Question

What was the major Greek contribution to astronomy?

Answer 1

Not only were the Greeks responsible for our current knowledge and theories of haleo-centric (may be thinking of the wrong word here- the theory that says the planets revolve around the sun) theory, but they also hypothesized first about the planets and stars being made from actual terrestrial substance such as earth and water rather than them being gods. for a full summary, look here: http://astro.wsu.edu/worthey/astro/html/lec-greeks.html

Answer 2

Not much. At best I'd say Greek philosophy in general made the contribution that the motions of the planets and stars could probably be understood by the same set of principles or laws that governed events on earth. But those principles themselves, e.g. as stated by Aristotle, were badly mistaken, and lacked good empirical grounding. The sound measurement of star and planet positions mostly occured in the Arab near east, in Babylon and later in Alexandria (which is where Ptolemy Almagest lived).