ASTR 101 Lecture 4

ASTR 101 Lecture 4 - Geocentric Models of the Solar System...

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Geocentric Models of the Solar System Basic Motions Stars appear to circle a fixed point each sidereal day, from E to W The Sun and Moon circle more or less with the stars, but take longer to complete the circle The result is that the Sun and Moon appear further E each day with respect to the stars The Sun and Moon also move North and South as they migrate eastward. Similarly, the planets usually appear further E each day, but occasionally appear further W (i.e. they circle the fixed point in less than one sidereal day) These occasional retrograde motions always occur at opposition for the planets Mars, Jupiter, and Saturn, and after maximum eastward elongation for Mercury and Venus. The planets do not move the same angular distance each day. (The phases of the moon are due to differing illumination) (The seasons are due to the changing North-South position of the sun) (By assuming the planets all move with the same speed, their relative angular speeds give relative distances correctly)
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Evidence for “Fixed Earth” 1. We throw something up, it comes back down in the same place (Aristotle taught that motion must be caused for earthly objects, and with no cause the motion of the object would stop, while the natural motion of the heavenly sphere would continue) 2. The stars do not show visible parallax (changing apparent position due to our changing point of view)
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Other Aristotelian Premises Uniform circular motion Perfectly spherical bodies
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ASTR 101 Lecture 4 - Geocentric Models of the Solar System...

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