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vernal equinox (s) (noun)
, vernal equinoxes (pl)
The point at which the sun's path crosses the celestial equator moving from south to north: The vernal equinox
marks the zero point in both the equatorial and ecliptic coordinate systems; horizontal angular distances are measured eastward from this point.
The vernal equinox occurs on March 20 or 21 and the autumnal equinox on September 22 or 23, marking the beginning of spring and autumn, respectively, in the Northern Hemisphere (and the reverse in the Southern Hemisphere).
The term "equinox" comes from Latin meaning "equal night" because the days on which an equinox falls have about equal periods of sunlight and darkness.
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The equinox, or celestial equator, which occurs about March 21.