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Fall equinox 2021: Exact time, start of autumn, and more for the September event

The Sun will be exactly above Earth's equator, marking the start of autumn.

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On September 22, the Sun will cross the celestial equator and mark the beginning of a new season and the September 2021 fall equinox.

For that brief moment during the autumn equinox, night and day are almost exactly equal before the day begins to get shorter. The seasonal change is actually due to our planet’s tilt on its axis, and a time when the Northern and Southern Hemispheres essentially trade places.

Inverse breaks down everything you need to know about this annual event.

What exact time is the September 2021 fall equinox?

The 2021 fall equinox will take place on Wednesday, September 22, 2021 at 3:21 p.m. Eastern.

During that time, the Sun will be exactly above the Earth’s equator and will appear overhead at noon as seen from the equator. This takes place around the same time every year, where the Sun shines directly over the equator and distributes the same amount of light on either side of the planet.

For the Northern Hemisphere, this marks the beginning of shorter days and colder weather.

What happens during the fall equinox?

The Earth is tilted on its axis by about 23.4 degrees. During the autumn equinox, the Southern and Northern Hemisphere trade places in receiving the amount of light and warmth from the Sun. This results in the change of the seasons.

In June, the Northern Hemisphere is titled more towards the Sun while the Southern Hemisphere tilts away from the star, marking the summer solstice. In December, the Southern Hemisphere receives more sunlight.

The illustration shows the different points at which the summer and winter season take place, with the equinoxes marking the change of seasons.NASA/JPL-Caltech

What’s the difference between a solstice and an equinox?

Throughout the year, the region of the Earth directly beneath the Sun, known as the subsolar point, shifts along a north to south axis. After it reaches its northernmost point during the Summer Solstice in June, it begins to change direction towards the south.

The subsolar point reaches the southernmost point in December for the Winter Solstice. In the middle of its journey, it crosses the equator which marks the equinox. This only takes place twice during Earth’s orbit around the Sun, marking the spring and autumn seasons.

During that time, Earth’s Northern and Southern Hemispheres are receiving the same amount of light from the Sun.

What happens after the fall equinox?

After that time of perfect symmetry, things start to shift again.

December 21 will mark the beginning of winter for the Northern Hemisphere, and summer for the Southern Hemisphere.

When is the next equinox?

The next equinox, the Spring Equinox, will be on March 20, marking the changing of the seasons once more from shorter winter days to longer summer days in the Northern Hemisphere and vice versa.

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