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The Ancient Greek astronomers had quite an obsession over uniform circular motion; I was wondering if there was a logical reason for this. Did it develop through actual observations of the stars? Do they appear to move uniformly? If so, why?

I've read a few things online that attributes this belief mainly to "Pythagorean mysticism", saying that the Ancient Greeks simply looked for an idealistically satisfying description of celestial motions. This too makes me wonder though:

1.) Why uniform?

2.) Why circular?

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+1ed just to eliminate the downvote . – centralcharge Jul 28 '13 at 15:22
up vote 2 down vote accepted

You have to remember the ancient greeks didn't have telescopes to make close observations of stars. They also believed that the Earth was the centre of the universe. It took the uncommon genius of Copernicus to figure out that the sun was actually the centre of earth's orbit and not the other way around.

With that in mind, it's easy to see why they thought the star's moved in circular orbits. Since the stars move across the sky, and the Earth was known to be spherical, it could have been easily postulated that the stars moved in circular orbits around the earth.

As for the uniformity - They stars rose and set at (approximately) the same time everyday, and moved across the sky at a uniform rate (since earth is spinning at a uniform speed).

I'm not sure how they explained the changing in times of rising of the stars everyday, though.

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The changing rise/set times according to a Solar clock didn't pose a problem for anyone who modeled the Sun as also moving around the Earth at a slightly different rate than the stars. – Chris White Dec 29 '12 at 21:07
I'm not sure what you mean by the stars rising and setting :-P – ProSteve037 Dec 30 '12 at 0:56
Well, that's just terminology. Like the sun rises and sets, the stars also rise and set. If you track a particular star/constellation for a few hours at night you'll notice that it has moved across the sky, and if you track it for long enough you'll see that it eventually sets. – Kitchi Dec 30 '12 at 10:26
Ah that makes sense. Thank you! – ProSteve037 Dec 30 '12 at 20:33

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