The Phenomena appears to be based on two prose works-Phenomena and Enoptron (Ἔνοπτρον, "Mirror", presumably a descriptive image of the heavens)-by Eudoxus of Cnidus, written about a century earlier.
The purpose of the Phenomena is to give an introduction to the constellations, with the rules for their risings and settings; and of the circles of the sphere, amongst which the Milky Way is reckoned.