Equinox: From the Latin aequinoctium, equal night; occurs when the center of the Sun is directly over Earth's equator. The Sun crosses the equator twice each year, once at the vernal equinox when it enters 0° Aries and again at the autumnal equinox when it enters 0° Libra. The days and nights are of equal duration all over the world on equinoctial dates. The vernal equinox occurs on the first day of spring, the autumnal equinox on the first day of autumn.

 

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Equinox: The point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. In the Northern Hemisphere, the vernal equinox falls on or about March 20 . . . View Full Definition

Precession Of The Equinoxes: The gradual westward shift (about 50' each year) of the equinoctial points along the ecliptic due to the ecliptic due to the rotational movement of the poles of Earth's axis. This phenomena . . . View Full Definition

Autumn Equinox: The Autumn equinox and the spring equinox are the two days in the year when the hours of daylight and darkness are exactly equal. Celebrated on September 22 or 23.

Vernal Equinox: Taking place in the Northern Hemispheric spring, it is the point at which the ecliptic intersects the celestial equator. Days and nights are most nearly equal in duration. It falls on or abo . . . View Full Definition