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AS101 Midterm Review (Chapters 1-3)

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Patrick Mc Graw

Midterm Review What to focus on (…for the final) Chapter 1 - Familiarity with units (au, ly) - Scientific notation - Rough idea of sizes of things Chapter 2 - Basic understanding of the cycles of the sky: o Daily rising/setting o Moon phases o Seasons o Eclipses - How do daily cycles depend on your location on Earth? - Constellations and Asterisms Chapter 3 - Geocentric (Ptolemaic) vs. Heliocentric (Copernican) models - Kepler: ellipses - Some evidence that led to adoption of heliocentric model o E.g. Galileo’s observations - Newton’s idea of universal laws that apply both on Earth and off 8  1 AU = average distance between Earth and Sun (150 mil12on km/ 1.5 x 113km)  1 light year = distance light travels in one year (9.46x10 km) nearly 10 AU  1 pc (parsec) = 3.26 ly = 3.09x10 km  1Mpc (mega parsec) = 1 million parsecs  Distance to nearest star (besides sun) = 4.37 ly / 1.34 pc  Nearest star system is Alpha Centauri, 44 trillion km away  Nearest STAR is Proxima Centauri, 4.2ly away from us (textbook)  Distance to nearest star is 4.37ly (ppt) RELATIVE SIZES  Sun > Earth by about 110 times in diameter  Moon’s diameter ¼ of Earth’s & Mercury a bit larger than the moon  Distance from Venus to Sun is about 0.7AU  Venus around Earth’s size  Mars, about 1.5 AU from the sun  Sun a bit larger than the average star Earth orbits the sun, inside a solar system in a galaxy, in a cluster in the universe  Our galaxy (Milky Way) includes ~100 billion stars o Great cloud of stars, gas and dust bound by combined gravity of the matter o Milky Way part of group of 40 galaxies called “Local Group”  Belongs to “Local/Virgo Supercluster” o Size = approx. 80 000 ly  There are about as many galaxies in the universe as there are stars in the milky way o Filament = o Void = PLANETS  Small nonluminous bodies, shine by reflecting light  Earth, Venus, Mercury are really close to the sun  Slightly elliptical orbits  Light from sun takes around 8 minutes SUN  Rises in the east, sets in the west STARS Constellations  88 official ones  48 ancient constellations still in use (from ancient Mesopotamia, Babylon, Egypt, Greece o Back then, just loose groupings of stars, faint ones not included o Named based on gods, heroes, shapes; myths were associated  Constellation now represents a section of the sky, any star within the region belongs only to that constellation  Like countries/provinces Asterisms  Less formally defined grouping  Big Dipper, Ursa Major etc.  Like a landmark within a place *the only thing that the stars have in common are that they are in approx. the same direction viewed from earth Naming Stars  Names derive from ancient Greek, Arabic or Latin  Originally descriptive or related to myths  Alternate naming: o Greek letter + Latin possessive form of constellation’s name o Letters in order from brightest -> faintest, but really faint ones don’t get a Greek letter o E.g. alpha Orionis o *BETA ORIONIS IS SLIGHTLY BRIGHTER THAN ALPHA ORIONIS  Aka Rigel ANGULAR DISTANCE  Angular distanOe/size measured in degrees  Full circle = 360  1 = 60’ (arcminutes) o Width of a finger at arm’s length  1’ = 60” (arcseconds)  Angular size & physical size proportional (double the size, double the angle)  Distance is inversely proportional  Object’s angular size = smaller if farther away Approximate angular sizes for comparison O  Size of moon/sun = 0.5  Smallest feature most human eyes can see: 1 arcminute (1/60 of a degree)  Jupiter (from earth) about 0.8 arcminute  If sun 1ly away, angular size about 0.03 arcsecond MAPPING THE SKY Zenith: point directly overhead, 90 degrees Horizon: all points 90 degrees away from the zenith Meridian: a line passing through zenith and connecting N and S points on horizon Sun crosses meridian around noon  Describing a point in the sky: o Altitude: angular distance from horizon (negative = below horizon) o Azimuth: direction, expressed as an angle (usually from south) Celestial Sphere  Imaginary sphere surrounding earth  Mapping  Contains stars, planets, sun, etc. Celestial Pole  Point on celestial sphere directly above one of earth’s poles (North and South)  Polaris close to North celestial pole Celestial Equator  Circle around celestial sphere directly above earth’s real equator Note:  Earth rotates around axis from WEST to EAST o To someone standing on earth’s surface, celestial sphere appears to rotate from EAST to WEST Declination: angular distance from celestial equator  Positive if north, negative if south  The latitude  -90 to +90 Right ascension: angle measured eastward from vernal equinox  The longitude  Measured in hours from 0-24  Tells us what time it crosses the meridian  If right ascension is bigger, that star crosses meridian later  Angular distance from horizon to north celestial pole always = latitude o So latitude 40 degrees north means:  North celestial pole 50 degrees from zenith  Celestial pole is 40 degrees away from horizon CIRCUMPOLAR STARS  Never sets, always above the horizon (or never rises)  Declinations > 90-latitude Daily Cycles  Sun rises East sets West  Earth rotates Eastwards o Causes stars, sun and moon to move westward in the sky  Terminator: great circle separating night side from light side of any planet or satellite o Standing at earth’s terminator: either sunrise or sunset Yearly Cycles SEASONS  Throughout the year, the sun moves eastward among stars following the ecliptic o Apparent path around the celestial sphere, of the sun among stars o Some constellations visible at a given time of night will change during the year (except circumpolar ones) Su
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