Chapter 21 Celestial Motion, Chapter 22 Moon r ical orbits around Sun 2. of stars, actually unrelated areas—such as Orion orthern sky 3. uator, in degrees N and S estial equator on the March 4. Mons on axis one complete rotation about 24 hours und Polaris daily out four minutes less iv. tories use sidereal day b. Rem from Sun curs about January 3 un, which varies r ii. Earth’s axis of rotation is inclined to our orbital plane around Sun l equator s in greater heating of b. e facing Sun results in less heating of c. result of Earth being closer to Sun 3. Sun apuator (23.5 ) on the Solstice: iii. Sun nst star backdrop lled ECLIPTIC t same plane of Earth around 1. Solar System a. Sun is the centeb. Planets have elliptc. Orbits function of inertia and gravity Constellations a. Apparent groupsb. 88 recognized divide sky into units to identifyc. Many bright stars have proper names—Sirius, Arcturus brightest in nPosition in sky also divided by geometry: celestial poles and equator extended from Earth’s poles and equator a. declination—from the eqb. right ascension—rising from where Sun crosses celequinox, in hours of Earth turning tion of Earth a. Rotation—turi. Turn on axis pointing at Polaris—Big Dipper ‘rotates’ aroii. Mean solar day—for Sun to get to High Noon again iii. Sidereal day—for star to get to same sky position: abthan mean solar day Astronomical observavolution—orbit around Sun i. Average of 150 million k1. Perihelion—147 million km; Oc2. Aphelion—152 million km; Occurs about July 4 3. This is a result of Earth’s elliptical orbit around Sfrom closest to a circle to about 5% from a circle in a 100,000 yeacycle 1. Results in the plane of the ecliptic at 23.5o angle to celestia2. Tilt of Earth’s axis results in seasons we have a. rotation with north pole facing Sun resultnorthern hemisphere rotation with south polnorthern hemisphere seasons are NOT the (notice distance vs. northern hemisphere seasons) pears to cross celestial equator on the Equinox: ~22 March and September o4. Sun furthest from celestial eq ~ 21 June and December appears to be displaced agai1. about 1 degree/day 2. path through stars ca3. planets and moon have orbits in abouSun, so they travel near the ecliptic also ES 104 2007.1002.CelestialMotionMoon Page 1 of 3c. Precession is the wobble of Earth’s axial tilt i. Slowly changing position in the sky—full circle 28,000 years 1. as axis changes position, it will bring seasonal change to differing coincidence with perihelion and aphelion 2. in 14,000 years, June solstice will occur nearer to perihelion, warming northern hemisphere more ii. Angle varies a slight amount also, between 21.5o and 24.5o—in a 41,000 year cycle. iii. These Earth-Sun variations can affect overall Earth temperature: see http://www.homepage.montana.edu/~geol445/hyperglac/time1/milankov.htm for a detailed explanation and competing hypotheses of Earth’s reaction to these variations 5. Motions of the Earth-Moon system a. Moon has an elliptical orbit around Earth i. 6% variation in distance throughout its cycle—it is on average 384,401 km ii. Orbit accounts for phases of Moon, and eclipses of Moon and Sun b. Phases of Moon— i. NewÆcrescentÆ1/4 MoonÆFullÆ3/4 MoonÆcrescentÆNew ii. Waxing for two weeks: greater amount illuminated each night iii. Waning for two weeks: lesser amount illuminated each night iv. Sunlight is reflected off of Moon’s surface 1. when Moon is opposite Sun, it is a full disc 2. when Moon is between Earth and Sun, it is a crescent, or not illuminated (New) 3. a Full Moon rises at sunset, and sets at sunrise, as a result of it orbital position to be shown as Full c. Lunar Motions i. It takes Moon 29 ½ days to come to the same position relative to the Sun—‘Synodic Month’: apparent period lengthened due to Earth’s orbit of Sun ii. However since Earth-Moon system has progressed 1/12 of the way around the Sun orbit, the period for Moon to go exactly all the way around Earth needs to be compared to a distant star. 1. it takes 27 1/3 days to go around Earth 2. ‘sidereal month’ 3. Moon also rotates on its axis, once every 27 1/3 days a. The same side of Moon always faces Earth b. Days and nights last two weeks on Moon c. Lack of moisture and atmosphere allow temperatures to vary widely during these extraordinarily long days and nights i. 127o C in day ii. -173o in night d. Eclipses—shadow effects of Moon and Earth i. Moon’s orbit is inclined about 5o to Earth’s orbit around Sun ii. Lunar eclipse occurs when Earth’s shadow falls on Moon 1. Earth is between Sun and Moon 2. occurs only when Moon is full 3. Moon is still visible because of some bending of light around Earth ES 104 2007.1002.CelestialMotionMoon Page 2 of 3iii. Solar eclipse occurs when Moon’s shadow falls upon Earth 1. Moon is between Earth and Sun 2. only occurs when Moon is new 3. Total eclipse is within the 275 km wide umbra a. Lasts at most for 7 minutes in any area b. Total eclipses are rare: next one in August 2017 4. partial eclipse over larger area in penumbra iv. Earth usually ‘misses’ Moon’s shadow, so on average, there are four eclipses per year: two lunar, and two solar 6. Moon a. Earth’s only natural satellite i. Large in reference to Earth, compared to other natural satellites of other planets 1. about ¼ of Earth’s diameter 2. 3475 km ii. Density similar to Earth’s mantle material, small iron core b. Surface—not protected by atmosphere i. Craters 1. impact of meteoroids a. ejecta b. rays 2. more common in early part of Moon’s history ii. lunar highlands—most of Moon’s surface, all of back side 1. original surface: intensely cratered 2. low-iron content compared to maria iii. maria—plural of mare 1. high iron content lowlands 2. younger than lunar highlands 3. created by large asteroid impact a. caused sub-crustal melting and basalt flows b. similar to Columbia Plateau basalts iv. lunar regolith 1. soil-like surface produced by numerous meteoroid impacts 2. fine dust, glass beads, breccia, igneous rock 7. Lunar history a. Earth impacted by large asteroid about 4.5 billion years ago i. Caused part of Earth to be ejected into orbit around Earth ii. Dust accreted into lunar body 1. Gravitational contraction caused melting and formation of crust, mantle and core. 2. original surface was the present
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