On 11 March AD 509, a lunar occultation of Aldebaran was observed in Athens, Greece. English astronomer Edmund Halley studied the timing of this event, and in 1718 concluded that Aldebaran must have changed position since that time, moving several minutes of arc further to the north. This, as well as observations of the changing positions of stars Sirius and Arcturus, led to the discovery of proper motion. Based on present day observations, the position of Aldebaran has shifted 7′ in the last 2000 years; roughly a quarter the diameter of the full moon. Due to precession of the equinoxes, 5,000 years ago the equinox (Northern Hemisphere)|vernal equinox was close to Aldebaran.