The 2017 Autumnal Equinox is on Friday, September 22. The 2018 autumn equinox will also fall on September 22. The Spring Equinox took place on Monday, March 20.
Equinoxes occur when the Sun is exactly above a planet’s equator. This causes the planet’s terminator, or dividing line between daytime and nighttime areas, to pass through the planet’s north and south poles.
Such an angle causes every area of the planet to experience the same amount of sunshine and darkness for one day twice a year.
The biannual phenomena are called the vernal and autumnal equinoxes. Along with a planet’s two annual solstices, the equinoxes mark the change in seasons.
In the Earth’s Northern Hemisphere, for example, the vernal equinox cues the beginning of spring around March 21 and the autumnal equinox indicates the beginning of fall around September 23. These seasonal cues are reversed in the Southern Hemisphere.