Page 1 of 5 1. Age of Reason a. Nicolaus Copernicus 1473-1543 i. Commenteriolus manuscript circulated from 1512 1. unpublished 2. Heliocentric hypothesis ii. ‘On the Revolutions of the Planets’ published year of his death 1. incorporated some epicycles to account for lack of perfect agreement with perfectly circular orbits around Sun (did not consider ellipse shape of orbits) 2. publisher’s preface declared it ‘hypothesis’ to predict location of planets 3. not much notice taken by Church at that time 4. Galileo began “Copernican Crusade” in early 1600’s that lead to it being banned from 1632-1835 by Catholic Church b. Tycho Brahe 1546-1601 Danish astronomer i. Observations very exact thorough 1. 1572 Supernova in Cassiopeia 2. Comet in 1577 convinced him Aristotle’s hypothesis was wrong—celestial sphere not solid and unchanging ii. “great quadrant” and corrections for refraction of atmosphere allowed him to plot very accurately 1. calculated Earth’s axial tilt to 1/100 of degree 2. tropical year length to one second accuracy iii. rejected Ptolemaic system, and Copernican system 1. Tychonian model a. Sun orbits Earth b. All other planets orbit Sun c. Earth is fixed—there is no stellar parallax iv. Took on assistant Johannes Kepler in 1600 c. Johannes Kepler 1571-1630 i. Used Tycho’s observations to formulate laws of planetary motion 1. Planets move in elliptical orbits around Sun 2. Planets sweep equal area in equal time period 3. distances of planets to Sun are proportional to period of orbit around Sun (p2=d3) ii. fanatic about finding ‘causes’ for natural observations—hindered him at times iii. Vigorous supporter of Galileo’s observations of the moons of JupiterPage 2 of 5 d. Galileo Galilei 1564-1642 i. Heard about lenses being used to magnify objects 1. created his own telescopes to 30 power—not the inventor! 2. looked at planets and Sun ii. Planetary observations 1. discovered planets are discs, not points 2. found Jupiter has moons a. implication that Earth not the only center of orbit b. disputes the argument that if Earth orbits Sun, then Moon would be left behind 3. Venus has phases, and this supports heliocentric hypothesis also. He noticed that when Venus is full, it appears smallest, because it is farther away than when it is crescent 4. Moon surface is cratered and mountainous 5. Sun has sunspots, that move around on Sun’s surface iii. Dialogue of the Great World Systems published in 1630, banned by Church, he was house-arrested for the rest of his life—exonerated in 1992 e. Isaac Newton 1642-1727 i. Formulated and tested the Law of Universal Gravitation—every body in the universe attracts every other body with a force proportional to their masses, and inversely proportional to the distance between them ii. Accounted for why Kepler’s laws worked iii. Also explains perturbations of orbits due to other bodies f. Foucault’s pendulum proved that Earth rotates on axis i. Pendulum continues to swing in same plane unless acted upon by outside influence ii. Pendulum set into motion changes apparent position over full day period iii. It doesn’t really swing in a changing plane, Earth is rotating under it. 2. Solar System a. Sun is the center b. Planets have elliptical orbits around Sun c. Orbits function of inertia and gravity 3. Constellations a. Apparent groups of stars, actually unrelated b. 88 recognized divide sky into units to identify areas—such as Orion c. Many bright stars have proper names—Sirius, Arcturus brightest in northern sky 4. Position in sky also divided by geometry: celestial poles and equator extended from Earth’s poles and equator a. declination—from the equator, in degrees N and S b. right ascension—rising from where Sun crosses celestial equator on the March equinox, in hours of Earth turningPage 3 of 5 5. Motion of Earth a. Rotation—turns on axis one complete rotation about 24 hours i. Turn on axis pointing at Polaris—Big Dipper ‘rotates’ around Polaris daily ii. Mean solar day—for Sun to get to High Noon again iii. Sidereal day—for star to get to same sky position: about four minutes less than mean solar day iv. Astronomical observatories use sidereal day b. Revolution—orbit around Sun i. Average of 150 million km from Sun 1. Perihelion—147 million km; Occurs about January 3 2. Aphelion—152 million km; Occurs about July 4 3. This is a result of Earth’s elliptical orbit around Sun, which varies from closest to a circle to about 5% from a circle in a 100,000 year cycle ii. Earth’s axis of rotation is inclined to our orbital plane around Sun 1. Results in the plane of the ecliptic at 23.5o angle to celestial equator 2. Tilt of Earth’s axis results in seasons we have a. rotation with north pole facing Sun results in greater heating of northern hemisphere b. rotation with south pole facing Sun results in less heating of northern hemisphere c. seasons are NOT the result of Earth being closer to Sun (notice distance vs. northern hemisphere seasons) 3. Sun appears to cross celestial equator on the Equinox: ~22 March and September 4. Sun furthest from celestial equator (23.5o) on the Solstice: ~ 21 June and December iii. Sun appears to be displaced against star backdrop 1. about 1 degree/day 2. path through stars called ECLIPTIC 3. planets and moon have orbits in about same plane of Earth around Sun, so they travel near the ecliptic also c. 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 variationsPage 4 of 5 6. 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
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