On the equinox there is nearly exactly 12 hours of daylight and night — the day and night are perfectly balanced. On these days the sun rises almost due East and sets due West. The Spring equinox and Fall equinox are the transition between the seasons of Winter to Spring or Summer to Fall, depending on if your days are getting longer or shorter. The other seasons transition at the solstices.
Technically, the equinox is the instant “the plane of Earth’s equator passes through the center of the Sun.” (wikipedia) This usually happens around the 20 March and 23 September.
The equinox exists due to the tilt of the Earth. See Seasons
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