Discovery of Precession

From the fourth to the second millennia BC, Taurus contained the Sun on the first day of spring. The position of the Sun against the background of fixed stars on the first day of spring is the vernal equinox. However, in the time of Hipparchos (2nd century BC), the Sun was in Aries at the vernal equinox. Its position then became known as "The First Point of Aries." Hipparchos discovered that the equinoctial point had moved westward from its earlier location in Taurus, sliding along the ecliptic, in a so-called "precession of the equinoxes."