equinox
the two days of the year on which neither hemisphere is tilted toward or away from the sun
An equinox is an astronomical event that occurs twice a year when the tilt of the Earth’s axis is inclined neither away from nor towards the Sun, resulting in a nearly equal duration of daylight and darkness at all latitudes. Specifically, during the equinox, the Sun appears to cross the celestial equator, an imaginary line in the sky that is directly above the Earth’s equator.
There are two types of equinox: the vernal (or spring) equinox and the autumnal (or fall) equinox. The vernal equinox occurs around March 20 or 21, marking the beginning of spring in the Northern Hemisphere and the beginning of fall in the Southern Hemisphere. The autumnal equinox occurs around September 22 or 23, marking the beginning of fall in the Northern Hemisphere and the beginning of spring in the Southern Hemisphere.
During the equinox, the amount of daylight and darkness is almost equal at all latitudes. However, the precise moment of an equinox varies from year to year due to differences in the length of a tropical year (the time it takes the Earth to make one complete orbit around the Sun), and can occur anytime between March 19 and 22 for the vernal equinox, and between September 21 and 24 for the autumnal equinox.
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