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Astronomy 101The Copernican Revolution
Early ObservationsPeople have been looking up and working toexplain the motion of cosmos for millennia.Early examples:Eratosthenes measures the size of the EarthPtolemaic (geocentric) system2
Eratosthenes and the Size of the EarthEratosthenes noticed that at noon on thesummer solstice theSun did not cast a shadowinAlexandria.BUT on the summer solstice theSun did cast ashadowinSyene.He measured the angle of the Sun at that time inSyene to be 7ofrom overhead.3
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Eratosthenes and the Size of the EarthHis conclusion: because 7ois 1/50 of a circle,the distance from Alexandria and Syene mustbe 1/50 of the whole circumference of theEarth.Alexandria to Syene = 5000 stadesCalculation:Modern value = 40,000 km.5circumference=50×5000 stades=250,000 stades=42,000 km
Early Philosophical Ideas of the HeavensEarly ideas about astronomy and the Earth’splace in the Solar System were deeply rooted inreligious and philosophical ideas:The heavens and heavenly bodies representperfection and thus are perfect.The heavens are unchanging.The circle is the perfect shape so all heavenlymotions must be circular.Earth is at the center and is immovable.6
Early Model of the Solar SystemEarth is at the center of the Universe(geocentric)Stars reside Celestial sphereCelestial sphere rotates about axis in 23h56mSun follows path called ecliptic going aroundEarth once in one yearAccounts formostobservations and agreeswith intuition about the Earth not moving.7
Axis ofRotationNorth StarCelestial SphereHow the Greeks modeled the heavens
The Complication: Wandering StarsPlanets were calledwandering stars(planete) because theymoved across the sky at a different rate than the stars.Retrograde motion: at times, a planet would appear tostop, go backwards, and start moving forward again.WestEastSouthMotionof Marsovermanymonths:.youtube.com/watch?v=1E3vlf-Esqk
Observations of the PlanetsOnly 5 planets are visible to the naked eye:Mercury, Venus, Mars, Jupiter and Saturn.All the planets exhibit retrograde motion.A successful model of the Solar System mustaccount for the retrograde motion of the planets.
Ptolemaic (Geocentric) ModelPtolemy came up with the firstsophisticatedgeocentricmodel toexplain the Solar System.Earth at the center, celestial objectsorbit the Earth in perfect circles.Tried to account for theretrograde motion of theplanets by placing planetsin circular orbits on orbits(epicycles)Ptolemy (100 – 170 A.D.)
Ptolemaic System and Retrograde Motion“Epicycles” to explain whysome planets appeared to gobackwards.Epicycle/lec03_files/image003.gif
“Epicycle upon Epicycle”In order to make theory agree with observations,Ptolemy just kept adding more and moreepicycles.

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Term
Winter
Professor
bennert
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