One of the Arab scientists’ major contributions was in astrolabes. Astrolabes were primarily invented by the ancient Greeks in 225 BCE by Apollonius based on the theories and the findings of Hipparchus. The main uses of astrolabes were to tell time during day or night, to identify the time of sunrise and sunset, and the length of the day, and to locate celestial objects in the sky. These uses were essential for astronomers, astrologers, and of course navigators.