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autumnal equinox

au·tum·nal e·qui·nox


noun 
Definition:
 
1. time when sun crosses equatorial plane: the first day of autumn, when the Sun crosses the plane of the Earth's equator and makes day and night approximately of equal length. It occurs about September 22 in the northern hemisphere and March 21 in the southern hemisphere.

2. sun's path crossing celestial equator: the position of the Sun during the autumnal equinox

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