ASTR 101 Lecture 3

# ASTR 101 Lecture 3 - Review You should have read chapters 1...

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Review You should have read chapters 1 and 2 of the text by now So far we have seen: Stars appear to circle a fixed point in the sky every 23h 56m 4s (sidereal day) The moon and sun move eastward with respect to the stars, which causes them to rise (and set) slightly later each day The planets generally move eastward each day with respect to the stars, but occasionally move westward (retrograde) The phases of the moon are related to the angle between the moon and sun in the sky The seasons occur because the yearly path of the sun takes it North and South of the celestial equator, and it illuminates the Earth's surface differently at different times

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Model Building and Mathematics Now we will stop collecting and memorizing facts about the motions of the heavenly spheres, and begin to make models for explaining them. We couldn't help doing some of this already just to categorize the motions, e.g. we pictured the bodies as attached to a transparent sphere that spun round the Earth once every sidereal day. We will extend this model to account for all the observed motions, but first we have to conquer some more basic concepts and review some math.
Angular Speed: The change in angular position with respect to time If you make an assumption about the motion of the planets, namely that they all move with the same speed, then ranking them by angular speed also puts them in order of distance. The assumption of same velocities is wrong, but not so wrong that it gives the wrong order. This allowed the ancient Greeks to correctly infer the relative distances to the planets. distance moved in 1 hour is

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## This note was uploaded on 09/29/2009 for the course ASTR 101 taught by Professor Christiansen during the Spring '08 term at UNC.

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ASTR 101 Lecture 3 - Review You should have read chapters 1...

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