ASTR 170B2 Lecture Notes - Lecture 4: Apparent Retrograde Motion, Aswan, Hipparcos
1 Oct 2017
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Document Summary
Angular size of the sun is same size as moon. Planets usually move slightly eastward from night to night relative to the stars. However during certain periods, planets move westward (backwards) relative to the stars for about a week (apparent retrograde motion) Apparent retrograde motion is straightforward for us to explain because we know the earth is not at the center of the solar system. Many ancient greeks believed that the earth was at the center (of everything) and the sun and planets orbited the earth. They also did not believe that the stars were as far away as they really are, or that the earth could be moving. These notions seemed contrary to their daily experiences. Stellar parallax is another form of apparent motion relative to the distant stars in the background. Parsec: (that other unit of distance commonly used in astronomy) the distance to an object that has a parallax angle of 1 arc second.
