Lecture 2: Our Place in Space •It is now clear that Earth is not central or special in its general properties (mass, distance from central star, etc.) •The Sun is an average star •The Milky Way is a typical spiral galaxy •This leads to the “principle of mediocrity” or “Copernican principle” Copernican Principle •The “principle of mediocrity” or “Copernican principle” states that physical laws are the same throughout the Universe •This principle can be tested using the scientific method! •So far, it seems to agree with observations perfectly •One of the key ideas is that we can use light from distant objects to understand conditions there, using the work of Isaac Newton Multiwavelength Astronomy •The color and energy of light are related to the Wavelength… •Different wavelengths of radiation tell us about objects with different temperatures… 1 Multiwavelength Astronomy Multiwavelength Astronomy •Using Newton’s techniques, combined with our knowledge of atomic structure, we can use light to study distant objects… X-ray optical composition of matter temperature of matter Scientific Method and Observation •Transformed the search for meaning into a quest for objective truth •We can deduce the properties of distant objects using observations •The scientific method originated in the renaissance, and is still used today: hypo thesi n s io tti iic d e r y p y p r r o o e o truth e truth h b h t s t e rr v a tt ii o n test 2 Copernican Principle •The validity of the “Copernican principle” is great news for science, because it means that we can learn about distant objects by analyzing the light that reaches us from them •This provides some of our best evidence for the existence of black holes… The Scale of Things •Human body: 1 meter •Mountain/Ocean: 1 km = 0.6 miles •Continent: 5,000km = 3,000 miles •Earth Diameter: 13,000 km = 8,000 miles •Solar Diameter: 1,400,000 km = 930,000 miles •Earth-Sun Distance: 140,000,000 km = 93,000,000 miles = 1 Astronomical Unit (AU) •Scientific Notation: 140,000,000 = 1.4 x 10^8 •Recall: 10^3 = 1,000 10^4 = 10,000 1.4 x 10^4 = 14,000 The Scale of Things 3 The Scale of Things Billions and Billions •There are billions of stars in a spiral galaxy •There are up to 1,000 galaxies in a Galaxy Cluster The Celestial Sphere 4 Towards the Big Dipper… Another Solar System… Towards the Big Dipper… Millions of other galaxies… Billions and Billions •There may be billions of clusters of galaxies! •There may even be billions of Super-Clusters… 5 The Sky at Night •Roughly 3,000 stars are visible to the naked eye on a clear, dark night (or about 100 in Fairfax!) •People organize stars into Constellations, which are chance alignments of stars into patterns •Constellations have historical and cultural significance •They are still useful for indicating general regions of the sky Constellations Constellations – Orion 6 Earth’s Seasons ecliptic (path of Sun) ecliptic (path of Sun) •The 23.5 degree tilt of Earth’s spin axis relative to its orbital axis around the Sun causes the seasons Celestial Sphere Celestial Coordinates •The path of the Sun through the sky is called the Ecliptic 7 Celestial Coordinates •Celestial Sphere animation •Ecliptic animation •View from the Northern Hemisphere •View from the Equator looking north •View from the Equator looking south •Retrograde motion of Mars •Solar System animation Seasonal Constellations •Due to the Earth’s motion around the Sun, different constellations are visible at night during the year Diurnal Motion •Earth’s daily rotation causes the stars to sweep out circular paths in the sky North Celestial Pole (star is Polaris) •This is called Diurnal Motion – objects rise in the east, set in the West •North Celestial Pole is extension of Earth’s spin axis into space 8 The Celestial Sphere Celestial Coordinates •Right Ascension (RA) works like Cosmic Longitude •Declination (DEC) works like Cosmic Latitude •Be careful with units! •DEC is measured using angular units: degrees (O), arc- minutes (’), arc-seconds (’’) •For example, for Betelgeuse in Orion, DEC = 17 degrees, 24 minutes •The range of DEC values is between –90 degrees (south celestial pole) to +90 degrees (north celestial pole) Degrees, Minutes, Seconds 9 Sidereal Day and Solar Day •RA is measured using time units: hours, minutes, seconds •This is because RA is related to the rotation of the Earth •The Earth rotates in 24 hours…but NOT solar hours! •We use Sidereal Time in astronomy…relative to the stars, the Earth 1 sidereal day later rotates once every 24 Sidereal Hours •1 Solar Day = 24 hours •1 Sidereal Day = 24 hours - 4 min •1 Sidereal Day=24 Sidereal Hours Phases of the Moon Orbit of the Moon •The Moon revolves around the Earth with a period of approximately one month •It always keeps the same face pointed towards Earth •This is a tidally locked orbit Earth Sun Orbit of Moon Moon 10 Orbit of the Moon •The Moon’s orbit is not in the same plane as Earth’s orbit around the Sun (it is tilted by 5.2o) •therefore “alignments” of the Earth, Moon, and Sun are not usually exact •The lunar orbit is not perfectly circular Earth Sun Orbit of Moon Moon •Eclipses occur during Eclipse Season, when the Earth and Sun are on the “Line of Nodes”, which happens twice a year •We do NOT see eclipses during every eclipse season because the Moon is usually not in the right place! Motion of the Moon •The time between stellar alignments (the sidereal period of the lunar orbit) is 27.3 (solar) days, or one “Sidereal Month” •The time between Earth-Moon-Sun “alignments” is 29.5 (solar) days, or one “Synodic Month” 1 synodic month later 1 sidereal month later Distant Star Sun Orbit of Moon starting point Moon Earth 11 Eclipses Eclipses Eclipses •Can occur only if the alignment is precise (Earth, Moon, and Sun line up exactly) •The Moon’s orbit is tilted by 5.2o relative to the ecliptic •Eclipses are infrequent because the Moon, Earth, and Sun must lie along the “Line of Nodes”, which is the intersection of the two orbital planes Line of Nodes Solar Sun Eclipse 12 Eclipses •Eclipses can be Lunar (Moon becomes darker) or Solar (Sun becomes darker) •Eclipses occur during Eclipse Season, when the Earth and Sun are on the “Line of Nodes”, which happens twice a year •We do NOT see eclipses during every eclipse season because the Moon is usually not in the right place! Line of Nodes No Eclipse Sun Solar Eclipse Solar Eclipse •The region of totality is called the “umbra” about 270 km across •The region in which the Sun appears partially blocked is called the “penumbra”, bout 7,000 km across •Solar Eclipse lasts about 7.5 minutes at one location 13 Annular Eclipse •The Moon’s orbit is slightly eccentric (non-circular) •If the Moon is far away in its orbit during an eclipse, it doesn’t completely block the Sun, resulting in an Annular Eclipse Lunar Eclipse •Lunar eclipses last about 100 minutes Solar Eclipse Tracks •Solar eclipses begin at sunrise in the West •The Moon’s shadow moves from West to East at 1,700 km per hour per hour due to the orbital motion of the Moon 14 Precession •The Earth spins like a top, and therefore it precesses due to the gravitational pull of the Moon and Sun •The period of precession is about 26,000 years •Precession changes the direction of the North Celestial Pole •This alters the coordinates of all celestial objects, and changes the time of year for the seasons •It also causes the Line of Nodes to rotate, making eclipse season about 20 days earlier each year 15.