Greek Cosmology

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Physics, Engineering Physics and Astronomy
ASTR 101

Greek Cosmology • Eratosthenes discovered the size of Earth, Aristotle discovered the Earth was a few times larger than the moon by observing Earth's shadow during eclipses and Aristarchus determined the sun was much farther than the Moon and much larger than Earth  Logically one would assume if the Sun is so much bigger and farther it must be the center of the solar system, but this was not the case • Ptolemy was an accepted geocentric model of the solar system which stated that the Earth was the center of the universe and everything orbited around it in circular patterns (since circles kept occurring in nature like planet shape, sun shape, etc)  Orbits of the sun, planets, and moon are independent of one another and occur at uniform speed Problems with Greek Cosmology: • The movements of the sun and moon are not uniform as their speeds against background stars are not constant  Modern Understanding: the Earth does not orbit the sun at constant speed; neither does the Moon orbit the Earth at constant speed! The apparent motion of the sun across the field of remote stars is caused by our changing perspective as we orbit around it  Our orbit is not a perfect circle centred on the sun, and we move faster when we are close to the sun in January than when we are farther away from it, say in August • Mercury and Venus were seen very close to the sun during dawn or dusk and could never be seen overhead at night like a planet like Mars could  Greeks insisted that Venus and Mercury were only independent from the sun to an extent  Modern Understanding: The sun is the center of the solar system and because Mercury and Venus are inner planets, the Earth could never be positioned between the sun and them like it could with Mars, therefore they could only be seen during specific times and orientations • Changes in sizes of the sun and moon seen during varying eclipses show that they do not travel in circular orbits  Eclipses vary from total t
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