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Equinoxes are astronomical events that occur twice a year, around 20 March and 22 September. The word itself has several related definitions. The oldest meaning is the day when daytime and night are of approximately equal duration. 

image: wikipedia

 The word equinox comes from this definition, derived from the Latin aequus (equal) and nox (night).

A geochron showing the daylight over Earth very near the Equinox. image: R.Conan-Davies

The equinox is not exactly the same as the day when period of daytime and night are of equal length for two reasons:

     Firstly, sunrise, which begins daytime, occurs when the top of the Sun's disk rises above the eastern horizon.  At that instant, the disk's center is still below the horizon. 

     ▪ Secondly, Earth's atmosphere refracts sunlight. As a result, an observer sees daylight before the first glimpse of the Sun's disk above the horizon.

To avoid this ambiguity, the word equilux is sometimes used to mean a day on which the periods of daylight and night are equal.