In the language of science, an
equinox is either of two points on the celestial sphere where the ecliptic and the celestial equator intersect.
For the rest of us, it's one of two times a year when the Sun crosses the
equator, and the day and night are of approximately equal length.
At the autumnal equinox (Sept 22, 2008; 11:44:18 A.M.
EDT), the Sun appears to cross the celestial equator, from north to south;
this marks the beginning of autumn in the Northern Hemisphere.
The vernal equinox, also known as "the first
point of Aries," is the point at which the Sun appears to cross the
celestial equator from south to north. This occurs about March 21,
marking the beginning of spring in the Northern Hemisphere.
On the
date of either equinox, the Sun is above the equator and night and day are
of approximately equal length; the word equinox is often used to
refer to either of these dates.
The equinoxes are not fixed points on the celestial
sphere but move westward along the ecliptic, passing through all the
constellations of the zodiac in 26,000
years. This motion is called the precession of
the equinoxes. The vernal equinox is a reference point in the equatorial coordinate system.
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