Equinox - Wikipedia, the free encyclopedia:
An equinox occurs twice a year (around 20 March and 22 September), when the plane of Earth's equator passes the center of the Sun. At this time the tilt of the Earth's axis is inclined neither away from nor towards the Sun. The term equinox can also be used in a broader sense, meaning the date when such a passage happens. The name "equinox" is derived from the Latin aequus (equal) and nox (night), because around the equinox, night and day are about equal length.
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