Perhaps the oldest of the sciences, Astronomy originated by observing the regularity of celestial movements. The earliest astronomers were priests who were often able to use the predictability of events to maintain their power. Ancient Greek students of Astronomy understood that the Sun was the center of the Solar System, made reasonable estimates for the size and distance of the Sun and Moon, and made a close estimate of the size of the Earth. These facts not rediscovered in the West for over one thousand years. Much later, Astronomy became important in timekeeping and Navigation.