Aristarchos' discovery of precession
Our knowledge of Aristarchos' discovery of precession is based on the existence of two ancient Greek manuscripts preserved in the Vatican Library that convey Aristarchos' values for the length of the sidereal and tropical years. The difference between these two values (as well as their accuracy) gives rise to the suspicion that he might have been aware of the phenomenon called precession of the equinoxes. The suspicion is confirmed when we consider that the value for the rate of precession that can be derived from Aristarchos' different year lengths is the same as that later adopted by (and wrongfully ascribed to) Hipparchos.