First Point of Aries

One of the two points on the celestial sphere where the ecliptic and the celestial equator cross. It is used as the zero point for right ascension. Because of the effects of precession, the equator is slowly sliding around the ecliptic, taking about 25,800 years to make one revolution. Though the intersection point was in the constellation Aries several thousand years ago, it is now in Pisces, but the name has not been changed. The Sun is at the First Point of Aries on the northern vernal equinox, and the expression "vernal equinox" is also used to describe this place on the celestial sphere.