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In the Northern Hemisphere, until the winter solstice on Dec. 21.

Equinoxes have been marked and celebrated worldwide for centuries. The fall equinox is often connected with harvest festivals in the Northern Hemisphere. For example, Dozynki, the harvest festival celebrated in Poland and other Eastern European countries, was thought to have been originally tied to the equinox. At the Mayan site Chichen Itza in Mexico, people gather during the equinox to watch the sun create a shadow pattern that resembles a serpent descending a building called El Castillo.

This year, there’s a bonus for some: Antarctica, New Zealand and a sliver of Australia may be able to see during their Monday.

But what is happening in the heavens? Here’s what to know about how we split up the year using the Earth’s orbit.

What is the 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.

During the equinox, the Earth’s axis and its orbit align so that both hemispheres get an equal amount of sunlight.

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.

The Northern Hemisphere’s fall — or autumnal — equinox can land between Sept. 21 and 24, depending on the year. Its spring — or vernal — equinox can land between March 19 and 21.

What is the solstice?

The solstices mark the times during the year when the Earth is at its most extreme tilt toward or away from the sun. This means the hemispheres are getting very different amounts of sunlight — and days and nights are at their most unequal.

During the Northern Hemisphere’s summer solstice, the upper half of the Earth is tilted in toward the sun, creating the longest day and shortest night of the year. This solstice falls between June 20 and 22.

Meanwhile, at the winter solstice, the Northern Hemisphere is leaning away from the sun — leading to the shortest day and longest night of the year. The winter solstice falls between Dec. 20 and 23.

What’s the difference between meteorological and astronomical seasons?

These are just two different ways to carve up the year.

While astronomical seasons depend on how the Earth moves around the sun, meteorological seasons are defined by the weather. Meteorologists break down the year into three-month seasons based on annual temperature cycles. By that calendar, spring starts on March 1, summer on June 1, fall on Sept. 1 and winter on Dec. 1.

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The Associated Press Health and Science Department receives support from the Howard Hughes Medical Institute’s Department of Science Education and the Robert Wood Johnson Foundation. The AP is solely responsible for all content.