Ancient astronomy: The sky.
Astronomers call the apparent dome of the sky the celestial sphere. The ancients, unlike modern astronomers, conceived of the earth as being at the center of the sphere with various celestial objects moving from east to west because of the rotation of the celestial sphere. Ancient astronomers also noted the rotation of celestial objects about two fixed points, the north and south celestial poles. In order to facilitate the observation of the motions of celestial objects, ancient astronomers named constellations of stars and the more prominent individual stars.
Cyclic phenomena. Since ancient times, people have observed and tried to predict cyclic phenomena of the heavens. Ancient astronomers tracked the path of the sun across the sky and recognized seasonal events on the basis of the sun's position. Phases of the moon, lunar and solar eclipses, and configurations of the planets are common earth-sky relationships carefully observed by ancient astronomers.
Geocentric solar system. By the time of Aristotle and Eratosthenes, natural philosophers seemed to have recognized that the earth was a sphere. Nevertheless, they also believed that the earth was an immovable globe surrounded by the sphere of the heavens. Movements of celestial bodies such as planets could be explained by attaching them to a movable sphere placed between earth and the celestial sphere. However, to make Ptolemy's geocentric model of the solar system work - especially to explain the apparent retrograde motion of certain planets - required elaborate combinations of uniform circular motions. Despite the complexity of the Ptolemaic model, the system dominated astronomical thought in Arab culture and medieval Europe.
