equinox

The equinox is defined as a day that occurs twice per year when the sun crosses the equator and the night and day are the same length.

(noun)

A day in March that is the beginning of spring and a day in September that is the beginning of fall, are examples of the equinox.

See equinox in Webster's New World College Dictionary

noun

the time when the sun in its apparent annual movement along the ecliptic crosses the celestial equator, making night and day of equal length in all parts of the earth: in the Northern Hemisphere the occurs about March 21 and marks the beginning of spring, and the occurs about September 22 and marks the beginning of autumn
either of the two points on the celestial sphere where the sun's path crosses the celestial equator
also called equinoctial point

Origin: ME < OFr equinoxe < ML aequinoxium < L aequinoctium < aequus (see equal) + nox, night

See equinox in American Heritage Dictionary 4

noun

Either of two points on the celestial sphere at which the ecliptic intersects the celestial equator.
Either of the two times during a year when the sun crosses the celestial equator and when the length of day and night are approximately equal; the vernal equinox or the autumnal equinox.

Origin:

Origin: Middle English

Origin: , from Old French equinoxe

Origin: , from Medieval Latin aequinoxium

Origin: , from Latin aequinoctium

Origin: : aequi-, equi-

Origin: + nox, noct-, night; see nek^w-t- in Indo-European roots

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