When is the first day of fall? The autumnal equinox, explained

It's fall, ya'll!

Sweater weather is upon us. The beginning of the autumn season marks the end of warm summer days and the start of longer chilly nights.

This year, the autumnal equinox is on Sept. 22, 2024. But what does that actually mean?

What is the autumnal equinox?

As the Earth travels around the sun, it does so at an angle.

For most of the year, the Earth’s axis is tilted either toward or away from the sun. That means the sun’s warmth and light fall unequally on the northern and southern halves of the planet.

When the Northern Hemisphere is tilted toward the sun, that's called the summer solstice. This marks the first day of summer in the North and the first day of winter in the South.

One hundred and eighty days later, when the Southern Hemisphere is tilted toward the sun, it is called the winter solstice. This marks the first day of winter in the North and the first day of summer in the South.

The halfway points between solstices are the autumnal and vernal equinoxes. The word equinox comes from two Latin words meaning equal and night. That’s because on the equinox, day and night last almost the same amount of time — though one may get a few extra minutes, depending on where you are on the planet. This provides approximately 12 hours of day, followed by 12 hours of night.

When is the 2024 autumn equinox?

The fall — or autumnal — equinox can land on Sept. 22 or 23 depending on the year. This year it lands on Sunday at 8:44 a.m. ET/ 5:44 a.m. PT in the Northern Hemisphere.

Why are there two kinds of fall?

There are two different ways to carve up the year: meteorological and astronomical seasons.

Meteorological seasons are defined by the weather. They break down the year into three-month seasons based on annual temperature cycles. By that calendar, fall already started on Sept. 1 and will run until Nov. 31.

But astronomical seasons depend on how the Earth moves around the sun.

Equinoxes, when the sun lands equally on both hemispheres, mark the start of spring and autumn. Solstices, when the Earth sees its strongest tilt toward or away from the sun, kick off summer and winter.

When is the 'Harvest Moon'?

The "Harvest Moon" is the full moon closest to the autumnal equinox. The name dates back to a time before electricity when farmers depended on the moon's light to harvest their crops late into the evening, according to NASA.

This year's harvest moon, the last supermoon of 2024, will emerge on Monday, Sept. 16, and reach peak illumination at 10:34 p.m. ET/7:34 p.m. PT on Tuesday, Sept. 17, according to The Farmer's Almanac.

The Associated Press contributed to this story.