Aside from the myths, the ancient Greeks were interested in rational explanations for the celestial phenomena they observed. They were influenced by Egyptian & especially Babylonian astronomy: Babylonian astronomers could predict the planets' motions with extraordinary accuracy.
The 2nd century BCE astronomer Hipparchos wanted to achieve the same level of accuracy as the Babylonians.
Hipparchos based his improved model for the Sun on observations of the equinoxes, which explained both changes in the speed of the Sun and differences in the lengths of the seasons. He is credited with the discovery of a phenomenon called precession of the equinoxes. According to Roman astronomer Claudius Ptolemaeus, Hipparchos measured the longitude of several bright stars and compared his measurements with data from his predecessors.