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Lecture 9

AS101 Lecture 9

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Patrick Mc Graw

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Lecture 9 2/4/2013 12:30:00 PM Does the universe revolve around you? Ancient Astronomy:  France: Cave paintings from 15, 000 BC may suggest knowledge of lunar phases (29 dots) and some star patterns.  Ancient people of central Africa apparently used the orientation of the crescent moon to mark the seasonal. A carved bone shows pictographs of the crescent moon.  This helped them to determine when to plant crops.  England: Stonehenge. Alignments of the stones mark the rising and setting points of the sun at the solstices.  China: earliest known records of supernova explosion od a day start. Also recorded solar, lunar eclipses continuously from 4 BC.  Eratosthenes measured the earths circumference by observing the sun from two places on the same day of the year. o Measurements:  At noon on the first day of summer,  Syene: sun directly over head  Alexandria: sun at 7˚ from vertical (zenith)  Distance from Syene to Alexandria must be 7/360 of the circumference.  Other Greek astronomer figured out distances and size of moon an sun, using careful observations and geometrical reasoning.  Principles underlying ancient Greek astronomy: o The earth is fixed at the centre of the universe. o The heavens are perfect and unchanging. o Circles are the most perfect shape—heavenly objects more in circles.  A few Greek philosophers believed that the earth moved: o Pythagoras:  Earth, sun and all planets move around an invisible fire. o Aristarchus of Samos:  Earth moves around the sun. o But these ideas were not widely accepted.  Principles underlying ancient greek astronomy: o Basic picture of the heavens:  The earth is fixed at the centre of the universe.  Uniform circular motion:  Moon, sun, planets, and stars are all attached to sphered of circles that rotate around the earth.  Problem: simple circles Reason for believing the Earth is Stationary:  First of all, it feels solid and unmoving.  If the earth moves, we would expect the stars to show apparent motion (parallax) due to different viewing angles. Constellations ought to change their apparent shapes.  Some argued that if the earth moved, it would leave the moon behind. Parallax:  An apparent change of position caused by viewing from different places (and different angles)  The farther away an object is, the smaller the parallax. The Ptolemaic Model:  Explained retrograde motion with epicycles.  Each planet moves on a smaller circle, epicycle.  The centre of each epicycle moves on a larger circle, the deferent.  Centres of deferents are slightly displaces from earth.  Epicycles also explain changed in apparent brightness: planets appear brighter when they are closer.  In order of distance: o Moon o Mercury o Venus o Sun o Mars o Jupiter o Saturn o Fixed stars  Ptolemy’s model was able to describe the motions of the planets quite accurately. (although imperfections became more noticeable over centuries.)  In Europe and the Arab world, it was considered the correct description for more than 1000 years.  Some historians consider it the first
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