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The heliocentric model marked the beginning of modern astronomy. But what were the reasons which led Copernicus to think that the earth orbited the sun. I've read that his model was based on observations made by some other astronomer. Can anyone describe how he got this idea?

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    $\begingroup$ The podcast Astronomycast recently discussed Copernicus and heliocentrism in a easygoing format. Heliocentrism was not a new idea, and Copernicus version of it was more complicated than the Ptolemaian geocentric model, and it didn't match observations any better! (Because orbits are not circular, they are elliptic which it took Keppler to realize based on the meticulous observations by Tycho Brahe) youtube.com/watch?v=Z4JQKd_LbWE $\endgroup$ – LocalFluff Apr 14 '14 at 8:57
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    $\begingroup$ You might find Aristarchus of Samos interesting as well. $\endgroup$ – called2voyage Apr 14 '14 at 14:12
  • $\begingroup$ I'd like to share this entertaining dialogue: -Why did people think that the Sun orbits Earth? -Well, of course because it looks as if it does! -So, how would it have looked if instead the Earth orbits the Sun? (which it DOES and which is how it actually LOOKS!) $\endgroup$ – LocalFluff Apr 16 '14 at 10:08
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The heliocentric model is historically thought first by Aristarchus. He eventually gave up on the idea due to not being able to give evidence for a concept called stellar parallax: If earth orbits the sun then we should see the closer stars shift in relation to the other stars (strong evidence for this didn't come until Galileo).

Copernicus became to accept the heliocentric model given by Aristarchus due to his study of the Ptolemaic model. His studies showed that planetary motion in a geocentric model was inaccurate.

His model, though, was still based on the fact that "heavenly objects" moved in complete circles. He had to add much complexity to account for this fact which is why the heliocentric model was still not widely accepted. Side note, it was Johannes Kepler, over 100 years later, that claimed that planets move in ellipses rather than circles.

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  • $\begingroup$ Nice answer. Can you please share some link which describes the problem of stellar parallax in detail? $\endgroup$ – Yashbhatt Apr 17 '14 at 16:42
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    $\begingroup$ @Yashbhatt I don't know if you have already found a link, but the Wikipedia article about stellar parallax is pretty good. $\endgroup$ – Padarom Apr 24 '14 at 9:26
  • $\begingroup$ Your answer could be interpreted as claiming that Copernicus achieved a higher precision than Ptolemy. I do not think this is accurate. As you pointed out, he needed to add epicycles to his model too so as to make up for the fact that orbits are ellipses and not circles. Fair enough, his ellipses were smaller than Ptolemy's, as it should be obvious, since at least he got out of the way a big mismatch! $\endgroup$ – user18466 Sep 29 '17 at 11:51

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