An equinox is an astronomical event in which the plane of Earth's equator passes the center of the Sun, making night and day of approximately equal length all over the planet. The equinoxes are the only times when the solar terminator (the "edge" between night and day) is perpendicular to the equator. As a result, the northern and southern hemispheres are equally illuminated.
In other words, the equinoxes are the only times when the subsolar point is on the equator, meaning that the Sun is exactly overhead at a point on the equatorial line. Equinoxes occur twice a year, around 21 March and 23 September. The subsolar point crosses the equator moving northward at the March equinox and southward at the September equinox.
The equinoxes, along with solstices, are directly related to the seasons of the year. In the northern hemisphere, the vernal equinox (March) conventionally marks the beginning of spring in most cultures, and the autumnal equinox (September) marks the beginning of autumn. In the southern hemisphere, the vernal equinox occurs in September and the autumnal equinox in March.
^ United States Naval Observatory (2015-09-21). "Earth's Seasons: Equinoxes, Solstices, Perihelion, and Aphelion, 2000-2025". Retrieved 2015-12-09.
^ "Equinoxes". USNO Astronomical Information Center FAQ. Retrieved 4 September 2015.