It is the moment when the Sun, throughout its apparent annual movement, crosses the plane of the celestial equator.
This happens twice a year: March 21, Spring Equinox and September 23, Autumn Equinox in the Northern Hemisphere and vice versa in the South. On these two dates, the duration of the day is equal to that of the night for all places on Earth.
The equinoctial situation would be perpetual if the plane of the Earth's orbit coincided with that of Ecuador; however, as is known, there is a lag of 23º 27 '. The Spring Equinox is also known as the "first point of Aries", and the autumn one "first point of Libra".
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