Day and night get equal billing on Tuesday. A guide to the fall equinox

The fall equinox is Tuesday, when light and darkness split the day evenly. It marks the beginning of fall for the Northern Hemisphere and the beginning of spring in the Southern Hemisphere.

On the equator, the sun will be directly overhead at noon. Equinoxes are the only times when both the north and south poles are lit by sunshine at the same time.

Equinoxes have been marked and celebrated around the world for centuries. Japan marks public holidays during the equinoxes. 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 pyramid called El Castillo.

Here’s what to know about the Earth’s orbit.

What is the equinox?

As the Earth travels around the sun, it does so at an angle, making the sun’s warmth and light fall unequally on the northern and southern halves of the planet for most of the year.

During the equinox, the Earth’s tilt is neither toward the sun nor away from the sun, so both the Northern and Southern hemispheres get an equal amount of sunlight. The sun rises almost exactly due east, and it sets almost exactly due west.

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. This year it is Tuesday at 8:05 p.m. Eastern time. Its spring — or vernal — equinox can land between March 19 and 21. The exact time of the equinox is the moment the sun is directly overhead at the equator.

What is the solstice?

The solstices mark the times when the Earth is tipped most extremely either 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.

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

The opposite happens at the Northern Hemisphere’s winter solstice: the Earth’s upper half leans the furthest 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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