Glossary: Only some selected terms are defined here; for a full glossary consult an astronomical dictionary or google the word you need defined.

Aphelion/Perihelion: Earth’s orbit is elliptical so the planet is farthest from the Sun at aphelion (first week of July) and closest at perihelion (first week of Jan).

Apogee/Perigee: Since orbits around the Sun or Earth are usually ellipses, the farthest and nearest distances use “apo” (far) and “peri” (near) to describe the maximum and minimum values. For Earth and its satellites, apogee is the farthest point and perigee is the nearest (“geo” = Earth). The same prefixes are applied to orbits around the Moon -”luna” (apolune and perilune) Sun -”helios” (aphelion/perihelion), etc.

Appulse: A close approach of two astronomical objects. i.e. minimum separation expressed in degrees, minutes and seconds of arc where 1 degree = 60 minutes and 1 minute = 60 seconds of arc. Note: the Moon and Sun are about 0.5 degrees or 30 minutes of arc across.

Bolides or Fireballs: A very bright meteor (shooting star) that often can light up the ground and produce meteorites. See Meteor Shower below for more.

Conjunction: The point in time when two stellar objects have the same Right Ascension. This is usually close to the minimum separation of the two objects but may not necessarily be minimum. (See also appulse above). When a planet is at Inferior Conjunction with the Sun it is between Earth and Sun and in Superior Conjunction, it is on the opposite side of the sun. At neither time are they easy (but not impossible) to see since they are near the Sun.

Dichotomy: The point when a planet or moon is exactly 50% illuminated by sunlight. For Earth’s Moon, synonymous with FQ and LQ phase.

Ecliptic: The path the Sun takes across the celestial sphere as seen from Earth. It follows the constellations of the zodiac generally except for a brief stint in a non-zodiac constellation in late November when it passes through Ophiuchus. The Sun spends about a month “in” each constellation during the year assuming each zodiac constellation covers 30° of sky, (which they don’t, -herein lies one conflict between astrology and astronomy).

Elongation (E or W): The apparent separation in the sky between two celestial objects, one usually the Sun. For ex. a Greatest Elongation East for Mercury means it is farthest east of the Sun on that date and best seen in the evening sky, east of the Sun after sunset. Similarly for Greatest Elongation W. Often abbreviated GEE and GEW.

Fireballs or Bolides: A very bright meteor (shooting star) that often can light up the ground and produce meteorites. See Meteor Shower below for more.

Focal length, focal ratio: The focal length is the distance between an optical element like a lens or mirror and the point where it produces a focused image. Modern telescope focal lengths fall in the range of a few hundred millimetres, like in a rich-field refractor for ex., to several metres like in a Schmidt-Cassegrain telescope of 2500 mm or more. Focal ratio is the mathematical ratio of the focal length of an optical element to its diameter or aperture. For ex. an 8-inch (200 mm) reflector mirror can have a focal length of 1200 mm, so the focal ration or f/ ratio is f/6 (1200/200 = 6). Lenses or mirrors are designated “fast” for f/ ratios less than 2 or 3 and “slow” at f/8 and f/10. The time refers to the time required to record an image with a camera.

Graze (or grazing occultation): When the Moon moving in its orbit passes a star so that it appears to skim along the top or bottom edge of the Moon. The Moon’s profile may cause the star to blink on and off a number of times as it passes behind mountains on the Moon’s edge. See also occultation. magnification,

Magnitude: The apparent brightness of a celestial object on an arbitrary numerical scale invented by the Greeks in antiquity. The range was from 1st to 6th magnitude or so for visual astronomy, so a star of 1st magnitude was considered brighter (twice as bright?) than one of 6th magnitude. It is a reverse scale with larger numbers representing fainter objects. The Greek astronomers identified specific stars like those in the Big Dipper, for example as mostly 2nd magnitude, with one rated as 3rd, and the faintest ones were down the list rated as 5th and 6th magnitude. Modern measurements have standardized the magnitude scale and the range from 1 to 6, i.e., 5 magnitudes is defined as a change in brightness of 100 times. This puts the jump between two whole number magnitudes as very close to a factor of 2 in brightness (actually 2.512). An attempt at standardizing the scale was made by assigning Vega a magnitude of 0 but that star was found to vary slightly in brightness. Note that brighter objects like the Moon and Sun have negative magnitudes, (Sirius, the brightest star in the sky is -1.46) and values greater than 6 would designate “invisible” objects requiring optical aids like binoculars and telescopes. The brightest object in the sky is the Sun at magnitude -26.75, next is the full moon at -12.7, then planets like Venus at -4.6 and Mars at opposition, -2.6 or so. At the other end of the scale are stars that can be detected by the largest telescopes like the 5 m Hale telescope which can see (meaning photograph) stars to 23 or so, and the Hubble Space Telescope with a limit of 32.

Meteor Shower: An occasion when a larger number (7 or 8 per hour) of meteors (shooting stars) appear to radiant from a specific point in a constellation. The constellation determines the name of the shower, for ex. the Perseids radiate from the general direction of the head of Perseus. This is purely a perspective effect much like snow flakes appearing to originate from a point as you drive into the storm. Meteors are commonly called shooting stars, but they are usually dust-sized bits of space debris that are entering our atmosphere and not stellar in any way. Larger fragments that survive the journey to land on Earth are called meteorites but rarely are these ever associated with a particular meteor shower. Most meteorites are random chunks of asteroid fragments and large ones are rare (and highly sought after by collectors). Occasionally, a very bright meteor (see Bolide above) may enter the Earth’s atmosphere, break up and drop fragments on the Earth that may be observed visually or more often by meteor camera networks such as the one set up by Western University ((UWO) across S. Ontario.

Occultation (or total occultation): When the Moon passes in front of a bright star or planet so that it occults (blocks the light of) the object. A star will wink out virtually instantly while planets may take several seconds, even minutes becasue they are disks not points. Total occultations on the leading eastward-moving edge of the Moon are followed some time later by a reappearance on the opposite limb of the Moon.

Opposition: A planet in opposition is located opposite the Sun from Earth (imagine looking down from above the solar system -the alignment is Sun-Earth-planet. The planet is on the same side of the solar system as Earth and so appears in our “midnight” sky. From Earth, the planet appears to rise in the east when the Sun sets in the west. Consequently, it is highest in our sky at midnight and then sets on the western horizon when the Sun is rising in the east. The planets disks are at maximum size at opposition as well so observing is recommended for then, especially for planets like Mars. A month either side of opposition is the best time to view planets as they are in dark sky for the longest period.

Perigee: The closest distance between the Moon (or man-made Earth satellites) and the Earth since the Moon’s orbit around Earth is an ellipse. See also apogee.

Radiant: The point in space from which meteors appear to radiate. This is purely a perspective effect like snowflakes appearing to come from a point ahead as you drive into falling snow or the road appearing to narrow in the distance.

Reflector: When a mirror is the primary image forming optical element the telescope is referred to as a reflector. These come in many different apertures (diameters), focal lengths and are mainly made from glass coated with a reflective material like aluminum. The mirror must have a curved shape, usually a parabola for it to focus an image, with the focal length determined by the “depth” of the parabola. Many variations besides the traditional simple parabola shape are used in reflector types named after their originators: Newtonian, Cassegrain, Schmidt-Cassegrain, Maksutov, Gregorian, etc. Note that lenses are also required in the eyepieces to magnify the primary image, so a reflector is in fact a compound type of telescope.

Refractor: When a lens, or combination of lenses, is the primary image forming optical element the telescope is referred to as a refractor. Like the reflector, refractors require additional lenses to create an image and then to enlarge it. The traditional sailor’s “spyglass” is a refractor and was the first type of telescope invented. Because of the multiple surfaces that need to be figured, refractors are usually more expensive than reflectors and come in smaller apertures. A typical 6 inch refractor might cost over $2000 while a 6-inch reflector would be only a few hundred dollars.

Sporadic: A meteor that is not part of a shower, i.e., a random shooting star. Usually 7 or 8 per hour can be seen as the Earth is constantly colliding with space debris.

Telescope: An optical device using lenses or mirrors or a combination of these, to enlarge, brighten and clarify astronomical objects. Telescopes come in two main types, the refractor which uses lenses and the reflector which uses mirrors to “create” images of distant objects. See also, focal length, focal ratio, magnification, refractor, reflector.

Transit: The passage of an object like a planet across the disk of another celestial object. Most common are transits of Mercury and Venus across the Sun. The ISS can be seen to transit the Moon or Sun and more rarely other planets like Jupiter or Saturn. Transits of planets across planets can happen but are extremely rare.