Spring equinox has begun, but what is it exactly?

Spring is almost here — officially, at least.

The vernal equinox arrives on Thursday, marking the start of the spring season for the Northern Hemisphere and the fall in the Southern Hemisphere. On the equator, the sun will be directly overhead at noon. Equinoxes are the only time when both the north and south poles are lit by sunshine at the same time.

The events have been marked and celebrated around the world for centuries. Nowruz, the Iranian New Year, is based on the spring equinox, for example. And 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.

Visitors hold their hands out to receive the sun's energy as they celebrate the Spring equinox atop the Pyramid of the Sun in Teotihuacan, Mexico, Thursday, March 21, 2019. (AP Photo/Marco Ugarte, File)

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 line up 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 spring — or vernal — equinox can land between March 19 and 21, depending on the year. Its fall – or autumnal — equinox can land between Sept. 21 and 24.

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.

This May 18, 1969, photo provided by NASA shows Earth from 36,000 nautical miles away as photographed from the Apollo 10 spacecraft during its trans-lunar journey toward the moon. (NASA via AP)

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 December 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. They 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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