The vernal equinox occurs around March 21st each year and is the moment in time when the Sun crosses the celestial equator. The Earth orbits the Sun once each year, but the Earth spins on its own axis which is tilted by 23 1/2 degrees. This means that, as seen from the surface of the Earth, during the course of a year the Sun's patch in the sky does not lie along the celestial equator. The celestial equator is the band of sky which you would see directly over your head if you were standing on the equator of the Earth looking up at the sky. The Sun's position during the year travels a path that is 23 1/2 degrees to the north of the celestial equator on or about June 21st, and 23 1/2 degrees south of this band on or about December 21st. At the equinoxes, the Sun is crossing the celestial equator on September and March 21st during the autumnal and vernal equinoxes.