When does fall officially begin? A visual guide to the autumnal equinox

This story was updated to add new information.

Farewell, flip-flops. And hello pumpkin spice.

Our long, hot summer finally comes to an end on Sunday at 8:48 a.m. EDT with the autumnal equinox, which marks the beginning of fall here in the Northern Hemisphere.

At a precise moment each September, usually on the 22nd or 23rd, the sun is directly above the equator, marking the autumnal equinox in the Northern Hemisphere.

South of the equator in the Southern Hemisphere, it's known as the vernal or spring equinox and marks the beginning of spring.

The Earth during the autumnal equinox

During the autumnal equinox, the Earth's axis is tilted neither toward nor away from the sun, resulting in a "nearly" equal amount of daylight and darkness at all latitudes, according to the National Weather Service .

The word "equinox" comes from the Latin words "aequalis" and "nox," meaning "equal night." On the autumnal (and the spring) equinox, day and night are roughly 12 hours long each in most of the world.

More: Druids and pagans gather at Stonehenge for fall equinox

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Seasons and the Earth's tilt

Earth has seasons because of its tilted axis, which is at 23.5 degrees to its orbit. As the planet revolves around the sun, the axis points in the same direction. On Sept. 22, day and night are roughly equal in length.

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What happens to daylight following the autumnal equinox?

For the next three months, until the winter solstice in December, the amount of daylight will continue to shrink each day.

The fall equinox has many effects, including:

• Agricultural industry : To ensure food storage for the winter, farmers gather crops according to the timing of the fall equinox, which traditionally marks the beginning of the harvest season.
• Changes in animal behavior: To survive the upcoming winter, many animal species adapt their feeding patterns, migrate, or both in reaction to the equinox's changing light conditions.
• Human health: Our biological circadian rhythm is influenced by daylight variations, and this can result in disruptions to our sleep and mood patterns. Seasonal Affective Disorder is a possible condition for some persons (SAD).

SOURCE NASA, National Weather Service, NOAA, HowStuffWorks.com and Sciencefocus.com

This article originally appeared on USA TODAY: When does fall officially begin? A visual guide to the autumnal equinox