September is the month of the autumnal equinox, which signifies the end of summer and the start of fall in the Northern Hemisphere.

On Sept. 22, the autumnal equinox, which signals the start of fall in the Northern Hemisphere, finally brings an end to our long, hot summer on Monday at 2:19 p.m. EDT. This happens at a precise moment each September, typically on the 22nd or 23rd, when the sun is directly above the equator.

According to the National Oceanic and Atmospheric Association, our planet will appear to have no tilt with respect to the sun. Earth's tilt aligns with its cycle around the sun twice a year, in March and September.

Those in the Southern Hemisphere, or south of the equator, will be experiencing the vernal or spring equinox, which marks the beginning of spring on Sept. 22.

Here's a visual guide to the equinox:

The Earth's orbit during the autumnal equinox

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 in most of the world.

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.

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Day and night aren't exactly 12 hours long on the day of the equinox because the Earth's atmosphere refracts, or bends, light in an optical illusion that results in more daylight. Because of this, the date when day and night are of exactly equal length is usually a few days after the autumnal equinox.

Seasons and the Earth's tilt

The tilt of Earth's axis, which is 23.5 degrees parallel to its orbit, causes seasonal changes, according to the NASA Earth Observatory. The axis of the planet points in the same direction as it spins around the sun.

Interesting fall equinox facts

On Sept. 22, the sun rises almost due east and sets nearly due west for most of the world, except for the north and south poles.

Although some people call the autumnal equinox the "official" start of fall, no administrative or political organization actually designates that.

In fact, though astronomers say summer ends Sept. 22, meteorologists and climatologists say summer already ended in our hemisphere on Aug. 31, the final day of the hottest months of the year.

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

Equinoxes – when day and night are roughly equal – occur in September and March and mark the astronomical beginning of autumn and spring in the Northern Hemisphere, respectively.

Solstices occur in December and June, which mark the beginning of astronomical winter and summer, respectively. The winter solstice occurs when the sun is at its lowest point in the sky, and the summer solstice is when the sun is at its highest point in the sky.

Darker days heading into winter

Because our hemisphere is exposed to the sun's rays less directly because of the Earth's tilt, days will become darker and shorter as the winter months approach.

The fall equinox affects many things on Earth, including:

  • 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, or SAD, is possible for some.
  • 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.
  • The 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.

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

This article originally appeared on USA TODAY: What is the autumn equinox? A planetary look at when the first day of fall arrives

Reporting by Janet Loehrke, USA TODAY / USA TODAY

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