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
Origin: ME < OFr equinoxe < ML aequinoxium < L aequinoctium < aequus (see equal) + nox, night
See equinox in American Heritage Dictionary 4
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 nekw-t- in Indo-European roots.
celestial sphere showing the positions of the autumnal and vernal equinoxes
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