This could be seen in the times of Thales of Miletus (ca. 624 to 547 B.C.).  He was the very first Greek astronomer who chronicled that he had measured the size of the sun and the moon in an approximate one and a half degree, gazing from the earth.  Thales was believed as well that he built up the relations of the same triangle which he used as a mean to rationalize relations in the cosmos and demonstrated that the ratio of two sides is the side for similar triangles. 

During the Hellenistic world, Greek language had become the lingua franca of scholars while pursuing the conquests of the Greek king named Alexander the Great of Macedon.  Alexander the Great generated one of the largest empires in ancient history.  This is also the time wherein Greek and foreign astronomers worked hand in hand.  They studied the Greek tradition at the Museum and the Library of Alexandria in Ptolemaic Egypt together.  The pre-Hellenistic phase is classified as the classical Greek astronomy.

In the new phase of astronomy, Eudoxus was spotted as the first to establish a model.  This model is according to what seems to be true complex motion of the heavenly bodies that indeed resulted from simple circular motion.  He then built an observatory in Cnidus upon observing the star called Canopus.

The word 'planet' in the early Greek astronomy comes from the Greek term planetes which means wanderer as astronomers of the Ancient Greek observed how definite lights moved above the sky in accordance to the other stars. 

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