Who were the first great astronomers?

Astronomical observation begins with the early civilizations of Mesopotamia, where prominent constellations are recognized and named soon after 3000 BC.

Babylonians
The sky-watchers of Mesopotamia identify the five wandering stars, which with the sun and moon form the seven original ‘planets’ (Greek for ‘wanderers’).
Within Mesopotamia the Babylonians, flourishing from the 18th century BC, are considered to be the first great astronomers. The minutes and seconds of modern astronomical measurement derive from their number system.

Greeks
From the 6th century BC the Greeks make significant advances in astronomy: their analytical approach to the heavens leads to early insights of great brilliance.
They are first to produce an astronomical theory in which a circular earth revolves on its own axis as well as moving in an orbit. The theory derives in part from the need to locate the great fire which they believe fuels the universe.