The First Point of Aries is the location of the vernal equinox, and is named for the constellation of Aries. It is one of the two points on the celestial sphere at which the celestial equator meets the ecliptic plane, the other being the First Point of Libra, located exactly 180° from it. Over its year-long journey through the constellations, the Sun crosses the celestial equator from south to north at the First Point of Aries, and from north to south at the First Point of Libra. The First Point of Aries is considered to be the celestial "prime meridian" from which right ascensions are calculated.