ASTRO-110 Lecture Notes - Lecture 2: Angular Distance, Ellipse, Atomism
9 Sep 2020
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Proposed by aristarchus in about 260 b. c. Made measurements that convinced him the sun was much larger than earth. Possibly concluded that it was more natural for the smaller earth to orbit the larger sun. Recognized that a sun-centred system offers a much more natural explanation of apparent retrograde motion. If the earth revolved around the sun, stars on the celestial sphere would appear to change angular separation, depending on how close to them the. Either stars are much further away than the greeks imagined. Stars are much further away than greeks imagined. Stellar parallax is easily observed with modern telescopes (annual shifts in stellar position) Planets usually move slightly eastward from night to night relative to the stars. But sometimes they go westward relative to the stars for a few weeks: Retrograde motion could easily be explained by having the sphere carrying a planet slow to stop, reverse its motion, and then resume spinning forward again.
