone Layer Layer of Earth's atmosphere at about 20 to 30 miles, marked by a high ozone (O3) content.


Pancam Camera This is the high-resolution stereo panoramic camera (pancam) that the Mars Exploration Rovers Spirit and Opportunity are

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equipped with. It is used to give a geological overview of a region and to navigate the Martian surface, as well as to choose what places are potentially interesting for the science instruments to study.

Panoptic See “Eyepiece”

Parabolic Mirror A mirror formed in a precise curve to cause the incoming light rays to be focused at the same point.

Paradigm A well-established example or model, an archetype.

Parallax (a) When used by itself, the word "" refers to trigonometric parallax, half the angle through which a star appears to be displaced when the earth moves from one side of the sun to the other, that is, through 2 A.U. The parallax of a star is inversely proportional to the distance to the star from the sun. (b) Some of the other ways of measuring distances, usually in those cases referred to with an adjective, as in spectroscopic parallax.

Parhelic Circle A luminous halo, each side of the sun parallel to the horizon, caused by refraction of sunlight through ice crystals.

Parsec A large distance often used in astronomy. A parsec is a parallax second, the distance at which 1 AU subtends an angle of 1 second of arc: equal to 30.857 x 1012 km = 3.2616 light years.

Peak to Valley Wavefront This is a reading of how smooth a parabolic mirror is by measuring the height difference from the centre (valley) of the mirror to the edge (peak) of it. It is typically a fraction of the wavelength of optical light.

Perchlorates These are a type of salt that have been found on the surface of Mars by the Phoenix lander. On Earth certain microbes are known to use perchlorates as an energy source.

Peculiar Velocity The velocity of a star with respect to the .

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Penumbra (a) For an eclipse, the part of the shadow from which the sun or other radiating body is only partially occulted. (b) of a sunspot, the outer region of the sunspot, not as dark as the central umbra.

Perfect Cosmological Principle The assumption that on a large scale the universe is homogeneous and isotropic in space and unchanging in time.

Periapse (Periapsis, Pericentre) The position in an orbital path that is the least distance from the primary body.

Periastron The near point of the orbit of a body to the star around which it is orbiting.

Perigee The point where (and when) an object's orbit about the earth in which it is closest to the earth; only applicable to objects orbiting the earth (not to objects orbiting the sun).

Perihelion The point in the orbit of a planet or other body where it is closest to the Sun.

Period A period of time in history seen as a single coherent entity; an epoch, era. The length of time during which the same characteristics of a periodic phenomenon recur, such as the repetition of a wave or the rotation of a planet.

Period-Luminosity Law A relationship that describes how the luminosity or absolute brightness of a Cepheid variable star depends on the period of time over which that brightness varies.

Periodic Comet A comet in a closed, elliptical orbit within our solar system. These comets typically have orbital periods of less than 200 years. Many comets have orbits that keep them in the inner solar system and allow their trajectories to be calculated with great accuracy and precision. Perhaps the best-known periodic comet is Halley’s comet, whose orbital period is 76 years.

Periodic Table A systematic arrangement of the elements in columns and rows; elements in a given column have similar properties.

Perturbations Gravitational influences ("tugging" and "pulling") of one astronomical body on another. Comets are strongly perturbed by the 4 October 2012 103 © Paul Thomas

gravitational forces of the major planets, particularly by the largest planet, Jupiter. These perturbations must be allowed for in orbit computations, and they lead to what are known as "osculating elements" (which means that the orbital element numbers change from day to day and month to month due to continued perturbations by the major planets, so that an epoch is necessarily stated to denote the particular date that the elements are valid.

Phase The apparent change in shape of the Moon and inferior planets as seen from Earth as they move in their orbits.

Phase Angle Is the elongation or angle between an orbiting body and the Sun as viewed from a particular perspective such as the Earth. It determines the amount of a planet or moon's visible surface that lies in shadow. Inferior planets such as Venus generally have a low phase angle as seen from Earth, so they are often viewed as a crescent. Superior planets such as Mars and Jupiter usually have a high phase angle, so little of the shadowed side is visible.

Photometer A device that measures the intensity of light from a particular source.

Photometry A measurement of the intensity of electromagnetic radiation emitted by a celestial body.

Photomultiplier An electronic device that through a series of internal stages multiplies the small current that is given off when light is incident on the device so that a relatively large current results.

Photon A packet of energy that can be thought of as a particle of light travelling at the speed of light. A photon of energy E is equivalent to an electromagnetic wave of wavelength λ=hc/E, where h is Planck's constant and c is the speed of light in a vacuum.

Photosphere The visible surface layer of the sun and stars; the layer from which continuous radiation escapes and where absorption lines form.

Photovoltaic Cell A solar cell; a cell that converts sunlight directly to electrical energy.

Physical Change A change in physical state or form.

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Physical Properties The qualities of a substance that can be demonstrated without changing the composition of the substance.

Pickering Scale A scale of Seeing conditions devised by William Pickering. 1 is very poor to 10 which is perfect.

Pinched Optics Mechanical stress affects reflective and refractive imaging element properties. If one of these elements comes under local stress from the support system, this can translate into artefacts in the image.

Pincushion Distortion An aberration of optical systems, in which magnification increases with distance away from the optical axis. With such a system, squares are imaged with their sides curved inward, looking sort of like pincushions.

Pixel A light-sensitive picture element on a charge-coupled device (CCD) or some other kind of digital camera. A pixel is a tiny cell that, placed together with other pixels, resembles the mesh on a screen door. The Hubble Space Telescope’s Wide Field and Planetary Camera 2 has four CCDs, each containing 640,000 pixels. Each pixel collects light from a celestial object and converts it into a number. The numbers (all 2,560,000 of them) are sent to ground-based computers, which convert them into an image. The greater number of pixels, the sharper the image.

Plage The part of a solar active region that appears bright when viewed in H0.

Planck Constant The numerical factor h relating the frequency v of a photon to its energy e in the expression e = hn. The Planck constant has the value h = 6.62620 x 10-27 erg sec.

Planck Function (also known as the Planck Law) The mathematical expression describing the continuous thermal spectrum of a glowing object. For a given temperature, the Planck function specifies the intensity of radiation as a function of either frequency or wavelength.

Planet A very large body in orbit around a star. Planets can be composed mainly of rock or of dense gases.

Planetarium A display museum in which images of stars and other astronomical phenomena are projected onto a domed ceiling. A computer program that displays celestial objects, that may also be used to control a telescope.

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Planetary Nebula A shell of gas surrounding a small, white star. The gas is usually illuminated by the star, producing a variety of colours and shapes.

Planetesimal A small (diameter up to several hundred kilometres) solar-system body of the type that first condensed from the solar nebula. are thought to have been the principal bodies that combined to form .

Planetoid Another term for a minor planet – the official name for asteroids, as used by the International Astronomical Union (IAU).

Planisphere A star chart with an overlay that shows you what stars are visible at a specific time, date and latitude.

Plasma A state of matter similar to a gas but composed of isolated electrons and nuclei rather than discrete whole atoms or molecules.

Plate Tectonics The theory that the Earth's continental and oceanic crust and outermost portion of the mantle is fractured into large plates that move relative to each other. Convective currents in the mantle provide the driving force for this motion. The plate motion is responsible for global mountain building, activity, and volcanism, all of which are most pronounced along plate boundaries.

Plossl See “Eyepiece”

Plutino form the inner part of the Kuiper belt and represent about a quarter of the known Kuiper belt objects. Plutinos are the largest class of the resonant trans-Neptunian objects (i.e. bodies in orbital with Neptune). For every 2 orbits that a makes, Neptune orbits 3 times.

Plutoid If an object in our Solar System has an orbit that takes it beyond Neptune, is near spherical and hasn’t cleared its orbit of detritus like small asteroids and , it’s now classed as a Plutoid or trans-Neptunian dwarf planet. There are now many known Plutoids: Eris, , , Sedna, and Pluto itself. The new category was invented by the International Astronomical Union (IAU). It follows the downgrading of Pluto, from planet to dwarf planet in 2006.

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Polar Alignment The process of getting the axis of an equatorial mounting that is supposed to point parallel to the Earth's axis of rotation, to do so.

Polar-Alignment Telescope A small telescope permanently attached to an equatorial mounting, aligned with the mounting axis that is supposed to point parallel to the Earth's axis of rotation. The polar-alignment telescope probably has markings visible through its eyepiece which can be aligned with stars near the north or south celestial poles, to aid in performing a polar alignment.

Polar Axis The axis of an equatorial telescope mounting that is parallel to the earth's axis of rotation.

Polarscope A small scope that goes into equatorial mounts to help you achieve an accurate polar alignment for your telescope. Accurate polar alignment is essential to take good quality, long-exposure images, for which the star field needs to be tracked.

Polarization The arrangement of electromagnetic waves so that all the planes in which the waves are oscillating are parallel to each other.

Polycyclic Aromatic Hydrocarbon Any of various organic compounds composed of a few to many six-membered rings of carbon atoms linked by an alternating sequence of single and double bonds and to which hydrogen atoms are attached.

Population I The class of stars with relatively high abundances of heavy elements. These stars are generally found in the disk and spiral arms of spiral galaxies, and are relatively young. The term Population I is also commonly applied to other components of galaxies associated with the star formation, such as the interstellar material.

Population II The class of stars with relatively low abundances of heavy elements. These stars are generally found in a spheroidal distribution about the galactic centre and throughout the halo, and are relatively old.

Population III Hypothetical class of 1st generation stars composed solely of H and He, with mass 100-1000 x solar and luminosity 1-30 million times greater. As usual in astronomy, the numbering is backward: these are the first stars.

Positron A subatomic particle with the same mass as the electron, but with a positive electrical charge; the antiparticle of the electron.

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Potential Energy Energy that is stored, and which may be converted into kinetic energy under certain circumstances. In astronomy, the most common form of potential energy is gravitational potential energy.

Prebiotic Relating to the chemical or environmental precursors of the origin of life.

Precession A rotation of the direction of the axis of rotation. Normally refers to the precession of the equinoxes, a consequence of the effect of the Sun's gravity on Earth's equatorial bulge. Earth's axis of rotation precesses with a period of about 25,770 years, during which time the equinoxes make a complete revolution about the celestial equator. Because the Vernal Equinox is the reference direction for the equatorial co-ordinate system, the co-ordinates of "fixed" objects changes with time and must therefore be referred to an epoch at which they are correct.

Primary Body The body that is being orbited. E.g. the Sun is the primary of the orbits of the planets and comets. With respect to multiple star systems, it is the most massive star.

Primary Cosmic Rays The cosmic rays arriving at the top of the earth's atmosphere.

Primary Mirror The principal light-gathering mirror in a reflecting telescope.

Prime Focus The location at which the main lens or mirror of a telescope focuses an image without being reflected or refocused by another mirror or other optical element.

Prime Meridian An imaginary line on the surface of a planet from which longitude is measured. On Earth, the line passes through the Royal Observatory at Greenwich, England.

Primordial Background Radiation Radiation detected in millimetre and sub-millimetre wavelength regions that is coming from all directions in space and interpreted to be the remnant of the big bang. Also known as 3° background radiation, background radiation, or the remnant of the primeval fireball.

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Primordial Nucleosynthesis Element building that occurred in the early universe when the nuclei of primordial matter collided and fused with one another. Most of the helium in the universe was created by this process.

Prism A prism is usually a triangular-shaped piece of glass used to refract, or bend, light. The shape of the glass causes the light to disperse, or spread out, as it bends, producing a rainbow of colours from the white light.

Prograde Motion Orbital or spin motion in the forward or "normal" direction; in the solar system, this is counter-clockwise as viewed looking down from above the north pole.

Project Cyclops A 10- design study sponsored by NASA, Stanford University, and the American Institute for Engineering Education, of possible means for detecting extra-terrestrial civilizations.

Prolate Having the diameter along the axis of rotation longer than the equatorial diameter.

Prominence An explosion of hot gas that erupts from the Sun's surface. Solar prominences are usually associated with sunspot activity and can cause interference with communications on Earth due to their electromagnetic effects on the atmosphere.

Proper Motion Motion across the sky with respect to a framework of galaxies or fixed stars, usually measured in seconds of arc per century.

Proto- A prefix from the Greek for "before." When used in conjunction with the name of a celestial body, means the state of the body just before it is considered to have formed.

Protogalaxy Matter that is beginning to come together to form a galaxy. It is the precursor of a present-day galaxy and is sometimes called a “baby galaxy.”

Protoplanet Astronomical objects, approximately the size of the Moon, formed from the mutual gravitational attraction of Planetesimals; they are thought to collide with each other and slowly form planets.

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Proton A positively charged elementary particle of mass equal to that of a neutron and almost two thousand times that of an electron; a component, with neutrons, of all atomic nuclei : The nucleus of the hydrogen atom : A hydrogen ion, H*, arising either through the removal of the peripheral electron from a hydrogen atom or, together with a hydroxyl ion, from the dissociation of a water molecule.

Protoplanetary Disc The disc of gas and dust surrounding a newly formed star. New planets may coalesce out of the matter in the protoplanetary disc.

Protoplanetary Nebula The initial stage in the formation of a ‘planetary nebula’ – a glowing cloud of gas and dust surrounding a star that has ceased nuclear fusion and blown off its outer layers of gas. The term is also sometimes used in astronomy to describe the disc of matter around a star in which planets may form.

Protostar A dense accumulation of dust and gas that is on the verge of collapsing to form a full blown nuclear burning star.

Pulsar A rapidly rotating neutron star that emits energy along its gravitational axis; an extremely dense collapsed star where the electrons have been forced into the protons. The object is thus made up mainly of neutrons and a few kilometres in diameter. Pulsars emit magnetic field constrained beams of radio, visible, x-ray and gamma radiation, which we perceive as pulses as these objects whirl, sometimes at hundreds of revolutions a second.

Pyrex A Trade Name for a glass having a low coefficient of expansion due to high silica content.

Pyroclastic The type of rock formed from fragments or ash resulting from volcanic activity, especially the hot, fast-moving dense clouds that occur during an eruption.


Q0 The deceleration parameter, a cosmological parameter that describes the rate at which the expansion of the universe is slowing up.

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Quadrature A point in the orbit of a superior planet where it appears at right angles to the Sun as seem from Earth.

Quantum The amount of energy associated with a photon, equal to hv, where h is the Planck constant, and v is the frequency. The quantum is the smallest amount of energy that can exist at a given frequency.

Quantum Mechanics The branch of 20th century physics that describes atoms and radiation; the theory involves bundles of energy known as quanta.

Quantum Efficiency The percentage of photons hitting the photo reactive surface that will produce an electron-hole pair (the electron in our bucket).

Quantum Theory This is the study and respective theories on how the Universe is put together on a tiny sub-atomic scale. It’s the dream of many scientists to combine quantum theory with the theories that explain the large scale Universe in one unified theory.

Quark One of the subatomic particles from which many modern theoreticians believe such elementary particles as protons and neutrons are composed. The various kinds of quarks have positive or negative charges of 1/3 or 2/3.

Quark Star A theoretical type of massive neutron star that is so dense that matter is broken down to its constituent particles. Quarks are the hypothetical constituent particles of subatomic particles, like neutrons.

Quasar An unusually bright object found in the remote areas of the universe. Quasars release incredible amounts of energy and are among the oldest and farthest objects in the known universe. They may be the nuclei of ancient, active galaxies.

Quasi-Stellar Object Any of a class of extragalactic objects also known as quasars, characterized by emission lines with very large . The quasi-stellar objects are thought to lie at great distances, in which case they existed only at earlier times in the history of the universe; they may be cores of young galaxies.

Quiet Sun The collection of solar phenomena that do not vary with the solar activity cycle.

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Rack and Pinion Focuser A focuser in which the tube containing the eyepiece has a straight row of gear teeth (the rack) running along one side of it, which are engaged by a more conventional-looking gear (the pinion), attached to a knob, in order to adjust the focus.

Radar The acronym for radio detection and ranging; an active rather than passive radio technique in which radio signals are transmitted and their reflections received and studied.

Radial Velocity The velocity of an object along a line (the radius) joining the object and the observer; the component of velocity toward or away from the observer.

Radian The unit of angular measure, defined as the ratio of a length of arc intercepted by two radii to the length of the radius. P radians = 180°.

Radiance The amount of power per unit area and per unit solid angle in a beam of light. (Think of it as the irradiance per unit solid angle.) Units: W m-2 sterad-1

Radiant The is the point in the sky where meteors (associated with a specific ) appear to come from. The constellation where the radiant is located determines the name of the meteor shower. So for example, the have their radiant in .

Radiation The process by which electromagnetic energy moves through space as vibrations in electric and magnetic fields. This term also refers to radiant energy and other forms of electromagnetic radiation, such as gamma rays and X-rays.

Radiation Era Collective name for the first three epochs of the history of the Universe (, hadron epoch, lepton epoch), lasting overall about 100 sec and dominated by radiation rather than matter.

Radiation Pressure Pressure created by light hitting a surface.

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Radiative Process An event involving the emission or absorption of radiation. For example, a hydrogen atom that absorbs a photon of light converts the energy of that radiation into electrical potential energy.

Radiative Transport The transport of energy, inside of a star or in other situations, by radiation.

Radioactive Having the property of spontaneously changing into another isotope or element.

Radioactive Dating A technique for estimating the age of material, such as rock, based on the known initial isotopic composition and the known rate of radioactive decay for unstable isotopes originally present.

Radioactive Decay Disintegration of an unstable atomic nucleus by spontaneous emission of radiation.

Radioactivity Spontaneous emission of alpha, beta, or gamma rays by the disintegration of the nuclei of atoms.

Radio Galaxy Any of a class of galaxies whose luminosity is greatest in radio wavelengths. Radio galaxies are usually large elliptical galaxies, with synchrotron radiation emitted from one or more pairs of lobes located on opposite sides of the visible galaxy.

Radionuclide Radioactive nuclear species or nuclide.

Radio Astronomy The branch of astronomy which utilizes radio waves through the use of radio telescopes to study celestial bodies and occurrences.

Radio Telescope An antenna or set of antennas, often together with a focusing reflecting dish, that is used to detect radio radiation from space.

Radio Waves Electromagnetic radiation with wavelengths longer than about one millimetre.

Ramsden See “Eyepiece”

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Rare Earth Elements Series of elements usually taken to include elements with atomic numbers 58 to 71 (lanthanum) and sometimes yttrium and scandium.

Ray (a) A light ray, a wave of electromagnetic radiation (b) on the surface of a moon or planet, a streak of material that is relatively light in shade, presumable representing material ejected when a crater was formed.

Rayleigh Criterion (Rayleigh Limit) Lord Rayleigh, a 19th century physicist, showed that a telescope optic would be sensibly indistinguishable from a theoretical perfect optic if the light (strictly, the wavefront) deviated from the ideal condition by no more than one quarter of its wavelength.

Receiver The part of the radio telescope that detects long wavelength electromagnetic radiation and converts it to an electrical signal so that we can sense it.

Recessional Velocity The velocity at which an object moves away from an observer. The recessional velocity of a distant galaxy is proportional to its distance from Earth. Therefore, the greater the recessional velocity, the more distant the object.

Recombination The addition of an electron to an ion, usually resulting in radiation (recombination lines) when the electron subsequently jumps down to lower energy states.

Recurrent Nova A star known to flare up in nova outbursts more than once. A recurrent nova appears to be a binary system containing a white dwarf and a mass-losing star, in which the white dwarf sporadically flares up when material falls onto it from the companion.

Red Dwarf Red dwarfs make up most of the stars in the Universe. They are small cool stars that usually have less than 10 per cent the mass of the Sun with surface temperatures of between 2,200 and 3,700°C.

Red Giant A stage in the evolution of a star when the fuel begins to exhaust and the star expands to about fifty times its normal size. The temperature cools, which gives the star a reddish appearance.

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Reddening The phenomenon by which the extinction of blue light by interstellar matter is greater than the extinction of red light so that the redder part of the continuous spectrum is enhanced.

Redshift The increase in the wavelength of visible light from a distant celestial body towards the red end of the electromagnetic spectrum. This is due to the Universe’s expansion, which causes the wavelength of light to increase as it travels through space.

Reducing Agent A substance that causes reduction and is itself oxidized.

Reducing Atmosphere Atmosphere comprised of substances that readily provide electrons.

Reduction The gain of one or more electrons or hydrogen atoms (electrons + protons) by an atom or molecule.

Reflection Reflection occurs when light changes direction as a result of "bouncing off" a surface like a mirror.

Reflector (Reflecting Telescope) A telescope that uses a concave mirror to gather light and form an image at a focal plane.

Reflection Nebula An interstellar cloud containing dust that shines by light reflected from a nearby star.

Reflex Finder (Sight) Also called Unit Magnification Finder. A finder which operates without magnification, as do many rifle 'scopes. Such a device is in essence a miniature head-up display: It projects a cross-hair, illuminated bulls-eye, or similar pattern on the sky, where the observer may view it easily with both eyes open. Reflex finders make it very easy to find objects which are bright enough to be seen with the naked eye, or which are close enough to naked-eye stars.

Refraction The bending of electromagnetic radiation as it passes from one medium to another or between parts of a medium that has varying properties. The index of refraction of a substance is the ratio of the speed of light in a vacuum to that in the substance.

Refractor (Refracting Telescope) A telescope that uses a transparent objective lens to refract, or bend, light in order to form an image at the focal plane.

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Refractoritis An amateur astronomer who is fond of refractors to the point of obsession, is said to have refractoritis.

Refractory The property of being able to exist in solid form under conditions of very high temperature. Refractory elements are characterized by a high temperature of vaporization; they are the first to condense into solid form when a gas cools, as in the solar nebula.

Refractory Grain A grain that has been formed at high temperatures. This type of grain was found in samples of cometary dust by the Stardust spacecraft, and hints at a process that might have deposited these grains in the far reaches of the Solar System.

Regolith The mantle of unconsolidated fragmental material that covers a land surface; i.e., and fractured rock.

Regression of the Nodes This is a slow movement of the lunar nodes caused by the gravitational pull of the Sun. The lunar nodes move slowly westward, taking 18.6 years to complete one revolution.

Relativistic Having a velocity that is such a large fraction of the speed of light that the special theory of relativity must be applied.

Relativity Either of the theories of relativity worked out by Albert Einstein. The special theory of relativity (1905) is a theory of relative motion. The general theory of relativity (1916) is a theory of gravitation.

Research A study or investigation to find new information.

Resolution The ability of an optical system to distinguish fine detail. This is the greatest amount of detail that a telescope is capable of delivering. A telescope’s resolution can be roughly estimated by observing binary stars. If a binary pair can just be made out to show two separate stars, the telescope’s resolution (or ‘resolving power’) is roughly equivalent to their angular separation in the sky.

Resolve The ability of a telescope to distinguish objects that are very close to each other as two separate objects.

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Resolving Power Another word for Resolution.

Rest Wavelength The wavelength of a spectral line as measured in a laboratory, when there is no relative motion between source and observer.

Retardation This is the time difference between Moon rise one day, and Moon rise the following day.

Reticle A system of lines and/or concentric circles at the focal plane of a telescope, used for positioning or guiding the telescope, or polar-aligning an equatorial mount. Is usually incorporated into an eyepiece and may be illuminated in order to render the lines visible against a dark background sky.

Reticule A system of lines forming a pattern of squares or ‘ticks’ at the focal plane of a telescope, used in micrometres.

Retrograde The rotation or orbital motion of an object in a clockwise direction when viewed from the north pole of the ecliptic. Any motion that is in the opposite sense from the great majority of solar system bodies.

Retrograde Motion The phenomenon where a celestial body appears to slow down, stop, then move in the opposite direction. This motion is caused when the Earth overtakes the body in its orbit.

Reversing Layer The layer of the Sun's atmosphere above the photosphere.

Revolution The orbiting of one body around another.

Right Ascension The east-west coordinate in the equatorial coordinate system. One element of the astronomical coordinate system on the sky, which can be though of as longitude on the earth projected onto the sky. Right ascension is usually denoted by the lower-case Greek letter alpha and is measured eastward in hours, minutes, and seconds of time from the vernal equinox. There are 24 hours of right ascension, though the 24-hour line is always taken as 0 hours. More rarely, one sometimes sees right ascension in degrees, in which case there are 360 degrees of right ascension to make a complete circuit of the sky.

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Rille A long, narrow depression on the surface of the Moon – and other planetary bodies – that resembles a channel. They are thought to be caused by collapsed lava flows just under the surface.

Ring Galaxy A galaxy that has a ring-like appearance. The ring usually contains luminous blue stars. Ring galaxies are believed to have been formed by collisions with other galaxies.

Ritchey-Chretien A telescope closely resembling a classical, two-mirror-only Cassegrain, except that the primary mirror is an hyperboloid. Slightly more strongly figured than the Cassegrain's paraboloid, and the secondary is a slightly stronger hyperboloid than that of the Cassegrain. Ritchey-Chretiens are corrected for coma as well as for spherical aberration, thus they can deliver relatively sharp images across a wide field of view. The Hubble is a Ritchey-Chretien.

Roche Limit The point near a massive body such as a planet or star, inside of which the tidal forces acting on an orbiting body exceed the gravitational force holding that body together. The location of the Roche limit depends on the size of the orbiting body.

Roche Lobe This is a theoretical spherical boundary around (for example) each star in a binary system. Matter is bound by the star’s gravitational attraction within each lobe. When two Roche lobes mix they may begin to transfer material.

Roentgen (obsolete) Unit of X-ray or gamma radiation dosage. The quantity of radiation which liberates by ionisation one esu of electricity per cm3 of air under normal conditions of temperature and pressure."Using 1 esu ≈ 3.33564×10−10 C and the air density of ~1.293 kg/m³ at 0°C and 101kPa , this converts to 2.58×10−4 C/kg. An exposure of 500 roentgens in five hours is usually lethal for human beings.

Ronchi Grating A Ronchi grating is a special type of diffraction grating used in optical tests. It’s used to find the quality of a scope’s mirror by studying the diffraction patterns produced by light bounced off the mirror.

Rosetta Stone A slab of black basalt stone found in 1799 that bears an inscription in hieroglyphics, demotic characters, and Greek, celebrated for having given the first clue to the decipherment of Egyptian hieroglyphics; from this, any breakthrough discovery of great magnitude.

Rotation The spin of a body about its axis. 4 October 2012 118 © Paul Thomas

RR Lyrae Variable A member of a class of pulsating variable stars named after the prototype star, RR Lyrae. These stars are blue-white giants with pulsation periods of less than one day, and are Population II objects found primarily in globular clusters.

Russell-Vogt theorem The theorem that the evolution of a star is completely determined by its mass and chemical composition.


S Wave A type of seismic wave that is a transverse, or shear, wave, and which can travel only through rigid materials.

Saber's Beads Lunar phenomenon seen on extremely young and old crescents. The striking resemblance to 2nd and 3rd contacts during a total solar eclipse was first noted by American amateur astronomer Stephen Saber.

Sampling General wisdom and rules of thumb suggests that the image scale (above) should optimally be 1/2 of the seeing though some people suggest this should be 1/3. In the UK, at sea level, seeing varies from about 2" per pixel (very good) to 6" per pixel (poor) but more typically ~3" per pixel. This suggests an optimal image scale of ~1"-1.5" per pixel. Undersampled: Based on the above, if the image scale is greater than 1/2 the seeing it is considered under sampled i.e. the imaging system may be holding back the resolution of fine detail in the image. Oversampled: The converse of the above. In oversampled images the imaging system has a higher resolution than atmospheric conditions permit. A degree of oversampling is, not necessarily, a bad thing as it allows you to take advantage of any period of better than average seeing.

Saros Cycle An 18 year, 11 day repeating pattern of solar and lunar eclipses caused by a combination of the tilt of the lunar orbit with respect to the ecliptic and the precession of the plane of the moon's orbit.

Satellite A natural or artificial body in orbit around a planet.

Scattering The random reflection of photons by particles such as atoms or ions in a gas, or dust particles in interstellar space.

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Schiefspiegler A particular type of off-axis compound telescope. The original Schiefspiegler has a primary mirror with a relatively long focal ratio.

Schmidt, Schmidt-Cassegrain, Schmidt Newtonian Forms of catadioptric telescope.

Schmidt Camera A type of telescope that uses a spherical mirror and a thin lens to provide photographs of a wide field.

Schmidt Corrector Plate The key piece of any Schmidt-type optical system, such as a , a Schmidt-Cassegrain, or any of many others. To the naked eye, a Schmidt corrector plate looks like a flat piece of glass, but its surface is actually figured in a subtle manner, to affect the spherical aberration of the system in which it is used.

Schröter Effect The strange observational effect phenomenon in which Venus’s disc reaches half phase a few days before or after the predicted date.

Schwarzschild Radius The radius that, according to Schwarzschild's solutions to Einstein's equations of the general theory of relativity, corresponds to the event horizon of a black hole.

Science A branch of knowledge based on the laws of nature.

Scientific Law A summary of experimental data; often expressed in the form of a mathematical equation.

Scientific Model A representation that serves to explain a scientific phenomenon.

Scintillation Acting like a lens, the Earth’s atmosphere diffracts the starlight passing through it causing the effect known as scintillation. It can be seen when stars appear to twinkle.

Secondary Cosmic Rays High energy particles that are generated in the earth's atmosphere by primary cosmic rays.

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Secondary Mirror The second mirror in a reflecting telescope (after the primary mirror), usually either convex, to reflect the image out of a hole in the bottom of the telescope to the Cassegrain focus or along the telescope mount axis to the coudé focus; or flat, to reflect the image out of the side of the telescope to the Newtonian focus.

Secondary Obstruction The secondary obstruction in a reflecting or Catadioptric telescope is a result of the necessary placement of a secondary mirror in the optical path. A large secondary obstruction will generally lower the brightness and contrast of images through the eyepiece at high magnifications.

Second Law of Thermodynamics The degree of randomness in the universe increases in any spontaneous process.

Seeing The steadiness of the earth's atmosphere as it affects the resolution that can be obtained in astronomical observations. Good seeing corresponds to a steady atmosphere, and bad seeing corresponds to an unsteady atmosphere.

SeeligerEffect When a reflective object such as a planet is in solar opposition it will appear brighter in the night sky than when it is not. This effect is named after the German astronomer Hugo von Seeliger.

Seismic Wave A wave created in a planetary or satellite interior, usually caused by an earthquake.

Seismology The study of waves propagating through a body and the resulting deduction of the internal properties of the body. The prefix "seismo-" comes from the Greek word for earthquake.

Selection Effect The tendency for a conclusion based on observations to be influenced by the method used to select the objects for observation. An example was the early that all quasars are radio sources, when the principal method used to discover quasars was to look for radio sources and then to find out whether they had other properties associated with quasars.

Selenography The study of the surface of the Moon.

Semi-Apochromatic Refractor A refracting telescope that uses three or more lenses to bring red, green and blue light to focus at nearly the same point.

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Semi-Major Axis Half the major axis, that is, for an ellipse, half the longest diameter.

Sensitivity Modern sensors are usable to very low light levels and therefore are considered highly sensitive. Many can operate down to near zero lux (lumens/sq. metre).

Sensor Noise This is noise generated by a digital camera’s sensor. It arises because there is a small amount of electrical activity in the sensor itself. Noise increases with temperature but its effects in images can be reduced by taking many exposures (to increase the signal to noise ratio).

Setting Circles Calibrated discs, in essence, protractors, attached at both the axes of an equatorial mount. The polar axis has a circle scale divided into 24 equal parts each equivalent to one hour right ascension. The declination axis’s circle has degrees of declination, from 90 degrees north to 90 degrees south. With the mount correctly aligned to the celestial pole, an object may be located by setting its co-ordinates on the circles.

Seyfert Galaxy A type of spiral galaxy that has a bright nucleus and whose spectrum shows emission lines. Historically, N galaxies and Seyfert galaxies were defined by different astronomers on the basis of different information, and the difference between them is not always clear cut.

Shadow Transit A shadow transit is when the shadow of a moon passes over the surface of another planet. The most obvious of these to amateur astronomers are the shadow transits of Jupiter’s moons across its disc. They can be observed with a medium to large telescope.

Shear Wave A wave that consists of transverse motions; that is, motions perpendicular to the direction of wave travel.

Shock Wave Discontinuity in the flow of a fluid (including a gas or plasma) marked by an abrupt increase in pressure, temperature, and flow velocity at the shock front.

Shooting Star A meteor, especially a streak of light in the night sky, caused by a meteoroid burning up as it enters the Earth’s atmosphere.

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Side by Side Guiding Where the two scopes (imaging and guiding) are mounted side by side. Has the advantage of reducing the lever arm of part of the weight of the payload thereby reducing the counterweight needed to achieve balance but the combination needs balancing around all axes which can be quite difficult to achieve.

Sidereal Of, relating to, or concerned with the stars. Sidereal rotation is that measured with respect to the stars rather than with respect to the Sun or the primary of a satellite. The length of a sidereal day is 23 hr, 56 min, 4.09 sec of mean solar time.

Sidereal Day The time it takes Earth to rotate once relative to the stars or 23 hours 56 minutes and 4 seconds, which is 4 minutes less than a solar day, since during one sidereal day the Sun also moves 1° east along the ecliptic, and Earth has to rotate 4 additional minutes to complete one rotation relative to the Sun in one 24-hour solar day.

Sidereal Drive A mechanism used to make a telescope follow stars across the sky as the Earth rotates. Modern sidereal drives for small telescopes are generally electrically powered and electronically regulated.

Sidereal Month The average period of revolution of the Moon around the Earth in reference to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of mean solar time.

Sidereal Period The period of revolution of a planet around the Sun or a satellite around its primary.

Sidereal Time The hour angle of the First Point of Aries. It is time measured with respect to the stars

Sidereal Year The time it takes Earth to complete one orbit of the Sun relative to the stars or 365 days 6 hours 9 minutes and 10 seconds. It is also the time it takes the Sun to travel once around the sky relative to the stars.

Siderophile Element Element with a weak affinity for oxygen and sulphur and readily soluble in molten iron (including iron, nickel, cobalt, platinum, gold, tin, and tantalum). Those measured digits that are known with certainty plus one uncertain digit.

Signal-to-Noise Ratio Those measured digits that are known with certainty plus one uncertain digit.

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Significant Figures Those measured digits that are known with certainty plus one uncertain digit.

Singlet One simple lens used all by itself, as opposed to doublet and triplet.

Singularity A point in space where quantities become exactly zero or become infinitely large; a singularity is present in a black hole.

Sinking The disappearance below the horizon of distant objects normally visible; the reverse of looming.

Skyglow Limit Every image will show some skyglow which is just the brightness of the background (or foreground if it's due to light pollution). The skyglow limit is the length of exposure where the skyglow swamps any actual data.

Small Magellanic Cloud An irregular galaxy, the smallest of two satellites of the Milky Way.

Snel’s Law This gives the quantitative change of direction of a ray of light in passing from one medium to another. The product n sin z is the same on both sides of a plane interface between two media, where n is the local , and z is the local angle the ray makes with the normal to the interface. In the curved atmosphere, Snel's law becomes nR sin z = constant, where R is distance from the local centre of curvature;

Sol The name given to our sun.

Solar Arrays Rigid, wing-like arrays of solar panels that convert sunlight directly into electricity to operate scientific instruments, computers, and radio transmitters. Some of the energy generated is stored in onboard batteries so the item can operate while in shadow.

Solar Cell A device used for converting sunlight into electricity; a photoelectric cell.

Solar Constant Rate at which radiant solar energy is received normally per unit area at the outer layer of Earth's atmosphere; its value is about 1.94 gram calories/cm2/min.

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Solar Cycle The 11-year period over which the activity of the Sun increases and decreases; phenomena such as sunspots and solar flares are most common during a time of peak activity, called a solar maximum, but may be absent entirely when activity is at a low ebb, a solar minimum.

Solar Day The synodic rotation period of the earth with respect to the sun; that is, the length of time from one local noon, when the sun is on the meridian, to the next local noon.

Solar Eclipse A phenomenon that occurs when the Earth passes into the shadow of the Moon. A total solar eclipse occurs when the Moon is close enough to completely block the Sun's light. An annular solar eclipse occurs when the Moon is farther away and is not able to completely block the light. This results in a ring of light around the Moon.

Solar Flare A bright eruption of hot gas in the Sun's photosphere. Solar prominences are usually only detectable by specialized instruments but can be visible during a total solar eclipse.

Solar Mass Mass of the Sun, 2x1030 kg, used commonly as a unit to measure the masses of stars.

Solar Maximum The midpoint in the solar cycle where the amount of sunspot activity and the output of cosmic particles and solar radiation is highest.

Solar Minimum The beginning and the end of a sunspot cycle when only a few sunspots are usually observed, and the output of particles and radiation is normal.

Solar Motion The deviation of the sun's velocity from perfect circular motion about the centre of the galaxy; that is, the sun's peculiar velocity.

Solar Nebula Cloud of gas and dust out of which a star condenses. The primordial gas and dust cloud from which the sun and planets condensed.

Solar System The Sun and its surrounding matter, including asteroids, comets, planets and moons, held together by the Sun’s gravitational influence. 4 October 2012 125 © Paul Thomas

Solar Telescope A special reflecting telescope designed to study our closest star, the Sun. Solar telescopes differ from normal telescopes in that they are stationary and use small tracking mirrors to direct sunlight into the primary mirror. This is necessary because the Sun appears to move across the sky due to Earth’s rotation.

Solar Time A system of time-keeping with respect to the sun such that the sun is overhead of a given location at noon.

Solar (Stellar) Wind A flow of charged particles that travels from the Sun out into the solar system.

Solid Angle A three-dimensional angle.

Solstice The time of the year when the Sun appears furthest north or south of the celestial equator. The solstices mark the beginning of the Summer and Winter seasons.

South Celestial Pole A direction determined by the projection of the Earth's South Pole onto the celestial sphere. The SCP is exactly 180 degrees from the North Celestial Pole and corresponds to a declination of -90 degrees.

Southern Hemisphere Half of a spherical or roughly spherical body; for example, the Southern Hemisphere of Earth is the half below the equator.

Space Velocity The velocity of a star with respect to the sun.

Spacetime The name often given to the combined ‘fabric’ of the Universe where both three dimensional space and time are linked in their four dimensions.

Spatial Resolution In astronomy, the ability of a telescope to differentiate between two objects in the sky that are separated by a small angular distance. The closer two objects can be while still allowing the telescope to see them as two distinct objects, the higher the spatial resolution of the telescope.

Special Relativity The physical theory of space and time developed by Albert Einstein, based on the postulates that all the laws of physics are 4 October 2012 126 © Paul Thomas

equally valid in all frames of reference moving at a uniform velocity, and that the speed of light from a uniformly moving source is always the same, regardless of how fast or slow the source or its observer is moving. The theory has as it consequences the relativistic mass increase of rapidly moving objects, time dilatation, and the principle of mass-energy equivalence which most people know by the equation E=mc2.

Specific Heat The amount of heat required to raise the temperature of 1g of the substance by 1°C.

Specific Gravity The density of a substance taking water to equal 1. For example, Saturn has a specify gravity, or density, of 0.71. Therefore an equal volume of water would weigh more than the planet - if you had a big enough bowl of water, Saturn would float in it! On the other hand, Earth has a specific gravity of 5.5, thus Earth would weigh 5.5 times as much as an equal volume of water.

Speckle Interferometry method of using short-exposure photographs to recover information down to the diffraction limit of large optical telescopes.

Spectral Analysis study of the distribution by wavelength or frequency of the radiation emitted by an object of interest.

Spectral Binary A binary system recognized as a binary because its spectrum contains lines of two stars of different spectral types.

Spectral Lines A rainbow shows the small slice of the entire electromagnetic spectrum that humans can see. When astronomers use instruments to break down the light from a distant star or galaxy they also produce spectra. These contain signatures of the chemicals and molecules present in an object

Spectral Type/Class Classification used to sort stars by photospheric temperature and intrinsic brightness. The seven spectral classes O-B-A-F-G-K- M, listed in order of decreasing temperature, include 99% of all known stars. Each spectral type is divided into a variable number of subtypes.

Spectrogram A photograph of a spectrum.

Spectrograph An instrument that spreads electromagnetic radiation into its component frequencies and wavelengths for detailed study. A spectrograph is similar to a prism, which spreads white light into a continuous rainbow.

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Spectroheliograph An instrument used in solar telescopes to photograph the Sun in a single wavelength of light. Different wavelengths reveal different features of the Sun’s surface.

Spectrometer The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.

Spectroscope An instrument allowing an observer to view the spectrum of a source of light.

Spectroscopic Binary Binary star that can be distinguished from a single star only through analysis of the Doppler shift of the spectral lines of one or both stars as they revolve about their common centre of mass.

Spectroscopic Parallax The distance to a star derived from comparison of its apparent magnitude with its absolute magnitude deduced from study of its position on an H-R diagram determined by observation of its spectrum (spectral type and luminosity class).

Spectroscopy A method of determining the chemical composition of a substance by the electromagnetic radiation it absorbs or emits. Through , scientists can discover the chemical components of distant celestial bodies.

Spectrum (a) Grass-like patterns of gas seen in the atmosphere of the Sun. (b) An arrangement of electromagnetic radiation according to wavelength.

Speed Of Light Speed of light: The speed at which electromagnetic radiation propagates in a vacuum. It is has been measured at 299,792,458 meters per second (186,212 miles per second). Einstein’s Theory of Relativity implies that nothing can go faster than the speed of light.

Speers-Waler See “Eyepiece”

Spherical Aberration This is an error in some lenses (and mirrors) that causes light rays coming from the edge of the lens to be brought into focus at a different point to those rays that are passing through the lens’s centre. It causes stars to appear bloated and not as pinpoints of light.

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Spherochromatism An aberration of optical systems, in which spherical aberration is different in different colours of light. That is, if you performed a knife-edge or star test of a system with spherochromatism, you would get different results in different colours of light.

Spicule A jet of matter ejected from the photosphere of the sun.

Spider The arms or struts, if any, which support a diagonal mirror or a secondary mirror in position.

Spin-Orbit Coupling A simple relationship between the orbital and spin periods of a satellite or planet, caused by tidal forces that have slowed the rate of rotation of the orbiting body. Synchronous rotation is the simplest and most common form of spin-orbit coupling.

Spiral Density Wave A spiral wave pattern in a rotating, , such as the or the plane of a spiral galaxy like the Milky Way

Spiral Galaxy A galaxy that contains a prominent central bulge and luminous arms of gas , dust, and young stars that wind out from the central nucleus in a spiral formation. Our galaxy, the Milky Way, is a spiral galaxy.

Sporadic Meteor A meteor that is not associated with a shower.

Sprites Gamma-ray flashes produced in Earth’s atmosphere by severe lightning storms and upper atmospheric events.

Stacking Stacking involves taking many short exposures and adding them together to make one long exposure. Good software will register (align) all of the images while it stacks them.

Standard Candle An object whose properties allow us to measure large distances through space. The absolute brightness of a standard candle can be determined without a measurement of its apparent brightness. Comparing the absolute brightness of a standard candle to its apparent brightness therefore allows us to measure its distance. For example, the distinct variations of Cepheid variable stars in other galaxies tell us their absolute brightness. By accurately measuring the apparent brightness of these stars, astronomers can precisely determine the distance to the galaxy in which they reside.

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Star A giant ball of hot gas that creates and emits its own radiation through nuclear fusion.

Star Cluster A large grouping of stars, from a few dozen to a few hundred thousand, that are bound together by their mutual gravitational attraction.

Star Diagonal A mirror on some refracting telescopes that allows more comfortable viewing than you get by looking along the telescope tube. The star diagonal bends the light by 90 degrees.

Star Magnitude The Star Magnitude or brightness is a rating on a scale that gives the brightest stars a low magnitude, and the faintest stars the highest number. Each step on the scale is 2.5 times difference in brightness; five steps on the scale indicate a difference of 100 time’s comparative brightness.

Starburst Galaxy A galaxy undergoing an extremely high rate of star formation. Starburst galaxies contain massive, deeply embedded stars that are among the youngest stars observed.

Starspot This is the equivalent of a sunspot on another star. It is a region where magnetic field lines of the star are breaking through the surface slowing the transport of heat and cooling the surface.

Star Test An optical test of a telescope performed using a star as the light source.

Static Random noise in a radio receiver. It can also be heard in telephone lines and cell phones.

Steady-State Theory The theory of cosmology in which the universe is thought to have had no beginning and is postulated not to change with time.

Stefan-Boltzmann Law The radiation law that states that the energy emitted by a black body varies with the fourth power of the temperature.

Stellar Atmosphere or Envelope Is the outermost region of a star. Although it forms only a small portion of the star's mass, for some evolved stars the stellar envelope can form a significant fraction of the radius. 4 October 2012 130 © Paul Thomas

Stellar Black Hole A black hole formed from the death of a massive star during a supernova explosion. A stellar black hole, much like a supermassive black hole, feeds off of nearby material - in this case, the dead star. As it gains mass, its gravitational field increases.

Stellar Epoch Sixth epoch in the history of the Universe, lasting perhaps 1010 yr. from the galactic era to the present, dominated by the formation of stars.

Stellar Evolution The process of change that occurs during a star’s lifetime from its birth to its death.

Stellar Nursery A region in space where stars are forming from a cloud of gas and dust.

Stellar Parallax The apparent annual shifting of position of a nearby star with respect to more distant background stars. The term is often assumed to mean the parallax angle, which is one- half of the total angular motion a star undergoes.

Stellar Wind The ejection of gas from the surface of a star. Many different types of stars, including our Sun, have stellar winds. The stellar wind of our Sun is also known as the Solar wind. A star's stellar wind is strongest near the end of its life when it has consumed most of its fuel.

Stones A stony type of meteorite, including the chondrites.

Stooping Vertical compression of the refracted image of a distant object; the opposite of towering.

Stradian The unit of solid angular measure, defined as the ratio of the surface area of that section of a sphere intercepted by a solid angle to the square of the radius. A full sphere subtends 4 P steradians from its centre.

Stratosphere One of the upper layers of the atmosphere of a planet, above the weather. The earth's stratosphere ranges from about 20 to 50 km in altitude.

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Stroboscopic Imaging A type of imaging technique that allows studies to be made of oscillating or pulsating celestial object such as a pulsar.

Strong (Nuclear) Force The nuclear force, the strongest of the four fundamental forces of nature.

Sub or Sub-Exposure An individual exposure. The duration of the sub-frame will be dictated by the stability of the imaging system to maintain accurate tracking and the background brightness of the sky (Sky Limited Exposure).

Sublimation The process of passing from gas to solid state (or vice versa) without becoming a liquid.

Subtend The angle that an object appears to take up in your field of view; actually, the angle between lines drawn from opposite sides of the object to your eye. For example, the full moon subtends ½°.

Subtypes Designated by numerals. Further, stars are sorted by intrinsic brightness into luminosity classes designated by the first five Roman numerals. In turn, these are subdivided into a small number of subclasses designated by the first few letters of the lower case English alphabet; for example, the Sun is a G2 V star (also sometimes denoted as a dwarf G2 star) and (a Orionis) is classified as M2 lab (i.e., intermediate between la and lb).

Summer The moment when the Sun reaches its greatest distance north of the celestial equator, on or about June 21. In the northern hemisphere this marks the first day of summer; in the southern hemisphere it marks the first day of winter.

Sun The star at the centre of our solar system. The energy from the sun supports almost all life on Earth. An average star in terms of size and mass, the Sun is a yellow dwarf of spectral type G2. It is about 5 billion years old, contains 2x1030 kilograms of material, has a diameter more than 100 times that of Earth, and a mean distance from the Earth of approximately 149.6 million kilometres (1 AU). Currently, the Sun (and therefore the Earth and the Solar System) may be found close to the inner rim of the Galaxy's , in the Local Fluff, inside the , in the , at a distance of 8.33 ± 0.35 kiloparsecs (27,200 ± 1,100 ly) from the Galactic Centre.

Sun Pillar A vertical column of light that appears above or below the Sun at sunset or sunrise.

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Sunspot Areas of the Sun's surface that are cooler than surrounding areas. The usually appear black on visible light photographs of the Sun. Sunspots are usually associated disturbances in the Sun's electromagnetic field.

Sunspot Cycle The roughly 11-year cycle of variation of the number of sunspots visible on the sun.

Super-Earths These are rocky found by extra-solar planet searches in recent years. They’re more massive than the Earth, but generally no larger than about 10 Earth masses.

Supercluster A cluster, of clusters of galaxies.

Supergiant The stage in a star's evolution where the core contracts and the star swells to about five hundred times its original size. The star's temperature drops, giving it a red colour. Supergiants fall in the extreme upper right of the H-R diagram.

Supergranulation The pattern of large cells seen in the sun's chromosphere, when viewed in the light of the strong emission line of ionized hydrogen.

Superior Conjunction A conjunction that occurs when a superior planet passes behind the Sun and is on the opposite side of the Sun from the Earth.

Superior Planet A planet that exists outside the orbit of the Earth. All of the planets in our solar system are superior except for Mercury and Venus. These two planets are inferior planets.

Supermassive Black Hole These objects are a million to a billion times more massive than black holes and are found, it is thought, in the centre of every galaxy.

Supernova A supernova is a cataclysmic explosion caused when a star exhausts its fuel and ends its life. Supernovae are the most powerful forces in the universe. All of the heavy elements were created in supernova explosions.

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Supernova Remnant An expanding shell of gas ejected at high speeds by a supernova explosion. Supernova remnants are often visible as diffuse gaseous nebulae usually with a shell-like structure. Many resemble "bubbles" in space.

Supernumeraries These are the faint rainbow bands seen on the inner edge of a rainbow. They appear when rain droplets in the rainbow are comparatively small and of a similar diameter to each other.

Super Wide Angle A well corrected eyepiece with large apparent fields of view (60 - 70 degrees).

Surface Brightness This is a measure of how bright something is (like a galaxy or a nebula) for each unit of area (a square arc-second).

Symmetrical Eyepiece A specific eyepiece design. The optics of a symmetrical eyepiece comprise two cemented doublets oriented face-to-face. Many designs sold as Plossl eyepieces are actually symmetrical, but true Plossls are not symmetrical.

Synchotron Self-Absorption Re-absorption of radiation from accelerated electrons by other nearby electrons; this is a possible source of low frequency turnovers observed in the radio spectra of compact sources.

Synchotron Emission Radiation from electrons constantly accelerated in a magnetic field at a rate great enough for relativistic effects to be important. Predicted long ago, this radiation was first encountered in the particle accelerator called the synchrotron. Much of the radiation observed by radio astronomers originates in this fashion.

Synchotron Rotation A situation in which the rotational and orbital periods of an orbiting body are equal, so that the same side is always facing the companion object.

Synergistic Effect An effect much greater than the sum of the expected effects.

Synodic Measured with respect to an alignment of astronomical bodies other than or in addition to the sun or the stars (usually the moon or a planet). For example, a synodic month depends on the positions of the sun, earth, and moon.

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Synodic Month The time it takes the Moon to complete one cycle of phases—from new moon to new Moon—an average of 29.53059 days.

Synodic Period The orbital or rotational period of an object as seen by an observer on the earth. For the moon or a planet, the synodic period is the interval between repetitions of the same phase or configuration. The synodic period of Mars depends on the relative positions of the earth and Mars as they orbit the sun.

Systems Analysis Analysis of the response to inputs of a set of interconnected units whose individual characteristics are known.

Syzygy An alignment of three celestial bodies. Sometimes applied more specifically to an alignment of the sun, earth, and moon.


T-Tauri Stars Luminous variable stars associated with interstellar clouds and found in very young clusters; they are believed to be still in the process of gravitational contraction from their protostellar phase and have not yet arrived at the Main Sequence and begun to burn hydrogen.

T Association A grouping of several stars, presumably formed out of the same cloud of interstellar dust and gas.

Tachocline A thin boundary that separates the Sun’s outermost layer, called the convective zone, from the denser internal regions. These layers rotate at different speeds to create the Sun’s magnetic activity.

Tail A tail is made up of dust and gas from a comet’s coma. A tail forms when the solar wind separates dust and gas from the coma, pushing it outward and away from the Sun in either a slightly curved path (for dust) or a straight path (for gas).

Takahashi See “Eyepiece”

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Technology The sum total of processes by which humans modify the materials of nature to better satisfy their needs and wants.

Tektites Small glassy objects found scattered around the southern part of the southern hemisphere of the earth.

Telecompressor The opposite of a telextender or a Barlow lens. A telecompressor multiplies the focal length of a telescope by a factor which is less than one, thus reducing the focal length.

Telescope An instrument used to collect large amounts of light from far away objects and increase their visibility to the naked eye. Telescopes can also enlarge objects that are relatively close to the Earth or on the Earth. A Telescope can also include any instrument designed to observe distant objects by their emissions of invisible radiation such as x-rays or radio waves.

Telextender Another name for a Barlow lens.

Telluric Stars These have nearly featureless continuum spectra that can be used to correct for the effect of telluric contamination of the Earth's atmosphere on the spectra of other stars. For example, water vapour in the atmosphere creates significant telluric absorption bands at wavelengths above 6800 Å. These features need to be corrected for in order to reach a more accurate spectrum.

TelRad A particularly popular brand of unit-magnification finder.

Temperature A measure of heat intensity, or how energetic the particles of a sample are.

Termination Shock This is where the solar wind confronts the interstellar wind and is slowed down considerably. The sudden deceleration of the gas causes it to become denser and to heat up.

Terminator The boundary between the light side and the dark side of a planet or other body.

Terra A rough upland or mountainous region of the moon with a relatively high albedo. The planet Earth. 4 October 2012 136 © Paul Thomas

Terrestrial A term used to describe anything originating on the planet Earth.

Terrestrial Planet A name given to a planet composed mainly of rock and iron, similar to that of Earth, Mars, Venus, Mercury.

Terrestrial Refraction The displacement of terrestrial objects from their geometric directions by atmospheric refraction. You may not notice it, but it is a major headache for geodesists and surveyors.

Theories Detailed explanations of the behaviour of matter based on experiments; may be revised if new data warrant.

Thermal Inversion On the average, the temperature in the lower atmosphere decreases with increasing height. (The average gradient is about 6.5 K per kilometer.) A region in which the warmer air lies above the colder air is said to have an “inverted” temperature gradient, and is called a “thermal inversion,” or “inversion layer”.

Thermal Radiation Radiation released by virtue of an object’s heat, namely, the transfer of heat energy into the Radiative energy of electromagnetic waves. Examples of thermal radiation are sunlight, the orange glow of an electric range, and the light from in incandescent light bulb.

Thermonuclear Reactions Radiation whose distribution of intensity over wavelength can be characterized by a single number (the temperature). Black- body radiation, which follows Planck's law, is an example of thermal radiation.

Thin Disk Population This refers to the layer of the Milky Way galaxy where the spiral arms are found and where most of the star formation takes place. It is about 300–400 parsecs (980–1,300 light-years) deep and cantered on the galactic plane. Stars belonging to this population generally follow orbits that lie close to this plane. This is in contrast to members of the population and halo stars.

Tidal Braking or Acceleration Is the transfer of momentum between an astronomical body and an orbiting satellite as the result of tidal forces. This can cause changes in the rotation periods for both bodies as well as modification of their mutual orbit. A satellite in a prograde orbit will gradually recede from the primary body, while slowing the rotation rate of both bodies.

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Tidal Force The differential gravitational pull exerted on any extended body within the gravitational field of another body.

Tidal Locking Is a net result of continued tidal braking wherein the satellite orbits with the same face always pointed toward its primary. The Moon is tidally locked with the Earth.

Time Dilation The increase in the time between two events as measured by an observer who is outside of the reference frame in which the events take place. The effect occurs in both Special and General Relativity and is quite pronounced for speeds approaching the speed of light and in regions of high gravity.

Topocentric Referred to a position on the surface of the Earth (cf geocentric, which is referred to the centre of the Earth.)

Total Eclipse Any eclipse in which the eclipsed body is totally blocked from view or totally immersed in shadow.

Total (Visual) Magnitude Total, integrated magnitude of a comet's head (meaning coma + nuclear condensation). This can be estimated visually, as the comet's "total visual magnitude". The variable m1, usually found in comet ephemerides, is used to denote the total (often predicted) magnitude.

Towering Abnormal vertical stretching of the image of a distant object; the opposite of stooping. Towering and stooping are refraction phenomena that distort but do not invert images; hence, they are not mirages.

Trans Neptunian Objects An object in the Solar System that orbits the Sun at a greater average distance (semi-major axis) than Neptune. The Kuiper Belt, Scattered Disk Objects, and Oort cloud, are three conventional divisions of this volume of space, though treatments vary and a few objects do not fit easily into any division.

The Kuiper Belt contains objects with an average distance to the Sun of 30 to about 55 AU, usually having close-to-circular orbits with a small inclination from the ecliptic. Kuiper belt objects are further classified into the following two groups:

‘Resonant Objects’ which are locked in an orbital resonance with Neptune. Objects with a 1:2 resonance are called Twotinos, and objects with a 2:3 resonance are called Plutinos, after their most prominent member, Pluto. ‘Classical Kuiper Belt Objects’ (also called Cubewanos) have no such resonance, moving on almost circular orbits, unperturbed by Neptune and sit in a band between the inner Plutinos and the outer Twotinos. 4 October 2012 138 © Paul Thomas

The Scattered Disk contains objects farther from the Sun, usually with very irregular orbits (i.e. very elliptical and having a strong inclination from the ecliptic).

Transient Lunar Phenomena These are colour changes and brightening’s on the Moon’s surface, reported by some lunar observers. Today TLPs also include the flashes produced when meteoroids impact the Moon, which have been recorded many times by CCD cameras.

Transit (i) The passage of a celestial body across an observer's meridian (ii) The passage of a celestial body across the disk of a larger one.

Transit Method A way of searching for by looking for the tell-tale dip in brightness of a star as a planet passes in front of it.

Transition Elements Metallic elements situated in the centre portion of the periodic table in the B groups.

Transit Telescope A telescope designed to point straight overhead and accurately measure the times at which stars cross the meridian

Transparency Transparency is a measure of the clarity of the sky.

Triple-Alpha Process A chain of fusion processes by which three helium nuclei (alpha particles) combine to form a carbon nucleus.

Triple Conjunction The close alignment of a planet and a star at three distinct times, caused by the retrograde motion of the planet. The planet passes the star once in its forward motion, once more in its retrograde motion, and a third time when it resumes its forward motion.

Triplet Three simple lenses used in combination, placed close together or in contact.

Tritium A rare radioactive isotope of hydrogen with two neutrons and one proton in the nucleus (a mass of 3 atomic mass units).

Trojan An object orbiting in the Lagrange points of another (larger) object. This name derives from a generalization of the names of

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some of the largest asteroids in Jupiter's Lagrange points. Saturn's moons Helene, Calypso and Telesto are also sometimes called Trojans.

Trojan Asteroids A group of asteroids that precede or follow Jupiter in its orbit by 60°.

Tropic of Cancer The line of latitude on the Earth’s surface that is 23 1/2 degrees north of the Equator. It marks the northernmost latitude in the northern hemisphere at which the Sun can appear directly overhead.

Tropic of Capricorn The line of latitude on the Earth’s surface that is 23 1/2 degrees south of the Equator. It marks the southernmost latitude in the southern hemisphere t which the Sun can appear directly overhead.

Tropical Year The length of time between two successive vernal equinoxes.

Troposphere The lowest layer of the Earth's atmosphere, lying at an average height of up to 11km (6.5 miles). Above the troposphere lies the stratosphere, above that the ionosphere, above that the exosphere. In the troposphere the temperature drops steadily except for localised layers of temperature inversion.

True Field of View The angular size (in degrees) of the actual area of the sky that you can view through a particular telescope with a particular eyepiece. (The TFOV depends on both, and is calculated by dividing the AFOV by the Magnification.)

Truss-Tube Dobsonian A Dobson-mounted telescope in which the telescope tube is a truss assembly, perhaps looking like part of a bridge, or a bird cage. Often designed for quick and easy assembly and disassembly in the field, and the disassembled parts are not very bulky. Thus a quite large telescope may be transported in modest space.

Tube Rings Also Mounting Rings. Rings that attach to a telescope’s mounting plate (see dovetail plate) and hold the tube assembly in place.

Turbulence Unstable and disorderly motion, as when a smooth, flowing stream becomes a churning rapid.

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Twilight The period of decreasing sky brightness after sunset, or of increasing sky brightness before sunrise. There are three definitions of twilight: Civil Twilight, Nautical Twilight, and Astronomical Twilight. Twilight lasts longer in higher latitudes.

Type Ia Supernova A supernova explosion that occurs when a white dwarf accretes enough matter from a companion star to exceed the Chandrasekhar limit.


Ultra-Low Expansion ULE, Corning Glass's successor to Pyrex as a material out of which telescope mirrors are made. U.L.E. expands or contracts very little when the temperature changes, and thus a mirror made out of it holds its shape as the temperature varies without distorting the image.

Ultraviolet The region of the spectrum between about 100 and 4000 angstroms; also used in the restricted sense of ultraviolet radiation that reaches the ground, namely, that between about 3000 and 4000 angstroms.

Ultraviolet Radiation Electromagnetic radiation that has a shorter wavelength and higher energy than visible light.

Ultra Wide Angle A specific eyepiece design. Ultra Wide Angle has a very large apparent field of view, of 80+ degrees.

Umbra The area traced on a planet during an eclipse where the eclipsed light source is completely blocked. Observers in the umbral shadow of a solar eclipse, for instance, see a total eclipse.

Unit-Magnification Finder A finder which operates without magnification. Such a device is in essence a miniature head-up display: It projects a cross-hair, illuminated bulls-eye, or similar pattern on the sky, where the observer may view it easily with both eyes open. Unit- magnification finders make it very easy to find objects which are bright enough to be seen with the naked eye, or which are close enough to naked-eye stars. Also called a Reflex Finder.

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Universal Time A measure of time used by astronomers; UT conforms (within a close approximation) to the mean daily (apparent) motion of the sun. UT is determined from observations of the diurnal (daily) motions of the stars for an observer on the earth. Coordinated Universal Time (UTC) is that used for broadcast time signals (available via shortwave radio, for example), and it is within a second of UT. Also known as .

Universe everything that came into existence at the moment of the Big Bang, and everything that evolved from that initial mass of energy, or everything we can in principle, observe.

Uranography ; the mapping of celestial bodies.

Uranology (obsolete) The study of the heavens and heavenly bodies, also books about the subject.

Uranometry A chart or catalogue of fixed stars, especially of stars visible to the naked eye.

UV light ultraviolet (UV) light.


Van Allen Belt Zones in the earth's magnetosphere where charged particles are confined by the earth's magnetic field. There are two main belts, one centred at an altitude of roughly 1.5 times the earth' radius, and the other between 4.5 and 6.0 times the earth's radius.

Vaporization The process in which a substance changes from the liquid to the gaseous (vapour) state.

Variable Star A star whose brightness fluctuates over time. This may be hours, days weeks or many years. The reasons for the fluctuations are varied, such as another star eclipsing it or shells of gas and dust being ejected.

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Velocity The measure of the speed and direction of an object; the distance travelled by an object per .

Velocity Curve A plot showing the orbital velocity of stars in a spiral galaxy versus distance from the galactic centre.

Velocity Dispersion A measure of the average velocity of stars in a group or cluster with random internal motions. In globular clusters and elliptical galaxies, the can be used to infer the central mass.

Vernal Equinox The moment when the Sun crosses the celestial equator travelling in a northward direction, on or about March 21. In the northern hemisphere, it marks the first day of spring. The term is also applied to the Sun’s position in the sky at that moment. It is one of two points where the ecliptic and the celestial equator intersect, the other being the autumnal equinox.

Very Large Array One of the world’s premier radio observatories, consisting of 27 antennas arranged in a huge “Y” pattern. The VLA spans up to 22 miles (36 km) across, which is roughly one and a half times the size of Washington, D.C. Each antenna is 81 feet (25 meters) in diameter. Located in Socorro, New Mexico, the telescopes work in tandem to produce a sharper image than any single telescope could record.

Very-Long-Baseline Interferometry The technique of using simultaneous measurements made with radio telescopes at widely separated locations to obtain extremely high resolution.

Vignetting An optical effect whereby the corners or edges of a telescope’s field of view are darkened. There are several ways Vignetting can appear, but it can be reduced by taking flat fields (using software to remove its appearance) and in some cases by stopping down an aperture.

Visible Light The part of the electromagnetic spectrum that human eyes can detect; also known as the . The colours of the rainbow make up visible light. Blue light has more energy than red light.

Virgo Cluster A gigantic cluster of over 2000 galaxies that is located mainly within the constellation of . This cluster is located about 60 million light years from Earth.

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Visual Binary Binary star system whose components can be identified with an optical telescope.

Visual Magnitude A scale used by astronomers to measure the brightness of a star or other celestial object. Visual magnitude measures only the visible light from the object. On this scale, bright objects have a lower number than dim objects.

Volatile The property of being easily vaporized. Volatile elements stay in gaseous form except at very low temperatures; they did not condense into solid form during the formation of the solar system.

Volatile Organic Compounds Class of organics that is easily vaporizable at low temperatures and pressures.

Volcano A break or vent in the crust of a planet or moon that can spew extremely hot ash, scorching gases, and molten rock. The term volcano also refers to the mountain formed by volcanic material.


Waning When the phase (illumination) of a planet or moon’s disc is decreasing in size. For example: ‘A waning crescent Moon can be seen tonight’.

Waning Crescent The phase of the Moon between third quarter and new moon. Waning means declining or fading.

Waning Gibbous The phase of the Moon between full moon and last quarter.

Wave A vibration in some media that transfers energy from one place to another. Sound waves are vibrations passing in air. Light waves are vibrations in electromagnetic fields.

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Waveguide Special resembling a pipe and often having a rectangular , inside of which radio waves may be propagated.

Wavelength The distance between consecutive crests of a wave. This serves as a unit of measure of electromagnetic radiation.

Waxing Crescent The phase of the Moon between new moon and first quarter. Waxing means increasing.

Waxing Gibbous The phase of the Moon between first quarter and full moon.

Waxing Moon Phases of the lunar cycle when the face of the Moon appears to be getting larger each day. In the waning parts of the lunar cycle, the face of the Moon appears to be getting smaller.

Weak Force One of the four fundamental forces of nature, weaker than the strong force and the electromagnetic force. It is important only in the decay of certain elementary particles.

Wedge The part that fits between the tripod or pillar and the fork of a fork-mounted telescope, which enables the fork to be equatorially aligned.

Weight The force determined by the gravitational pull on a mass.

WGS84 (Geographical Coordinates) Geographical coordinates are given by the angles longitude (Lon), latitude (Lat), and altitude in meters (Alt). A place north of the equator at marked by N or +, places south of the equator by S or -. The longitude from the meridian of Greenwich is counted positive towards east (E). Places west from Greenwich are marked W or by -. The geographical coordinates refer to an ellipsoid, which fits the true shape of the Earth (geoid). The geoid corresponds to calm sea surface. The keyword "Geographic:" uses the local ellipsoid as reference system. WGS84 mark coordinates referring to the WGS84 ellipsoid. The difference in altitude to the geoid sums up to 100 meters and is called geoid undulation. This is corrected for when tagged "MSL" (mean sea level), such that the origin of the height system is at sea level.

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Whistler Electromagnetic ultra-low frequency radiation observed in planetary magnetospheres; energized by lightning and other discharges.

White Dwarf A very small, white star formed when an average sized star uses up its fuel supply and collapses. This process often produces a planetary nebula, with the white dwarf star at its centre. It has a mass less than the Chandrasekhar limit, which is about 3 x 1030 kg.

White Hole A theoretically possible but physically highly unlikely singularity from which matter and energy are able to escape; the antithesis of a black hole.

Wien’s Law An experimentally discovered law applicable to thermal continuum radiation, which states that the wavelength of maximum emission intensity is inversely proportional to the absolute temperature.

Winter Solstice The moment when the Sun reaches its greatest distance south of the celestial equator, on or about December 22. In the Northern Hemisphere this marks the first day of winter; in the Southern Hemisphere it marks the first day of summer.

Wm-2 (or W/m2 ) Watts per square meter of incident signal flux per whatever resolution bandwidth is in use. It is the total signal flux if the receiving bandwidth equals or is greater than the bandwidth of the signal.

Wm-2 Hz-1 (or W/m2 Hz) Spectral flux density (see Jansky).

Wolf-Rayet Star A type of O star whose spectrum shows very broad emission lines.

Worm Drive Probably the most common drive on equatorial mounts. It consists of a spirally cut cylinder (the "worm") which rotates longitudinally such that its thread engage with the specially shaped teeth on the circumference of a disc (the "worm wheel"), which in turn drives the shaft of the mount.

W Virginis Star A type of II Cepheid, one of the fainter class of Cepheid variable stars characteristic of Cepheids in globular clusters.

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X-Power Identifies the magnifying power of a lens or mirror. For example, a 50-power telescope makes the image 50 times larger than it is when viewed without the telescope.

X Radiation Electromagnetic radiation in the range of approximately 0.05-100 A.

X-Ray Electromagnetic radiation of a very short wavelength and very high-energy. X-rays have shorter wavelengths than ultraviolet light but longer wavelengths than cosmic rays. A photon of electromagnetic radiation in the wavelength interval between about 1 Angstrom and 100 Angstroms.

X-Ray Astronomy The field of astronomy that studies celestial objects by the x-rays the emit.

X-Ray Sources Celestial objects that give off X-rays. These exotic objects are producing very energetic radiation and include black holes, neutron stars (pulsars), supernovae remnants, and the centres of galaxies.

X-Ray Telescope A special telescope used to detect X-rays - high-energy electromagnetic radiation. The high energy of X-rays means they will go through rather than bounce off a “normal” telescope mirror. Instead, the mirrors are arranged so the X-rays skip across them much like a stone skips across the surface of a lake.

X-Ray Star A bright celestial object that gives off x-rays as a major portion of its radiation.


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Yarkovsky Effect This is an effect that can change the movement or rotation of tiny rocky particles and small asteroids in space, caused by the absorption and reemission of photons of light. The result is that asteroids can be slightly perturbed in their orbits by this effect.

Year The period of revolution of a planet around its central star; more particularly, the earth's period of revolution around the sun.

Year, anomalistic The period for successive perihelion passages of the Earth, a little less than 5 minutes longer than the sidereal year.

Year, calendar The mean length of the year according to the , 365 days, 5 hours, 49 minutes and 12 seconds.

Year, sidereal The period taken by the Earth to complete one orbit of the Sun, 365.26 days.

Year, tropical The period taken for successive passages of the Sun across the Vernal Equinox, 365.24 days.

Yellow Dwarf An ordinary star such as the Sun at a stable point in its evolution.

Yoke Mounting A form of equatorial mounting in which the polar axis consists of a yoke in which the telescope tube is mounted.


Zeeman Effect The broadening or splitting of spectral lines caused by the presence of a (strong) magnetic field in the gas where the lines are formed.

Zenith The point on the celestial sphere that is directly above the observer. Holding a balloon overhead places the balloon at your zenith. Although celestial objects appear to rise and set as they move across the sky, they rarely reach the zenith point.

Zenith Distance The angular distance of an object from the zenith. The zenith distance of an object is the complement of its altitude.

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Zenith Hour Rate (Zenithal) This is a number for the current or estimated activity of a meteor stream: the number is the hypothetical number of meteors in the sky for an observer with the radiant in the zenith and a limiting magnitude of 6.5

Zero-Age Main Sequence The main sequence in the H-R diagram formed by stars that have just begun their hydrogen-burning life- times, and have not yet converted any significant fraction of their core mass into helium. The zero-age main sequence forms the lower left boundary of the broader band representing the general main sequence.

Zircon A silicate mineral, ZrSiO4, diamond like in appearance, that contains silica silicon dioxide, and the element zirconium. Some zircons in meteorites contain material from the period before the Solar System formed.

Zodiac An imaginary belt circling the celestial sphere along the ecliptic, broad enough to encompass the paths of all the planets visible to the naked eye

Zodiacal Light A faint cone of light that can sometimes be seen above the horizon after sunset or before sunrise. is caused by sunlight reflecting off small particles of material in the plane of the solar system.

Zone A definite area on the mirror, measured from the centre.

Zoom An eyepiece design whereby you can vary the focal length by twisting the barrel of the eyepiece.

Zulu Another name for Universal Time, or GMT

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Astronomy Acronyms & Abbreviations

Acronym Description Δf Offset frequency (Delta frequency) μ micro- μC-SiC microcrystalline silicon carbide μm micrometre; micron α-Si:H hydrogenated amorphous silicon β-BaB2O4 beta barium borate +D Positive-Dispersion -D Negative-Dispersion 1D One Dimensional 2.5G Enhanced Second Generation 2A2OA 2-aminoadipate: 2-oxoglutarate aminotransferase 2D or 2-D Two-Dimensional 2D-BPM Two-Dimensional Beam Propagation Method 2G Second Generation 2P-OBIC Two-Photon Optical Beam-Induced Current 3D BDS Three-Dimensional Beam Delivery System 3D OCT Three-Dimensional Optical Coherence 3D or 3-D Three-Dimensional 3DTV Three-Dimensional Television 3D-BPM Three-Dimensional Beam Propagation Method 3D-FISH Three-Dimensional Fluorescence In Situ Hybridization 3D-MID Three-Dimensional Moulded Interconnected Device 3D-STORM Three-Dimensional Stochastic Optical Reconstruction Microscopy 3G Third Generation 3PLSM Three-Photon-Excitation Laser-Scanning Fluorescence Microscopy 3R Re-amplify, Re-shape, And Re-time 3T Three-Transistor 4G Fourth Generation 4GLS Fourth-Generation Light Source 4HG Fourth Harmonic Generation 5-ALA 5-aminolevulinic acid 5G Fifth Generation 5HG Fifth Harmonic Generation 4 October 2012 150 © Paul Thomas

AC Astrochat (.com Forums) ADU Analog Digital Unit AGN Active Galaxy Nucleus Alt Altitude AO or A/O Adaptive Optics AP Astro Photography Apo Apochromatic Refractor APOD Astronomy Picture Of The Day AR Anti-Reflection (Coated Surface) ASCOM Astronomy Common Object Model ATM Amateur Telescope Maker AU Astronomical Unit Az Azimuth Bins Binoculars BRB Be Right Back c Constant for the Speed of Light CA Chromatic Aberration CCD Charge Coupled Device CETI Communication With Extra-Terrestrial Intelligence CXO Chandra X-ray Observatory. CHZ Continuously Habitable Zone CLS City Light Suppression (type of filter) CMBR Cosmic Microwave Background Radiation CME Coronal Massive Ejection CMOS Complementary Metal–Oxide–Semiconductor CNO Carbon, Nitrogen, Oxygen Dec Declination DN Diffuse nebulae Dob Dobsonian Telescope DSI Deep Sky Imager (Meade Product) DSLR Digital Single Lens Reflex (Type Of Camera) DSO Deep Sky Object DSS Deep Sky Stacker (Image Processing Software) DTC Digital Telescope Computer (eg ) ED Extra Low Dispersion EFN Extremely Faint nebulae EFR Emerging Flux Region (On Our Sun)

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EP Eye Piece ER Eye Relief EQ Equatorial EQDIR Equatorial Direct (Software From Shoestring Astronomy) EQMOD GEM Control Software ESA European Space Agency ESO European Southern Observatory ET EXIF Exchangeable Image File (Format) FAQ Frequently Asked Questions FFT Fast Fourier Transform FIR Far Infra-Red FITS Flexible Image Transport System FL Focal Length FLI Finger Lakes Instrumentation Co. FLO First Light Optics (retail Outlet) FMC Fully Multi Coated FN Faint Nebulae FOC Faint Object Camera FOS Faint Object Spectrograph FOV Field Of View FPN Faint Planetary nebulae FSQ Flat Field Super Quadruplet (Takahashi 4 Element Refractors) FTA Field Transition Arch (On Our Sun) FTL Faster Than Light FTP File Transfer Protocol FUV Far Ultra Violet FW Filter Wheel FWHM Full Width, Half Maximum G Gravitational Constant GC Globular Cluster / Gaseous Comets GCVS General Catalogue Of Variable Stars GEM German Equatorial Mount (Aka EQ Mount) GEO Geosynchronous Earth Orbit GEOS Geosynchronous Earth Orbit Satellite GIF Graphics Interchange Format GLOB Globular Cluster

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GLP Green Laser Pointer GMT Greenwich Mean Time GPS Global Positioning System GR General Relativity GRB Gamma Ray Burst GRS Great Red Spot (On Jupiter) GSO Guan Sheng Optical GUT Grand Unified Theory H-R Hertzsprung-Russell H0 The Hubble Constant HA (or Ha) Hydrogen Alpha (First Line Of The Balmer Series - 6563 Angstroms) Hb Hydrogen Beta (Light Spectrum Emission Line) HC Hand Controller HD Henry Draper (DSO Catalogue) HDF Hubble Deep Field HDRI High Dynamic Range Imaging (Or Just HDR) HF High Frequency HR High Resolution / Hertzsprung-Russell HST Hubble Space Telescope HTH Hope That Helps (Forum Abbreviation) Hz Hertz (Frequency In Cycles Per Second) Habitable Zone ? IAU International Astronomical Union IDP Interplanetary Dust Particle IC Index Catalogue (List Of Objects) ICNR In Camera Noise Reduction IR Infra-Red (Light) IIS IceInSpace (Forum) IMO In My Opinion IMHO In My Humble Opinion (Usually Not Being Very Humble At The Time) ISM InterStellar Medium ISS International Space Station JPG or JPEG Joint Photographic Experts Group (Very Common Picture File Format) JPL Jet Propulsion Laboratory KBO Kuiper Belt Object Km Kilometre LAN Local Area Network (Of Computers) Lappy A Laptop Computer

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LB Light Bridge (Meade DOB Design) LBT Large Binocular Telescope LGM Little Green Men LIDAR Laser Imaging Detection and Ranging LMC LMFAO Laugh My Fcuking Arse Off (Forum Abbreviation) LMST Local Mean Sidereal Time LOL Laugh Out Loud (Forum Abbreviation) / Lots Of Love LP Light Pollution LPI Lunar Planetary Imager (Meade Product) LPR Light Pollution Reduction LRGB Luminance Red Green Blue LRGB-C Luminance Red Green Blue Clear LSB Low Surface Brightness (ie, Dim!) LST Local Sidereal Time LV Lanthanum Vixen LVW Lanthanum Vixen Widefield M Messier (DSO Catalogue) Object Prefix MA Modified Achromat / Modern Astronomy (Retail Outlet) Mak Maksutov-Cassegrain Telescope MAS My Astro Shop MASAR Microwave Attenuation by Stimulated Absorption of Radiation MASER Microwave Amplification by Stimulated Emission of Radiation MCT Maksutov-Cassegrain Telescope MIL Mother In Law (A Super Massive Blackhole) MN or MNT Maksutov-Newtonian (Type Of Telescope) MPC Mega Parsec MPCC Multi-Purpose Coma Corrector. MUSA Multichannel Spectrum Analyzer mW Milliwatt, = 0.001 Watt NASA National Aeronautics And Space Administration (U S A) NB Narrow Band NCP North Celestial Pole NEO Near Earth Object (Orbit) NELM Naked Eye Limiting Magnitude Newt Newtonian Telescope (Reflector)

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NGC New General Catalogue (Of DSO’s) NIB New In Box (But Opened) NLV New Lanthanum Vixen (Eyepiece) NOAA National Oceanic And Atmospheric Administration NPB Narrow Pass Band NPL New Plossl (Vixen Eyepiece) OAG Off Axis Guide OC OSC One-Shot Colour (Camera) O-III Oxygen III (Or 3) (Doubly Ionized Oxygen Emission Line) OT Off-Topic (Forum Post) OTA Optical Tube Assembly PAE Pointing Accuracy Enhancement (Feature Of Goto Mount) PAH Polycyclic Aromatic Hydrocarbon PE Periodic Error PEC Periodic Error Correction / Printed Electronic (Component or Circuit) PC Parsec PGC Principal Galaxies Catalogue PHD Push Here Dummy (Astrophotography Software) PI Pixinsight PITA Pain In The Arse PN Planetary Nebula PM Private Message (Between Two Forum Members) PME Paramount ME (Mount By Software Bisque) PMX Paramount MX (Mount By Software Bisque) PN Planetary Nebulae POTH Plain Old Telescope Handset PS Photoshop PST Personal Solar Telescope (Coronado Product) QE Quantum Efficiency (CCD Camera Type) QSO Quasi-Stellar Object RA Right Ascension RACI Right Angle Correct Image RADAR Radio Detection and Ranging RAM Random Access Memory RAW Not An Acronym, It Just Means Raw Imaging Data RC Ritchey-Chretien

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RCOS RC Optical Systems Co. RCT Ritchey-Chretien Telescope RCW Rodgers Campbell Whiteoak Catalogue (Of DSO’s) RDF Red Dot Finder RFT Rich Field Telescope RGB Red Green Blue RGBb Red Green Blue Light Blue RGO Royal Greenwich Observatory RK Rank Kellner (Type Of Eyepiece - Cheap) RN Reflection Nebula ROFL Roll Over Fcuking Laughing (Forum Abbreviation) RVO Rother Valley Optics (Retail Outlet) S2 or S-II Type Of Filter SA Spherical Aberration Smithsonian Astrophysical Observatory SB Surface Brightness SBIG Santa Barbara Instrument Group SC Star Clusters SCP South Celestial Pole SCT Schmidt-Cassegrain Telescope SD Super-Low Dispersion SDM Size Does Matter (Australian Telescope Maker Peter Read's Motto) SEB South Equatorial Belt (On Jupiter) SETI Search For Extra Terrestrial Intelligence SG Spiral Galaxies SGL Stargazerslounge (.com Forum) SII Singly Ionized Sulphur Emission Line SLR Single Lens Reflex (Camera) SMA Super Modified Achromat SMC SN or SNT Schmidt-Newtonian (Type Of Telescope) SNP Starry Night Pro (Astronomy Software) SNR Super Nova Remnant / Signal To Noise Ratio SP Super Plossl SPED Saxon Premium ED SPM Sir Patrick Moore SR Super Ramsden Or Symmetric Ramsden

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SQM Sky Quality Meter (Brightness Reading) SSC Smithsonian Stellar Catalogue (Not A Preferred Term) STP Standard Temperature (0 °C) And Pressure (1 Atm) STV SBIG Television (Camera) SV Super View SWA Super Wide Angle SWMBO She Who Must Be Obeyed (Forum Abbreviation) SXV Starlight Xpress Video (Type Of Camera) TAK Takahashi (Manufacturer) TBD To Be Decided / Too Bloody Difficult TBH To Be Honest (Forum Abbreviation) TEC Telescope Engineering Company TFH Too Fcuking Hard TFOV True Field Of View TH Telescope House (.com) TLA Three Letter Acronym (Like This) TLAO Turn Left At Orion (Book) TMB Thomas M. Back (Of TMB Optical Co) TNO Trans-Neptunian Object TOA Triplet Orthoscopic Apochromat (Takahashi Design) TSA Triplet Super Apochromat (Takahashi Design) UFO Unidentified Flying Object UHC Ultra High Contrast UHF UO University Optics USB Universal Serial Bus (Computer Data Transport) UT Universal Time UTA Upper Tube Assembly (Of Truss Dobs) UTC Co-ordinated Universal Time UV Ultra Violet (Light) UVBY system using 3 filters in the UV, Blue, and Green-Yellow UWA Ultra Wide Angle VHF Very High Frequency VLA Very Large Array VLBI Very-Long-Baseline Interferometry VLT Very Large Telescope WDS Washington Double-Star Catalogue

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WO William Optics Z Zulu (GMT / UT) ZAMS Zero-Age Main Sequence ZLM Zenith Limiting Magnitude ZHR Zenith Hour(ly) Rate

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Spectral Star Classes

The Harvard classification system is a one-dimensional classification scheme. The classes are appropriate only for stars on the main sequence portion of their , and so are not appropriate for red giants etc. The spectral classes O through M are subdivided by Arabic numerals (0–9). For example, A0 denotes the hottest stars in the A class, and A9 denotes the coolest ones. The Sun is classified as a G2.

Surface Mass Radius Hydrogen Fraction of all Class temperature Conventional colour Apparent colour (solar (solar radii) lines main-sequence stars (kelvin) masses)

O ≥ 33,000 blue blue ≥ 16 ≥ 6.6 Weak ~0.00003%

B 10,000–33,000 white to blue white blue white 2.1–16 1.8–6.6 Medium 0.13%

A 7,500–10,000 white white to blue white 1.4–2.1 1.4–1.8 Strong 0.6%

F 6,000–7,500 yellowish white white 1.04–1.4 1.15–1.4 Medium 3%

G 5,200–6,000 yellow yellowish white 0.8–1.04 0.96–1.15 Weak 7.6%

K 3,700–5,200 orange yellow orange 0.45–0.8 0.7–0.96 Very weak 12.1%

M 2,000–3,700 red orange red ≤ 0.45 ≤ 0.7 Very weak 76.45%

L 1,300–2,000 purple-red red Unknown Unknown Extremely weak ≥ 100.00%

T 700-1,300 brown purple-red Unknown Unknown Extremely weak ≥ 100.00%

Y <700 dark brown brown Unknown Unknown Extremely weak ≥ 100.00%

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Stellar Evolutionary Paths

4 October 2012 160 © Paul Thomas

The Solar System

Sun (Sol) ☉ The star at the centre of our solar system. An average star in terms of size and mass, the Sun is a yellow dwarf of spectral type G2. It is about 5 billion years old, contains 2x1030 kilograms of material, and has a diameter more than 100 times that of Earth. Mercury ☿ The closest planet to the Sun. The temperature range on Mercury’s surface is the most extreme in the solar system, ranging from about 400° C (750° F) during the day to about –200° C (–300° F) at night. Mercury, which looks like Earth’s moon, has virtually no atmosphere, no moons, and no water. Venus ♀ An inner, terrestrial (rocky) planet that is slightly smaller than Earth. Located between the orbits of Mercury and Earth, Venus has a very thick atmosphere that is covered by a layer of clouds that produce a “greenhouse effect” on the planet. Venus’s surface temperature is roughly 480° C (900° F), making it the hottest planet in the solar system. Venus has no moons. Earth ♁ ⊕ The third planet from the Sun and one of four terrestrial planets in the inner solar system. Earth, the only planet where water exists in large quantities, has an atmosphere capable of supporting myriad life forms. The planet is 150 million kilometres (93 million miles) away from the Sun. Earth has one satellite — the Moon. Moon ☽ A large body orbiting a planet. On Earth’s only moon, scientists have not detected life, water, or oxygen on this heavily cratered body. The Moon orbits our planet in about 28 days. Mars ♂ The fourth planet in the solar system and the last member of the hard, rocky planets (the inner or terrestrial planets) that orbit close to the Sun. The planet has a thin atmosphere, volcanoes, and numerous valleys. Mars has two moons: and .

Asteroid Belt A belt of small pieces of rocks and metals that orbit the sun. Possibly from the remains of a planet that was destroyed, or material left over from a planet that was never formed. The numbers change all the time as new ones are made from bits broken off during collisions, and some are lost, as their orbit is changed due to the collisions, and head off into space. See the section (Asteroids from the main belt) for a breakdown of the largest of them. 4 October 2012 161 © Paul Thomas

Jupiter ♃ The fifth planet from the Sun and the largest planet in our solar system, twice as massive as all the other planets combined. Jupiter is a gaseous planet with a very faint . Four large moons and numerous smaller moons orbit the planet. Jupiter is more than five times the Earth’s distance from the Sun. It completes an orbit around the Sun in about 12 Earth years. Saturn ♄ The sixth planet in the solar system, noted for its obvious ring structure. Saturn is almost ten times the Earth’s distance from the Sun. The planet completes a circuit around the Sun in about 30 Earth years. Saturn is the second largest and the least dense planet in our solar system. The planet has more than 21 moons, including , the second largest known moon in our solar system. Uranus ♅ The third largest planet in the solar system and the seventh from the Sun. Uranus is 19 times the Earth’s distance from the Sun and completes a circuit around the Sun in about 84 Earth years. This gaseous, giant outer planet has a visible ring system and over 20 moons, the largest of which is . Uranus is tipped on its side, with a rotation axis in nearly the same plane as its orbit. Neptune ♆ The eighth planet and the most distant giant gaseous planet in our solar system. The planet is 30 times the Earth’s distance from the Sun, and each orbit takes 165 Earth years. Neptune is the fourth largest planet and has at least eight moons, the largest of which is . Neptune has a ring system, just like all the giant gaseous outer planets. Pluto ♇ n A dwarf planet whose small size and composition of ice and rock resembles the comets in the Kuiper Belt, a region beyond Neptune’s orbit where Pluto resides. Pluto’s orbit is more elliptical than those of the eight solar system planets. Pluto was considered the ninth planet until August 2006, when the International Astronomical Union reclassified it as a dwarf planet. It is now referred to as a Plutoid. It has five satellites. Kuiper Belt Is a region of the Solar System beyond the planets extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger - 20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies, or remnants from the Solar System's formation. While most asteroids are composed primarily of rock and metal, Kuiper belt objects are composed largely of frozen volatiles (termed "ices"), such as methane, ammonia and water. Currently, it is estimated that there are over 100,000 objects over 100km in diameter. See the section (Asteroids from the Kuiper belt) for a breakdown of the largest of them.

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Planetary Information

Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto (Our Moon)

Radius (km) 2,439 6,052 6,378 3,396 71,492 60,268 25,559 24,764 1,153 1,738.00

Mass (Earth=1) 0.0558 0.8150 1.0000 0.1074 317.8300 95.1590 14.5000 17.1470 0.0020 0.0123

Density (g/cm3) 5.43 5.24 5.52 3.94 1.33 0.69 1.30 1.64 2.03 3.3464

Escape Velocity 4.30 10.40 11.20 5.00 59.50 35.60 21.22 23.60 1.10 2.38 (km/s)

Gravity (m/s2) 3.70 8.87 9.78 3.71 24.79 10.44 8.69 11.15 0.66 1.622

Rotation (hrs) 1047.6 - 5832.5 23.93 24.62 9.93 10.66 - 17.24 16.11 - 153.29 655.73

Year (Earth=1) 0.24085 0.62 1 1.88 11.86 29.46 84.32 164.8 248.09

Orbit (days) 87.969 224.698 365.256 686.971 4,332.590 10,759.220 30,799.095 60,190.030 90,613.305 27.321

Min Distance 46,001,200 107,477,000 147,098,290 206,669,000 740,573,600 1,353,572,956 2,748,938,461 4,452,940,833 4,437,000,000 362,570 from Sun (km) Max Distance 69,816,900 108,939,000 152,098,232 249,209,300 816,520,800 1,513,325,783 3,004,419,704 4,553,946,490 7,311,000,000 405,410 from Sun (km)

Axial Tilt (deg) 2.11 177.30 23.45 25.19 3.13 26.73 97.77 28.32 119.59 6.68

Inclination to 7.00 3.39 1.85 1.30 2.49 0.77 1.77 17.14 ecliptic (deg)

Albedo 0.142 0.670 0.367 0.170 0.520 0.470 0.510 0.410 0.500 0.136

Surface Temp 80/700 735 184/331 130/308 165 134 49/76 72 33/55 70/390 Min/Max (°K)

Natural Satellites 0 0 1 2 61 ? 31 ? 25 ? 13 ? 1 -

H, He, CH4, H, He, CH4, Atmosphere CO2 N, CO2, H2O N, O, CO2, H2O N, CO2, H2O H, He, CH4 H, He, CH4 CH4 ? Ar, He, Na NH3 NH3

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Asteroids from the Main Belt

Name Symbol Dia (km) Date Discovered Class

1 Ceres 952 1 January 1801 G 544 28 March 1802 B 525 29 March 1807 V

10 Hygiea 431 12 April 1849 C 326 2 October 1910 F

52 Europa 301 4 February 1858 C

511 Davida 289 30 May 1903 C

87 Sylvia 286 16 May 1866 X

65 Cybele 273 8 March 1861 C

15 Eunomia 268 29 July 1851 S 258 1 September 1804 S 256 1 September 1854 C

624 Hektor 241 10 February 1854 D

88 Thisbe 232 15 June 1866 B

324 Bamberga 229 25 February 1892 C

451 Patientia 225 4 December 1899

532 Herculina 222 20 April 1904 S

48 Doris 222 19 September 1857 C

375 Ursula 216 18 September 1893

107 Camilla 215 17 November 1868 C

45 Eugenia 213 27 June 1857 F

7 Iris 213 13 August 1847 S

29 Amphitrite 212 1 March 1854 S 209 7 December 1896 C

19 Fortuna 208 22 August 1852 G

13 Egeria 206 2 November 1850 G

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24 Themis 198 5 April 1853 C

94 Aurora 197 6 September 1867 C

702 Alauda 195 16 July 1910

121 Hermione 190 12 May 1872 C

372 Palma 189 19 August 1893

128 Nemesis 188 25 November 1872 C

6 Hebe 186 1 July 1847 S

16 Psyche 186 17 March 1852 M 174 10 April 1872 C

41 Daphne 174 22 May 1856 C

9 Metis 174 25 April 1848 S 153 11 May 1850 S 152 19 May 1851 S

18 Melpomene 141 24 June 1852 S 128 18 October 1847 S 120 1 March 1854 S 5 Astraea 119 8 December 1845 S 113 13 September 1850 S 108 5 October 1855 S 103 19 April 1855 C 95 5 May 1853 S 90 17 April 1852 S

Does not include any natural satellites of the above objects, which in themselves, are candidates for this list.

NB. Where there is more than one symbol after the object, the first one is usually the oldest version, and the last one, is the latest, more common, or usually accepted one.

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Asteroids from the Kuiper Belt

Name Symbol Dia (km) Date Discovered Class Neptune Triton Captured Moon 2700 10 October 1846 Moon

136199 Eris ¥ ¦ ² ³ È 2330 5 January 2005 SDO Pluto Pluto o n 2330 18 February 1930 Plutino 136108 Haumea À Á 1500 28 December 2004 Plutino 136472 Makemake ¼ ½ 1440 31 March 2005 Cubewano 225088 2007 OR 10 1420 17 July 2007 SDO r [ ] ´ 1000 14 November 2003 84522 2002 TC 302 1150 9 October 2002 SDO 50000 Quaoar ( ` µ 980 5 June 2002 Cubewano d I 850 17 February 2004 Plutino 208966 2003 AZ 84 730 13 January 2003 Plutino 307261 2002 MS 4 730 18 June 2002 Cubewano 55565 2002 AW 197 730 10 January 2002 Cubewano 55637 2002 UX 25 680 30 October 2002 Cubewano c E F G \ · 650 22 May 2001 Plutino 613 22 September 2004 Cubewano = º 528 19 November 1998 Cubewano # _ 500 28 November 2000 Cubewano

Does not include any natural satellites of the above objects, which in themselves, could be candidates for this list.

NB. Where there is more than one symbol after the object, the first one is usually the oldest version, and the last one, is the latest, more common, or usually accepted one.

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Modern Constellations

Constellation Sign Pronunciation Abbrv Meaning Origin

Andromeda an-DROM-eh-da And The Chained Maiden ancient (Ptolemy) Antlia ANT-lee-uh Ant The Air Pump 1763, Lacaille Apus APE-us Aps The Bird of Paradise 1603, , Keyser & de Houtman h ack-KWAIR-ee-us Aqr The Waterbearer ancient (Ptolemy) Aquila ack-WILL-lah Aql The Eagle ancient (Ptolemy) Ara AY-rah Ara The Altar ancient (Ptolemy) Aries ^ AIR-ease Ari The Ram ancient (Ptolemy) or-EYE-gah Aur The Charioteer ancient (Ptolemy) Bootes bow-OH-tease Boo The Herdsman ancient (Ptolemy) Caelum SEE-lum Cae The Chisel 1763, Lacaille ca-MEL-oh-PAR-dal-iss Cam The Giraffe 1613, Plancius[5] Cancer a KAN-surr Cnc The Crab ancient (Ptolemy) KAN-es veh-NAT-ih-see CVn The Hunting Dogs 1690, Firmamentum Sobiescianum, Hevelius Canis Major KANE-es MAY-jer Cma The Great Dog ancient (Ptolemy) Canis Minor KANE-es MY-ner Cmi The Little Dog ancient (Ptolemy) Capricornus g CAP-rih-CORN-us Cap The Sea Goat ancient (Ptolemy) Carina car-EE-na Car The Keel 1763, Lacaille, split from Argo Navis Cassiopeia KASS-ee-oh-PEE-ah Cas The Queen ancient (Ptolemy) Centaurus sen-TOR-us Cen The Centaur ancient (Ptolemy) SEE-fee-us Cep The King ancient (Ptolemy) SEE-tus Cet The Sea Monster ancient (Ptolemy) Chameleon kah-ME-lee-un Cha The Chameleon 1603, Uranometria, Keyser & de Houtman Circinus SIR-sin-us Cir The Drawing Compass 1763, Lacaille Columba ko-LUM-ba Col The Dove 1592, Plancius, split from Canis Major CO-ma bare-uh-NYE-sees Com Berenice’s Hair 1603, Uranometria, split from Leo Corona Australis coe-ROW--nah ow-STRAHL-iss CrA The Southern Crown ancient (Ptolemy) coe-ROW--nah BOR-ee-AL-iss CrB The Northern Crown ancient (Ptolemy) Corvus CORE-vuss Crv The Raven ancient (Ptolemy) 4 October 2012 167 © Paul Thomas

Crater CRAY-ter Crt The Cup ancient (Ptolemy) Crux Kruks Cru The Southern Cross 1603, Uranometria, split from Centaurus Cygnus SIG-nus Cyg The Swan ancient (Ptolemy) Delphinus del-FYE-nus Del The Dolphin ancient (Ptolemy) Dorado doh-RAY-doh Dor The Goldfish 1603, Uranometria, Keyser & de Houtman DRAY-ko Dra The Dragon ancient (Ptolemy) Equuleus eh-KWOO-lee-us Equ The Little Horse ancient (Ptolemy) Eridanus eh-RID-uh-nuss Eri The Celestial River ancient (Ptolemy) Fornax FOR-naks For The Furnace 1763, Lacaille Gemini ` GEM-in-eye Gem The Twins ancient (Ptolemy) Grus GROOS Gru The Crane 1603, Uranometria, Keyser & de Houtman HER-kyou-leez Her The Strongman ancient (Ptolemy) Horologium hor-uh-LOW-gee-um Hor The Clock 1763, Lacaille HY-druh Hya The Sea Serpent ancient (Ptolemy) Hydrus HY-drus Hyi The Water Snake 1603, Uranometria, Keyser & de Houtman Indus IN-dus Ind The Indian 1603, Uranometria, Keyser & de Houtman Lacerta la-SIR-ta Lac The Lizard 1690, Firmamentum Sobiescianum, Hevelius Leo b LEE-oh Leo The Lion ancient (Ptolemy) LEE-oh MY-ner Lmi The Little Lion 1690, Firmamentum Sobiescianum, Hevelius Lepus LEE-puss Lep The Hare ancient (Ptolemy) Libra d LEE-bra Lib The Scales ancient (Ptolemy) Lupus (ex Therion) LOUP-us Lup The Wolf ancient (Ptolemy) links Lyn The Lynx 1690, Firmamentum Sobiescianum, Hevelius LIE-rah Lyr The Lyre ancient (Ptolemy) Mensa MEN-sa Men The Table Mountain 1763, Lacaille MY-krow-SKOH-pee-em Mic The Microscope 1763, Lacaille Monoceros mon-OSS-er-us Mon The Unicorn 1613, Plancius Musca MUSS-ka Mus The Fly 1603, Uranometria, Keyser & de Houtman Norma NOR-ma Nor The Level 1763, Lacaille Octans OCK-tens Oct The Octant 1763, Lacaille Oaf-ih-YOU-kus Oph The Serpent Bearer ancient (Ptolemy) Orion oh-RYE-un Ori The Hunter ancient (Ptolemy)

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Pavo PAY-vo Pav The Peacock 1603, Uranometria, Keyser & de Houtman Pegasus PEG-uh-suss Peg The Winged Horse ancient (Ptolemy) PURR-see-u Per The Hero ancient (Ptolemy) Phoenix FEE- Phe The Phoenix 1603, Uranometria, Keyser & de Houtman Pictor PICK-tor Pic The Painter 1763, Lacaille i PIE-sees Psc The Fish ancient (Ptolemy) Piscis Austrinus PIE-sees oss-TREE-nus PsA The Southern Fish ancient (Ptolemy) Puppis PUP-iss Pup The Poop Deck 1763, Lacaille, split from Argo Navis PICK-sis Pyx The Compass 1763, Lacaille Reticulum reh-TICK-yuh-lum Ret The Net 1763, Lacaille Sagitta suh-JIT-uh Sge The Arrow ancient (Ptolemy) Sagittarius f sa-jih-TARE-ee-us Sgr The Archer ancient (Ptolemy) Scorpius e SKOR-pee-uss Sco The Scorpion ancient (Ptolemy) Sculptor SKULP-tor Scl The Sculptor 1763, Lacaille Scutum SCOOT-um Sct The Shield 1690, Firmamentum Sobiescianum, Hevelius Caput SIR-pens CAP-ut Ser The Snake’s Head ancient (Ptolemy) Serpens Cauda SIR-pens KAW-dah Ser The Snake’s Tail ancient (Ptolemy) SEX-tens Sex The Sextant 1690, Firmamentum Sobiescianum, Hevelius _ TOR-us Tau The Bull ancient (Ptolemy) Telescopium tell-es-SCOPE-ee-um Tel The Telescope 1763, Lacaille Triangulum tri-ANG-yuh-lum Tri The Triangle ancient (Ptolemy) tri-ANG-yuh-lum aus-TRAY-lee TrA The Southern Triangle 1603 Uranometria, Keyser & de Houtman Tucana too-KAY-nah Tuc The Toucan 1603 Uranometria, Keyser & de Houtman Ursa Major URR-sah MAY-jer UMa The Great Bear ancient (Ptolemy) Ursa Minor URR-sah MY-ner UMi The Little Bear ancient (Ptolemy) Vela VEE-la Vel The Sail 1763, Lacaille, split from Argo Navis Virgo c VER-go Vir The Maiden ancient (Ptolemy) Volans VO-lans Vol The Flying Fish 1603, Uranometria, Keyser & de Houtman Vulpecula vul-PECK-yoo-la Vul The Fox 1690, Firmamentum Sobiescianum, Hevelius

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The Greek Alphabet

Lower Upper Name Case Case α alpha Α β beta Β γ gamma Γ δ delta Δ ε epsilon Ε ζ zeta Ζ η eta Η ϑ theta Θ ι iota Ι κ kappa Κ λ lambda Λ μ mu Μ ν nu Ν ξ xi Ξ ο omicron Ο π pi Π ρ rho Ρ σ sigma Σ τ tau Τ υ upsilon Υ ϕ phi Φ χ chi Χ ψ psi Ψ ω omega Ω

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Meteor Showers

Speed Name Dates Peak RA Dec ZHR Parent Object Notes (km/s) Antihelion Source 10 Dec - 10 Sep multiple varies varies 30 4 Faint.

Quadrantids 28 Dec – 12 Jan 4 Jan 15 +49 41 120 Comet C/1490Y1 & C/1385U1 Blue & yellow, med speed. Radiant in Bootes. Capri.-Sagitt. 13 Jan - 29 Feb 2 Feb 21 -23 29 7

Alpha Centaurids 28 Jan – 21 Feb 8 Feb 14 -59 56 6 Bright. Theta Centaurids 23 Jan – 12 Mar 10 Feb 15 -44 66 3

Gamma Normids 25 Feb – 22 Mar 14 Mar 16 -50 56 6 Bright. Kappa Serpentids 1 Apr - 7 Apr 5 Apr 15 +18 46 4

Virginids 22 Mar – 26 Apr 17 Apr 12 +10 20 5

Lyrids 16 Apr – 25 Apr 22 Apr 18 +34 49 18 Comet Thatcher Swift, bright meteors. 15 Apr – 28 Apr 23 Apr 7 -45 18 var Comet 26P/Grigg-Skjellerup Bright. Every 5 years. Eta 19 Apr - 28 May 5 May 23 -1 66 65 Comet 1P/Halley Bright. Omega Cetids 5 May - 9 Jun 7 May 1 -7 38 8

Eta 3 May - 14 May 8 May 19 +44 43 3 Faint. Daytime May 4 May - 6 Jun 15 May 3 +18 28 4 showers. Try about 5am. Daytime Eps. Arietids 4 May - 6 Jun 15 May 3 +21 23 4 Daylight showers. Try about 5am. Alpha Scorpiids 21 Apr – 26 May 15 May 16 -29 33 3 Low activity from several radiants. South Omega Scorpiids 23 May – 15 Jun 31 May 17 -22 26 5

North Omega Scorpiids 23 May - 15 Jun 31 May 17 -15 23 5

Arietids 22 May – 2 Jul 7 Jun 3 +24 37 54 Comet 96P/Machholz

Daytime Zeta 20 May - 5 Jul 9 Jun 4 +28 27 20

June Lyrids[1] 11 Jun – 21 Jun 16 Jun 19 +44 20 3

South June Aquilids 9 Jun - 2 Jul 16 Jun 19 -5 39 3

Pi Cetids 16 Jun – 4 Jul 26 Jun 2 -12 68 4

June Bootids 22 Jun – 2 Jul 27 Jun 15 +48 18 var Comet 7P/Pons-Winnecke Bright. Unpredictable. Periodic. Daytime Beta 5 Jun - 17 Jul 28 Jun 6 +24 31 10

Tau Aquariids 27 Jun – 6 Jul 28 Jun 23 -12 66 7

July 24 Jun – 18 Jul 12 Jul 2 -48 48 4

North Delta Aquariids 15 Jul - 25 Aug 26 Jul 23 0 42 4 Comet 96P/Machholz

Piscis Austrinids 15 Jul – 10 Aug 27 Jul 23 -30 35 5 Faint.

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Southern Delta Aquariids 12 Jul – 23 Aug 29 Jul 23 -16 41 16 Faint. Beta Cassiopeids 3 Jul - 19 Aug 29 Jul 24 59 52 10

Alpha Capricornids 3 Jul – 5 Aug 29 Jul 20 -10 23 5 Comet 169P/NEAT Slow, bright, yellow meteors. Fireballs seen. Eta Eridanids 3 Aug – 14 Aug 9 Aug 3 -13 65 6

Perseids 17 Jul – 24 Aug 12 Aug 3 +58 59 100 Comet 109P/Swift-Tuttle Bright. 3 Aug – 25 Aug 17 Aug 19 +59 25 3 M Planet 2008 ED69 Faint. Gamma Doradids 27 Aug – 3 Sep 28 Aug 4 -50 42 5

Alpha 28 Aug – 5 Sep 31 Aug 6 +39 66 6 Comet C/1911N1 (Kiess) Bright. September Epsilon Perseids 5 Sep – 21 Sep 9 Sep 3 +40 64 5 Faint. Daytime Sextantids 26 Sep - 3 Oct 1 Oct 10 -2 33 20

Draconids 6 Oct – 10 Oct 8 Oct 17 +54 20 var Comet 21P/Giacobini-Zinner Medium. Very feeble shower. Periodic Southern Taurids 10 Sep – 20 Nov 10 Oct 2 +9 27 5 Bright. Delta Aurigids 10 Oct – 18 Oct 11 Oct 6 +44 64 2 Faint. Epsilon 14 Oct – 27 Oct 18 Oct 7 +27 70 3 Faint. Orionids 2 Oct – 7 Nov 21 Oct 6 +16 66 25 Comet 1P/Halley Bright. Fast meteors with fine, persistent trains. Leo Minorids 19 Oct – 27 Oct 24 Oct 11 +37 62 2 Faint.

Northern Taurids 20 Oct – 10 Dec 12 Nov 4 +22 29 5 M Planet 2004 TG10 Bright. Nov. Iota Aurigids 1 Nov – 23 Nov 15 Nov 5 +33 36 8

Leonids 6 Nov – 30 Nov 17 Nov 10 +22 71 15 Comet 55P/Tempel-Tuttle Bright. Alpha 15 Nov – 25 Nov 21 Nov 8 +1 65 var Unknown Bright. Periodic

Phoenicids 28 Nov – 9 Dec 6 Dec 1 -53 18 var Comet D/1819W1 (Blanpain) Medium. Periodic Puppid-Velids 1 Dec – 15 Dec multiple 8 -45 40 10 Medium. Monocerotids 27 Nov – 17 Dec 8 Dec 7 +8 42 2 Faint Sigma Hydrids 3 Dec – 15 Dec 11 Dec 8 +2 58 3 Faint. Geminids 7 Dec – 17 Dec 13 Dec 7 +33 35 120 M Planet Rich shower of medium bright/speed meteors. Comae Berenicids 12 Dec – 23 Dec 15 Dec 12 +18 65 3 Faint. Dec. 5 Dec - 4 Feb 19 Dec 11 +30 64 5 Faint. 17 Dec – 26 Dec 23 Dec 14 +76 33 10 Comet 8P/Tuttle Faint. Weak shower.

Dates will change slightly each year. Up to date info on the web !

Showers in Orange = Views from the Southern Hemisphere.

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Significant People in Astronomy

Brahe, Tycho (1546-1601) A Danish astronomer (also known as Tyge Ottesen) whose accurate astronomical observations of Mars in the last quarter of the 16th century formed the basis for Johannes Kepler’s laws of planetary motion. Brahe lost his nose in a dual at Rostock in 1566 with Manderup Parsberg (a fellow student and nobleman) over a dispute as to who was the better mathematician, and for the rest of his life is said to have worn a metal replacement. In 1601, rather than commit the social faux pas of leaving a banquet before it concluded, he stayed put until his bladder ruptured. He died several days later, not from his urinary mishap, however, but rather from the high levels of mercury in his , a result of the medicine he took after falling ill.

Copernicus, Nicolaus (1473-1543) A Polish astronomer who advanced the theory that Earth and the other planets revolve around the Sun—the heliocentric theory. This was highly controversial at the time, since the prevailing Ptolemaic model held that the Earth was at the centre of the universe, and all objects, including the Sun, circled it. The Ptolemaic model had been widely accepted in Europe for 1000 years when Copernicus proposed his alternative. Although, it should be noted, that the heliocentric idea was first put forth by Aristarcus of Samos in the third century B.C., a fact known to Copernicus but long ignored by others prior to him.

Einstein, Albert (1879-1955) The German-American physicist who developed the Special and General Theories of Relativity that—along with quantum mechanics—are the foundation of modern physics.

Galilei, Galileo (1564-1642) An Italian scientist renowned for his contributions to physics, astronomy, and scientific philosophy. He is regarded as the chief founder of modern science. He developed the telescope, with which he found craters on the Moon and discovered the largest . Galileo was condemned by the Catholic Church for his view of the cosmos, which was based on the heliocentric theory of Copernicus.

Halley, Edmond (1656–1742) Halley was born in Haggerston, Shoreditch, England. His father, Edmond Halley Sr., came from a Derbyshire family and was a wealthy soap-maker in London. He studied at St Paul's School, and then, from 1673, at The Queen's College, Oxford. While an undergraduate, Halley published papers on the Solar System and sunspots. Halley stated his belief that the comet sightings of 1456, 1531, 1607, and 1682 related to the same comet, which he predicted would return in 1758. He did not live to witness the

4 October 2012 173 © Paul Thomas

comet's return, but when it did, the comet became generally known as Halley's Comet. Halley held many positions of authority, and finally succeeded in 1720 as Astronomer Royal, a position Halley held until his death.

Herschel, William (1738-1822) Sir William Herschel was a renowned astronomer who first detected the infrared region of the electromagnetic spectrum in 1800.

Hertz, Heinrich (1857-1894) A German physics professor who performed the first experiments that generated and received electromagnetic waves, in particular radio waves. In his honour, the unit with which we measure the frequency of these waves is called a hertz.

Hubble, Edwin P. (1889-1953) An American astronomer whose observations proved that galaxies are what he called “island ” outside our galaxy and not nebulae within our galaxy. His greatest discovery, called Hubble’s Law, was the linear relationship between a galaxy’s distance and the speed with which it is moving. The Hubble Space Telescope is named in his honour.

Huygens, Christiaan (1629-1695) A Dutch physicist who was the leading proponent of the wave theory of light. He also made important contributions to mechanics, stating that in a collision between bodies, neither loses nor gains “motion” (his term for momentum). In astronomy, he discovered Titan (Saturn’s largest moon) and was the first to correctly identify the observed elongation of Saturn as the presence of Saturn’s rings.

Kepler, Johannes (1571-1630) A German astronomer and mathematician. Considered a founder of modern astronomy, he formulated three famous laws of planetary motion that comprised a quantitative formulation of Copernicus’s theory that the planets revolve around the Sun.

Messier, Charles (1730-1817) The 18th-century French astronomer who compiled a list of 110 fuzzy, diffuse objects that appeared at fixed positions in the sky. Being a comet-hunter, Messier compiled this list of objects which he knew were not comets. His list is now well known to professional and amateur astronomers as containing the brightest and most striking nebulae, star clusters, and galaxies in the sky.

Moore, Patrick (1923 - ) Sir Patrick Alfred Caldwell-Moore, CBE, FRS, FRAS is a British amateur astronomer who has attained prominent status in astronomy as a writer, researcher, radio commentator and television presenter. He is a former president of the British Astronomical Association, co-founder and former president of the Society for Popular Astronomy (SPA), author of over 70 books on astronomy, and presenter of the world's longest-running television series with the same original presenter, The Sky at Night

4 October 2012 174 © Paul Thomas

on the BBC. As an amateur astronomer, he became known as a specialist on observing the Moon and creating the Caldwell catalogue. In 1982, asteroid 2602 Moore, was named in his honour.

Newton, Isaac (1642-1727) The English cleric and scientist who discovered the classical laws of motion and gravity. The bit with the apple is probably apocryphal.

Ptolemy (circa 100 to circa 170 AD) Also known as Claudius Ptolemaeus. Ptolemy believed the planets and the Sun orbited the Earth in the order Mercury, Venus, the Sun, Mars, Jupiter, Saturn. This system became known as the Ptolemaic system and predicted the positions of the planets accurately enough for naked-eye observations (although it also made some ridiculous predictions, such as the distance to the moon should vary by a factor of two over its orbit). He authored a book called Mathematical Syntaxis (widely known as the Almagest). The Almagest included a star catalogue containing 48 constellations, using names we still use today.

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SI Units and Specifics

Prefix Symbol 10n Decimal Short scale Long scale Googolplex - 10googol 1010 100 Googolplex Googol - 10100 1 with 100 zeros Ten Duotrigintillion Ten Sexdecilliard --- Universe - 9.3x1026 930,000,000,000,000,000,000,000,000 The probable size of the universe. Yotta Y 1024 1,000,000,000,000,000,000,000,000 Septillion Quadrillion Zetta Z 1021 1,000,000,000,000,000,000,000 Sextillion Trilliard Exa E 1018 1,000,000,000,000,000,000 Quintillion Trillion --- LightYear ly 9.46x1015 9,460,000,000,000,000 One light year. Peta P 1015 1,000,000,000,000,000 Quadrillion Billiard Tera T 1012 1,000,000,000,000 Trillion Billion Giga G 109 1,000,000,000 Billion Milliard Mega M 106 1,000,000 Million kilo k 103 1,000 Thousand hecto h 102 100 Hundred deca da 101 10 Ten - - 100 1 One deci d 10−1 0.1 Tenth centi c 10−2 0.01 Hundredth milli m 10−3 0.001 Thousandth micro μ 10−6 0.000,001 Millionth nano n 10−9 0.000,000,001 Billionth Milliardth pico p 10−12 0.000,000,000,001 Trillionth Billionth femto f 10−15 0.000,000,000,000,001 Quadrillionth Billiardth atto a 10−18 0.000,000,000,000,000,001 Quintillionth Trillionth zepto z 10−21 0.000,000,000,000,000,000,001 Sextillionth Trilliardth yocto y 10−24 0.000,000,000,000,000,000,000,001 Septillionth Quadrillionth --- Plank Length l 1.6162x10- 0.000,000,000,000,000,000,000,000,000,000,000,016 Nothing physically shorter than this. p 35 googolth - 10 -100 0. with 99 zeros then a 1 UK Archaic, EU, Binary prefixes have an i after the Symbol (ie. Mi, Gi, Ti, etc.) USA, UK Modern India, RotW

4 October 2012 176 © Paul Thomas

Unofficial Units and Specifics

Prefix Symbol 10n Decimal Short Scale luma 1063 1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 minga 1060 1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 nena 1057 1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 ocha 1054 1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 pepta 1051 1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 quexa or ultra U or ut 1048 1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 quindecillion rinta or quinsa Q or qu 1045 1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 quattuordecillion sorta or cata C or ca 1042 1, 000,000,000,000,000,000,000,000,000,000,000,000,000,000 tredecillion treda or astra A or at 1039 1, 000,000,000,000,000,000,000,000,000,000,000,000,000 duodecillion uda or vela V or ve 1036 1, 000,000,000,000,000,000,000,000,000,000,000,000 undecillion vunda or vendeko or besa Bor be 1033 1, 000,000,000,000,000,000,000,000,000,000,000 decillion weka or weko or sansa B or s 1030 1,000,000,000,000,000,000,000,000,000,000 nonillion xona or xenno or hella Xor n 1027 1,000,000,000,000,000,000,000,000,000 octillion

xonto or xenna or tiso x or t 10-27 0.000,000,000,000,000,000,000,000,001 octillionth wekto or weka or vindo w or v 10-30 0.000,000,000,000,000,000,000,000,000,001 nonillionth vunkto or vendeka v 10-33 0.000,000,000,000,000,000,000,000,000,000,001 decillionth unto 10-36 0.000,000,000,000,000,000,000,000,000,000,000,001 undecillionth trekto 10-39 0.000,000,000,000,000,000,000,000,000,000,000,000,001 duodecillionth sotro 10-42 0.000,000,000,000,000,000,000,000,000,000,000,000,000,001 tredecillionth rimto 10-45 0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,001 quattuordecillionth quekto 10-48 0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,001 quindecillionth pekro 10-51 0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,001 otro 10-54 0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,001 nekton 10-57 0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,001 mikto 10-60 0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,001 lunto 10-63 0.000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,001

Note: Prefixes in normal text are in use and have been proposed to the Consultative Committee on Units. The prefixes in italics, are in use, but have not been proposed. 4 October 2012 177 © Paul Thomas

Formulas Telescope Formulae

Aperture D = F/f

Where: D is the aperture of the objective F is the focal length of the objective f is the f-number (f/) of the objective

Magnification: By Fields M = Alpha/Theta

Where: M is the magnification Alpha is the apparent field Theta is the true field

Apparent Field: the closest separation eye can see is 4', more practically 8-25', 1-2' for good eyes. The Zeta Ursae Majoris double (/) is 11.75'; is 3'.

True Field (in °) = 0.25 * time * cos of the declination (in ') = 15 * time * cos of the declination where time is the time to cross the ocular field in minutes. A star therefore moves westward at the following rates: 15° /h (1.25°/5 min) at 0° declination 13° /h (1.08°/5 min) at 30° declination 7.5°/h (0.63°/5 min) at 60° declination.

Magnification: By Focal Lengths M = F/f

Where: M is the magnification F is the focal length of the objective f is the focal length of the ocular

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At prime focus (ground glass), magnification is 1x for each 25 mm of F

Magnification: By Diameter And Exit Pupil M = D/d

Where: M is the magnification D is the diameter of the objective d is the exit pupil (5-6 mm is best; 7 mm may not produce a sharp outer image)

The scotopic (dark-adapted) aperture of the human pupil is typically 6 (theoretically 7, 5 if over age 50) mm. Since the human pupil has a focal length of 17 mm, it is f/2.4 and yields 0.17 per mm of aperture. 2.5 mm is the photopic (light-adapted) diameter of the eye.

Exit Pupil d = f/f-number

(by substituting F/f for M) where: d is the exit pupil f is the focal length of the ocular f-number is the f-number (f/) of the objective

By substituting d=7 (the scotopic aperture of the human pupil) and multiplying it by the f-number, the longest useful focal length of the ocular is given.

Low-Power Law For Limiting Magnification M = D/6 = 17*D

(by substituting 6 mm for d and taking the reciprocal) where: M is the minimum magnification without wasting light for a dark-adapted eye (17x per mm of aperture) D is the diameter of the objective in mm

High-Power Law For Limiting Magnification M = D/0.63 = 158*D

(by substituting 0.63 mm, the minimum diameter to which the average pupil can contract, for d and taking the reciprocal) where: M is the maximum theoretical magnification (158x per mm of aperture); the maximum practical magnification is +50%).

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Limiting Visual Magnitude (Light-Gathering Power) m = 6.5-5 log Delta+5 log D = 2.7+5 log D

(assuming transparent dark-sky conditions and magnification >= 1D in mm) where: m is the approximate limiting visual magnitude Delta is the pupillary diameter in mm (accepted as 7.5) D is the diameter of the objective in mm

Relative Light Efficiency (Twilight Factor) Relative Brightness Value = d2 = (D/M)2 where the larger the relative brightness value, the better the instrument (e.g., binoculars) is for viewing in twilight or for astronomical use at dusk (low light conditions only): d is the diameter of the exit pupil D is the diameter of the objective M is the magnification

Angular Radius Of Airy (Diffraction) Disc r = (1.12*Lambda*206265)/D = 127.1/D

(the second formula is based on Lambda = 0.00055 for yellow) where: r is the angular radius (one-half the angular diameter) of the Airy disc (irreducible min. size of a star disc in ") Lambda is the wavelength of the light in mm 206265 is the number of " in a radian D is the diameter of the objective in mm

Linear Radius Of Airy (Diffraction) Disc r = 0.043*Lambda*f

Where: r is the linear radius (one-half the linear diameter) of the Airy disc in mm Lambda is the wavelength of light in mm (yellow 0.00055) f is the f-number (f/) of the objective

Depth Of Field (Photovisual) c = (Distance*Lambda)/D2

4 October 2012 180 © Paul Thomas

Where: c is the depth of field in mm Distance is that to the objective from the object viewed or imaged (in mm) Lambda is the wavelength of light in mm (yellow light 0.00055 for peak of human visual sensitivity) D is the diameter of the objective in mm

Dawes Limit (Smallest Resolvable Angle, Resolving Power) Theta = 115.8/D

Where: Theta is the smallest resolvable angle in " D is the diameter of the objective in mm

Atmospheric conditions seldom permit Theta < 0.5". The Dawes Limit is one-half the angular diameter of the Airy (diffraction) disc, so that the edge of one disc does not extend beyond the centre of the other). The working value is two times the Dawes Limit (diameter of the Airy disc), so that the edges of the two stars are just touching.

Magnification Needed To Split A M = 480/d

Where: M is the magnification required 480 is number of seconds of arc for an apparent field of 8 minutes of arc d is the angular separation of the double star

About the closest star separation that the eye can distinguish is 4 minutes of arc (240 seconds of arc). Twice this distance, or an 8-minute (480- second) apparent field angle, is a more practical value for comfortable viewing. In cases where the comes is more than five magnitudes fainter than the primary, you will need a wider separation: 20 or 25 minutes of arc, nearly the width of the moon seen with the naked eye.

Resolution Of Lunar Features Resolution = (2*Dawes Limit*3476)/1800) Dawes Limit * 38.8

Where: Resolution is the smallest resolvable lunar feature in km 2*Dawes Limit is the Airy disc (a more practical working value is twice this) 1800 is the angular size of the moon in " 3476 is the diameter of the moon in km 4 October 2012 181 © Paul Thomas

Light Grasp Light Grasp = (D/d)2*pi = 7*D2

Where: Light Grasp is times that received by the retina D is the diameter of the objective in mm d is the diameter of the eye's pupillary aperture in mm (accepted value 7.5) pi is the transmission factor (approximately equal to 62.5% for the average telescope, up to approximately 180 mm) To compare the relative light grasp of two main lenses used at the same magnification, compare the squares of their diameters


Resolving Power (in arcseconds) Pr = 120/Do

Magnification M = fo/fe or M = Do/Dep

Focal Ratio fr = Fo/Do

Scopes Field of View FOVs = FOVe/M

Diameter of Exit Pupil Dep = Do/M or Dep = fe/fr

Focal Length Of Scope fo = Do * fr

Focal Length Of Eyepiece fe = Dep * fr

Gain In Visible Star Magnitudes Gmag = 5 log(Do/7)

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Where: Dep = Diameter of the exit pupil Do = Diameter of the Objective fe = Focal length of the eyepiece fo = Focal length of the objective fr = f-Ratio FOVe = Field of view of the eyepiece FOVs = Field of view of the scope Gmag = Gain in visible star magnitudes M = Magnification Pr = Resolving power


Astrophotography Formulae

F-Number: Prime Focus (Erect Image) f/ = F/D

Where: f/ is the f-number of the system (objective) F is the focal length of the objective D is the diameter of the objective

F-Number: Afocal, Eyepiece-Camera Lens (Reversed Image) f/ = F minutes/D = (M*Fc)/D = ((F/Fe)*Fc)/D = (F/D)*(Fc/Fe) = (M/D)*Fc

Where: f/ is the f-number of the system F' is the effective focal length of the system Fe is the focal length of the ocular (divided by Barlow magnification) D is the diameter of the objective

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M is the magnification Fc is the focal length of the camera F is the focal length of the objective Fc/Fe is the projection magnification M/D is the power per mm

The diameter of the first image equals the film diagonal (44 mm for 35 mm film) divided by the magnification

F-Number: Eyepiece Projection, Positive Lens (Reversed Image) f/ = F minutes/D = (F/D)*(B/A) = (F/D)*(((M+1)*Fe)/A) = (F/D)*((B/Fe)-1)

Where: f/ is the f-number of the system F' is the effective focal length of the system D is the diameter of the objective F is the focal length of the objective (times any Barlow magnification) B is the secondary image ("throw"), the distance of the ocular centre from the focal plane of the film, equal to ((M+1)*Fe)/A A is the primary image, the distance of the ocular centre from the focal point of the telescope objective M is the projection magnification, equal to (B/Fe)-1 Fe is the focal length of the ocular

F-Number: Negative Lens Projection (Erect Image) f/ = F minutes/D = (F/D) * (B/A)

Where: f/ is the f-number of the system F' is the effective focal length of the system D is the diameter of the objective B is the distance of the Barlow centre from the focal plane of the film A is the distance of the Barlow centre from the focal point of the telescope objective B/A is the projection magnification (Barlow mag.)

Exposure Comparison For Extended Objects Exposure Compensation = (f/S)2/(f/E)2 = ((f/S)/(f/E))2 (the ratio of intensities of illumination is squared according to the inverse square law)

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Where: Exposure Compensation is the exposure compensation to be made to the example system f/S is the f-number (f/) of the subject system f/E is the f-number (f/) of the example system

Exposure Comparison For Point Sources Exposure Compensation = De2/Ds2 = (De/Ds)2

Where: Exposure Compensation is the exposure compensation to be made to the example system De is the objective diameter of the example system Ds is the objective diameter of the subject system

Light-Recording Power Of A System Power = r2/f2 (the light-recording power is directly proportional to the square of the radius of the objective and inversely proportional to the square of the f-number)

Where: Power is the light-recording power of the system r is the radius of the objective f is the f-number (f/) of the system

Example: a 200-mm f/8 system compared with a 100-mm f/5 system (1002)/82 compared with (502)/52 156.25 compared with 100, or 1.56 times more light-recording power

Efficiency Of Lens For Photographing An Average Meteor Efficiency = F/f2

Where: Efficiency is the efficiency of the lens for photographing an average meteor (in a meteor shower) F is the focal length of the lens f is the f-number (f/) of the lens

Print's Effective Focal Length Print EFL = Camera FL * Print Enlargement

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Where: Print EFL is the print's effective focal length Camera FL. is the camera's focal length Print Enlargement is the amount of enlargement of the print (3x is the standard for 35-mm film)

Guidescope Magnification Guidescope M ~ f/12.5

Where: Guidescope M is the magnification needed f is the photographic focal length in mm

Experience indicates that the minimum guiding magnification needed is about f divided by 12.5, precisely what a 12.5 mm guiding ocular used in an off- axis guider for prime-focus photography yields. (Since visual magnification is the ratio of the objective to ocular focal length, the combination of prime-focus camera and off-axis guider with a 12.5-mm ocular gives a guiding magnification of f/12.5. f/7.5 (as with a typical focal reducer that reduces the effective focal length by a factor of 0.6) is a significant improvement. f/5 or higher magnification is for top-quality guiding.

Guidescope M = Guidescope EFL / Print EFL

Where: Guidescope M is the guidescope's magnification (should be >= 1, preferably 5-8) Guidescope EFL is the guidescope's effective focal length, the guidescope's focal length times any Barlow magnification (should be >= to the focal length of the primary and the guidescope's magnification, 0.2x per mm of focal length of the objective, 0.1x per mm of the camera lens Print EFL is the print's effective focal length

Guiding Tolerance Guiding Tolerance = 0.076 * Guidescope M

Where: Guiding Tolerance is in mm 0.076 is one " at a 254-mm reading distance from the print (a crosshair is usually 0.05 mm)

Maximum Allowable Tracking (Slop) Error S ~ 8250/(F*E)

Where: S is the error ("slop") in " 4 October 2012 186 © Paul Thomas

F is the focal length in mm E is the amount of enlargement of the print (3x is the standard for 35-mm film)

The slop is derived from the formula Theta = k*(h/F), with k = 206256 (the number of seconds in a radian) and h = 0.04 mm of image-drift tolerance (an empirical value from astrophotographs).

Conversion Of Plate Scale To Effective Focal Length EFL = mm per degree * 57.3 = 206265/" per mm

Where: EFL is the effective focal length in mm 57.3 is the number of degrees in a radian 206256 is the number of " in a radian

Resolving Power Of A Photographic System Resolving Power = 4191"/F

Where: Resolving Power is the resolving power of a photographic system with Kodak 103a or colour film F is the focal length of the system in mm

Maximum Resolution For A Perfect Lens Maximum Resolution = 1600/f

Where: Maximum Resolution is the resolution for a perfect lens f is the f-number (f/) of the lens

Most films, even fast ones, resolve only 60 lines/mm; the human eye resolves 6 lines/mm (less gives a "woolly" appearance). 80 lines/mm for a 50-mm lens is rated excellent (equal to 1 min of arc); a 200-mm lens is rated excellent with 40 lines/mm. 2415 films yields 320 line pairs (160 lines)/mm (equal to 1 second of arc); Tri-X yields 80 lines/mm.

Minimum Resolution Necessary For Film Minimum Resolution = Maximum Resolution * Print Enlargement

Where: Minimum Resolution is the min resolution necessary Maximum Resolution is the max resolution for a perfect lens Print Enlargement is the amount of enlargement of the print (3x is the standard for 35-mm film) 4 October 2012 187 © Paul Thomas

Size Of Image (Angular) h = (Theta*F)/k Theta = k*(h/F) F = (k*h)/Theta

Where: h is the linear height in mm of the image at prime focus of an objective or a telephoto lens Theta is the object's angular height (angle of view) in units corresponding to k F is the effective focal length (focal length times Barlow magnification) in mm k is a constant with a value of 57.3 for Theta in degrees, 3438 in minutes of arc, 206265 for seconds of arc (the number of the respective units in a radian)

The first formula yields image size of the sun and moon as approximately 1% of the effective focal length (Theta/k = 0.5/57.3 = 0.009).

The second formula can be used to find the angle of view (Theta) for a given film frame size (h) and lens focal length (F). Example: the 24 mm height, 36 mm width, and 43 mm diagonal of 35-mm film yields an angle of view of 27 deg, 41 deg, and 49 deg for a 50-mm lens.

The third formula can be used to find the effective focal length (F) required for a given film frame size (h) and angle of view (Theta).

Size Of Image (Linear) i = (h/D)*F h = (D*i)/F D = (h*F)/i F = (D*i)/h

Where: i is the linear image size in mm of the image at prime focus of an objective or telephoto lens (for terrestrial objects, equal to 24 mm divided by the amount of enlargement of the print [3x is the standard for 35-mm film] for the smallest dimension of 35-mm film]) h is the linear height of the object in units corresponding to D D is the distance of the object in units corresponding to h F is the effective focal length (focal length times Barlow magnification) in mm

The last formula gives the focal length necessary to photograph a recognizable celestial (Linear Width in km) or terrestrial (Linear Width in m).

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Size Of Image (Celestial) h = (Theta*F)/K Theta = K*(h/F) F = (K*h)/Theta

Where: h is the linear height in mm of the image at prime focus of an objective or a telephoto lens Theta is the object 's angular height (angle of view) in units corresponding to K F is the effective focal length (focal length times Barlow magnification) in mm K is a constant with a value of 57.3 for Theta in degrees, 3438 in minutes of arc, 206265 for seconds of arc (the number of the respective units in a radian)

The first formula yields image size of the sun and moon as approximately 1% of the effective focal length (Theta/K = 0.5/57.3 = 0.009).

The second formula can be used to find the angle of view (Theta) for a given film frame size (h) and lens focal length (F). Example: the 24 mm height, 36 mm width, and 43 mm diagonal of 35 mm film yields an angle of view of 27 degrees, 41 degrees, and 49 degrees for a 50-mm lens.

The third formula can determine effective focal length (F) required for a given film frame size (h) and angle of view (Theta).

Size Of Image (Terrestrial) h = (Linear Width / Distance) * F Linear Width = (Distance * h) / F Distance = (Linear Width * F) / h F = (Distance * h) / Linear Width

Where: h is the linear height in mm of the image at prime focus of an objective or telephoto lens Linear Width is the linear width of the object in m Distance is the distance of the object in m F is the effective focal length (focal length times Barlow magnification) in mm

Length Of A Star Trail On Film Length = F*T*0.0044

Where: Length is the length in mm of the star trail on film F is the focal length of the lens in mm 4 October 2012 189 © Paul Thomas

T is the exposure time in minutes 0.0044 derives from (2*Pi)/N for minutes (N = 1440 minutes per day)

Exposure Time For Star Trail On 35-Mm Film T = 5455/F

Where: T is the exposure time in minutes for a length of 24 mm (the smallest dimension of 35-mm film) F is the focal length of the lens in mm

Maximum Exposure Time Without Star Trail T = (1397/F)

Where: T is the maximum exposure time in seconds without a star trail 1397 derives from 1' at reading distance (254 mm), the smallest angular quantity that can be perceived by the human eye without optical aid ("limiting resolution") and is equal to < 0.1 mm. This quantity also applies to the moon. 2-3x yields only a slight elongation. Use 20x for a clock drive. F is the focal length of the lens in mm

The earth rotates 5' in 20 s, which yields a barely detectable star trail with an unguided 50-mm lens. 2-3' (8-12 s) is necessary for an undetectable trail, 1' (4 s) for an expert exposure. Divide these values by the proportional increase in focal length over a 50-mm lens. For example, for 3' (12 s), a 150-mm lens would be 1/3 (1' and 4 s) and a 1000-mm lens would be 1/20 (0.15' and 0.6 s). Note that to compensate for these values, the constant in the formula would be 1000 for a barely-detectable trail, 600 for an undetectable trail, and 200 for an expert exposure.

N.B. The above formulae assume a declination of 0o. For other , multiply lengths and divide exposure times by the following cosines of the respective declination angles: 0.98 (10°), 0.93 (20°), 0.86 (30°), 0.75 (40°), 0.64 (50°), 0.50 (60°), 0.34 (70°), 0.18 (80°), 0.10 (85°).

Exposure Duration For Extended Objects E = f2/(S*B)

Where: e is the exposure duration in seconds for an image size of >= 0.1 mm f is the f-number (f/) of the lens S is the film's ISO speed B is the brightness factor of the object (Venus 1000, Moon 125, Mars 30, Jupiter 5.7)

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Thus, a 2-minute exposure at f/1.4 is equivalent to a 32-minute exposure at f/5.6 (4 stops squared times 2 minutes), ignoring the effects of reciprocity failure in the film, which would mean that the 32-minute exposure would have to be even longer.

Surface Brightness Of An Extended Object ("B" Value) B = 100.4(9.5-M)/D2

Where: B is the surface brightness of the (round) extended object M is the magnitude of the object (total brightness of the object), linearized in the formula D is the angular diameter of the object in seconds of arc (D2 is the surface area of the object)

Exposure Duration For Point Sources e = (100.4(M+13))/S*a2

Where: e is the exposure duration in seconds for an image size of >= 0.1 mm M is the magnitude of the object S if the film's ISO speed a is the aperture of the objective

Focal Length Necessary To Photograph A Recognizable Object F = (Distance / Linear Field) * Image Size

Where: F is the focal length in mm necessary to photograph a recognizable object Distance is the distance of the object in m Linear Field is the linear field of the object in m Image Size is the image size in mm (equal to 24 mm divided by the amount of enlargement of the print [3x is the standard for 35mm film] for the smallest dimension of 35mm film)

Miscellaneous Formulae

Hour Angle H = Theta - Delta

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Where: H is the hour angle Theta is sidereal time Delta is right ascension

The Hour Angle is negative east of and positive west of the meridian (as right ascension increases eastward).

Bode's Law (4 + 3(2n))/10 in AU at aphelion

Where: n is the serial order of the planets from the sun (Mercury's 2n =1, Venus's n = 0, Earth's n = 1, asteroid belt = 3)

Apparent Angular Size Of An Object Theta = (h/D)*k

Where: Theta is the object's apparent angular size in units corresponding to k h is the linear height of the object in units corresponding to D D is the distance of the object in units corresponding to h Theta is the object's angular height (angle of view) in units corresponding to k k is a constant with a value of 57.3 for Theta in degrees, 3438 in minutes of arc, 206265 for seconds of arc (the number of the respective units in a radian)

A degree is the apparent size of an object whose distance is 57.3 times its diameter. The formula holds for celestial or terrestrial objects. e.g., for the width of a quarter at arm's length: (57.3*25 mm)/700 mm = 2°.

Under ideal conditions, the human eye can resolve anything subtending more than a 1' angle, i.e., see an object as an extended object or see a double star as two stars rather than a single point of light, provided that the two components are of nearly equal brightness. A more practical value would be 4'; 8' is an even more practical value for comfortable viewing. The best earthbound telescopes are usually limited by atmospheric effects to objects 1" or larger (0.25" with excellent seeing) in apparent size (before magnification). In theory, a telescope could see everything with a magnification of 60x (1" magnified to 1').

Angular Size Theta = (55*h)/d

Where: Theta is the angular size of the object in degrees h is the linear size of the object in m 4 October 2012 192 © Paul Thomas

d is the distance from the eye in m e.g., for the width of a quarter at arm 's length: (55*0.254)/0.711 = 2 degrees

Twilight Performance Twilight Performance = (D * M)

Where the greater the Twilight Performance is, the better suited the instrument (e.g., binoculars) will be for viewing in twilight or for astronomical use after dusk (low light conditions only). This is only valid when comparing optics of similar quality; consider for example how a 5.0 litre Ferrari engine will perform differently than a Ford 5.0 litre engine: = Square Root D is the diameter of the objective M is the magnification

Relative Light Efficiency Relative Brightness Value = d2 = (D/M)2

Where the larger the relative brightness value, the better the instrument (e.g., binoculars) is for viewing in twilight or for astronomical use after dusk (low light conditions only). Again this is only valid when comparing optics of similar quality: d is the diameter of the exit pupil D is the diameter of the objective M is the magnification

Length Of A Meteor Trail h = (Theta*D)/57.3

Where: h is the linear height of the meteor in km Theta is the object's apparent angular size in degrees D is the distance of the object in km

Measuring Angles With A Micrometre Reticle To calibrate a micrometre’s linear scale LS = 206265/F

Where: LS is the Linear Scale division spacing) in seconds of arc F is the focal length of the telescope objective lens

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Estimating Angular Distance

1p piece, 4 km distant 1" Sun, Moon 30' Width of little finger at arm's length 1° 5p piece at arm's length 1° 2p piece at arm's length 2.5° Width of Orion's belt 3° Alpha Ursae Majoris (Dubhe) to (Merak) 5° Alpha Geminorum () to Beta Geminorum () 5° Width of fist at arm's length 10° Alpha Ursae Majoris (Dubhe) to (Megrez) 10° Height of Orion 16° Length of palm at arm's length 18° Width of thumb to little finger at arm's length 20° Alpha Ursae Majoris (Dubhe) to (Alkaid) 25° Alpha Ursae Majoris (Dubhe) to Alpha Ursae Minoris (Polaris) 27°

Estimating Magnitudes

Big Dipper (from cup to handle) Alpha (Dubhe) 1.8 Beta (Merak) 2.4 Gamma (Phecda) 2.5 Delta (Megrez) 3.4 Epsilon (Alioth) 1.8 Zeta (Mizar) 2.2 Eta (Alkaid) 1.9

Little Dipper (from cup to handle) Beta (Kochab) 2.0 Gamma (Pherkad) 3.1 Eta 5.0 Zeta 4.3 Epsilon 4.4 Delta (Pherkard) 4.4 Alpha (Polaris) 2.0

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Range Of Useful Magnification Of A Telescope

D = diameter of aperture in mm

Minimum useful magnification for better contrast 0.13*D 0.2*D Best visual acuity 0.25*D Wide views 0.4*D Lowest power to see all detail (resolution of eye matches resolution of telescope) 0.5*D Planets, Messier objects, general viewing 0.8*D Normal high power, double stars 1.2*D to 1.6*D Maximum useful magnification 2.0*D Close doubles 2.35*D Sometimes useful for double stars 4.0*D Limit imposed by atmospheric turbulence 500

The rotational period of the Earth: 86164.09890369732 seconds or, 23 hours 56 minutes and 04.09890369732 seconds of Universal Time

The rate of rotation of the Earth: 15.04106717866910 seconds of arc per second of time

The mean Sidereal day: 23 hours 56 minutes 04.090524 seconds of Universal Time

The day of Universal Time: 24 hours 03 minutes 56.5553678 seconds of mean Sidereal Time

* The lengths of a day of UT and a day of mean Sidereal time vary slightly with variations in the Earth's rotation. However, the ratio of UT to mean sidereal time, is unaffected by these variations.

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The Periodic Table

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The Electromagnetic Spectrum

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Lens & Mirror Characteristics

Spherical Aberration

Chromatic Aberration

Focal Points & Lengths

(by kind permission NASA & STScl)

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4 October 2012 199 © Paul Thomas