An Etymological Dictionary of Astronomy and Astrophysics
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The point of intersection between the ecliptic and the celestial equator at which the Sun passes from south to north of the celestial equator during its apparent annual motion. The instant of this event. It occurs on March 20, 21 or rarely 19. At the vernal equinox, as with the → autumnal equinox, night and day are equal in length world over. Several thousands years ago the vernal equinox was in Aries, but because of precession it has now slid west into Pisces. Right ascension and celestial longitude are measured from the vernal equinox. Also known as spring equinox. → First Point of Aries.

→ vernal; → equinox.