Measures and Units:
First Point of Aries
astronomy The celestial point on the extended line passing from Earth through the Sun at the moment of the northern spring equinox, and the accepted starting moment of the tropical year and of the zodiac (q.v. for further explanation), hence the zero point for expressing right ascension and celestial longitude (see latitude). It is a slowly moving point relative to the stars, originally in the star pattern Aries, which in classical times signified the opening of the year that began at that equinox.