Another demonstration-- Earth was round: Eratosthenes (250 B.C) •the altitudes of stars change with latitude—Greeks sailed the •measured the seas! circumference of the •Erasthatenes (who?) first realized Earth--Used "the shadow in this several decades earlier the well" method. •At S, sun is straight overhead. •At A, sun is at an angle to zenith (about 7o) •Here, α is the same as β. •Proportion: AS/β = 360/(circumference of Earth).
Star Trails Above Mauna Kea Credit & Copyright: Peter Michaud (Gemini Observatory), AURA, NSF Star trails from the Oregon Star party: http://www.keteu.org/images/astro/osp-trails.web.jpg
Aristarchus of Samos (300 B.C.): But….Aristotle argued for the •"Heliocentric" arguments- determined relative . Geocentric Model distances of the Earth and the sun,
•Based on angles between Observed• no stellar parallaxes Earth Moon and Sun at Earth Moon and Sun at no• winds—shouldn’t 1000 km/hr various phases. wind blow as earth spins •From size of eclipse shadow, determined through it?
relative sizes of Earth spinning• Earth would hurl its moon…and sun! inhabitants into space. •Deduced: sun is 90 x •Deduced: sun is 90 x No• mechanism to keep it in a farther than moon, moon circular orbit. is 1/3 size of earth, sun is Click on image to play MPEG move on about 30 x as large. the web. • angles are harder to From: www-astronomy.mps.ohio-state.edu measure than it looks here! •Concluded earth must orbit much larger sun!
Main difficulty with Geocentric Ptolemy’s Geocentric Model Model: Retrograde motion •Plato, Eudoxes, Aristotle, and finally •Why do some celestial Ptolemy (100AD) objects move in developed geocentric backward loops? “systems” •“obvious” solution was •These used Deferents “wheels upon wheels” (large orbits) and •But how many wheels? epicycles (small orbits” •The trick was to use these to make accurate predictions of positions From: http://www.earth.uni.edu/~morgan/astro/course/retrograde_mars.jpg in the future •i.e. Mars will be near Antares in Scorpius on winter solstice…etc.
1 Dark Ages-Aristotelian beliefs Ptolemy’s was most accurate… Mandatory •Ptolemy (pronounced Tolemy) •The defeat of the Rome and was professor in Alexandria the empire by Germanic tribes during the Roman Era. (Aric the brave!) led to •A mathematician, physicist, withdrawal of roman troops astronomer, and foremost across Europe geographer of his era. •Infrastructure, commerce, •Created elaborate system of 88 education, and defense of cities http://www.coam.org.uk/Events/event2006/Dark%20Ages.jpg epicycles and deferents, not crumbled to dust…. precisely centered on earth, to •Highly divided Christian Europe model the motion of the was battling Vikings from the heavens. North, Mongols from the east, •Proved accurate in making and the Plague. predictions for over a thousand •The Church began to seize years power through various means…
http://www.newizv.ru/images/photos/other/20050126210812_4-VIKINGS.jpg
Copernicus (1473 - 1534) So was Copernicus or Ptolemy’s
• by 1400 the planetary positions were model correct? no longer predicted by the •Tycho Brahe, Johanes Kepler, and “almagest” • Copernicus Proposed all the Galileo were the greatest contributors following “fix”: to the debate. 1. Earth spins on its axis once every 23 •Brahe (pronounced Bray) was the last hrs, 56 min and greatest pre-telescopic 2. Earth and all known planets orbited astronomer. the sun in circular orbits with sun at center. •Brahe felt that better observations 3. distant stars were so far that no were needed . parallax could be seen . •interested in proving that the “Tyconic 4. Polar axis precessed every 26,000 Universe" was correct years. 5. All the above just a mathematical model to make accurate predictions…easier than updating the Ptolemaic model
Johannes Kepler (1571 - 1630) Kepler and Brahe...continued •Mathematician...sought out Brahe for his famous data, and was hired by Brahe to fit data to the tyconic •Brahe died and family wouldn't model universe. But…. release data after Brahe died •Kepler was trying to fit data to the (don’t ask how he died). : “Kepler model" Universe •Eventually Kepler "acquired" •the orbits of the six known planets Brahe's data and found that: fit into the largest spheres which •--the orbit of Mars just isn't a could be inscribed into the six circle! regular geometric solids •Plato was wrong! •--crazy by today's standards, but at •The door to a true least the orbits were centered on the understanding of the solar sun! system was now wide open! Brahe’s tombstone, from: www.nada.kth.se/~fred/tycho/tychotomb.jpg •Wrote: Harmony of the worlds relating music, geometry, astronomy
2 Kepler’s results Kepler’s third law • From Brahe’s data, Kepler 1. The cube of a planets semi- deduced three laws: major axis is proportional to 1. Planets orbit the sun in the square of its orbital Ellipses with the sun at one period a3 = T2 focus 2. a = semi-major axis –also 2. A line joining a planet and average distance from the sun sweeps out equal planet to sun (also written as areas in equal times d or r). Units are AU’s. 3. T = orbital period (also written as P). Units are years. These three relations are now known as Kepler's three laws. 4. Newton later used this A graph of semi-major axis cubed on discovery to develop and vertical axis, and orbital period squared on Extra for experts: x and y are the positions of the Earth with 0,0 at intersection of prove the law of universal horizontal axis. Clearly not a coincidence! major and minor axis. gravitation.
After Kepler Galileo(1564 -1642)
•an accurate description of the •Mathematician, physicist, Solar system astronomer •definitive proof that the Earth •If you take a course in either orbited the sun, and spun on its Area, Galileo’s name comes up axis (most of that due to often Galileo—stay tuned! •Heard about telescope and
Missing:• an explanation of Why built one to explore the heavens the planets orbited the sun and •Proved the Copernican model why Kepler's mysterious laws and developed physics to were true! NASA’s Project explain Earth could orbit sun, Kepler—to detect etc. terrestrial planets •Paid a price for his curiosity orbiting other stars!
What did Galileo observe with his Conclusion? telescope? •Venus had phases like the •Venus orbited Moon: sun: •Size always •Not the Earth changed with •Clearly not in phase keeping with the •At Crescent official Geocentric Phase Venus was model! always closest to •What else could Earth be wrong? •At Full Phase, Venus was always The Trial of Galileo farthest from: http://astro.ysc.go.jp/venus-phases.jpg
3 Jupiter has moons that Orbit Jupiter( not the Sun) The Sun has spots
photo from: http://www.ladeltascience.com/astronomy/kisatchie04/ju piterand moons.jpg
The Galilean http://solar.physics.montana.edu/canfield/ssu_images/sunspots.gif Moons
the Moon has craters Plato to Galileo: review
Write down one important idea or contribution •So the Moon was for each of the following people. One a world like the sentence each. Give approximate date as Earth… well. •Not a perfect 1.Aristotle geometric object 2.Plato as Plato taught. 3.Brahe 4.Copernicus 5.Galileo 6.Kepler 7.Ptolemy
Important terms—define in one or two sentences • Parallax shift • Sidereal day and month • Epicycle • Geocentric • Heliocentric • Precession cycle (include period) • Retrograde motion • Centripetal acceleration • Kepler’s laws • Phases of Venus
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