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Every planet in the solar system has an equinox. Earth has two every year: vernal and autumnal. Learn about the 2017 autumnal equinox.

What is an Equinox? What are equinoxes? What causes these astronomical events? Find out how they influence the seasons and hours of daylight on each planet.

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 happen two times in a year when day and night are of equal length. Every planet in the solar system experiences equinoxes.

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.

Solar System 101 How many planets are in the solar system? How did it form in the Milky Way galaxy? Learn facts about the solar system's genesis, plus its planets, moons, and asteroids.

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.