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The December solstice is here. Here’s how it works and why it starts winter and summer at the same time.

earth winter summer solstice december 21 2018 illustration google earth proAn illustration of Earth during the December solstice in 2018.

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  • The 2022 winter solstice happens on Wednesday 21 December at 4:48 p.m. ET.
  • It is the winter solstice in the northern hemisphere but the summer solstice in the southern.
  • This is due to Earth’s tilted axis as it orbits around the sun. Here’s why. 

The December solstice of 2022 happens at 4:48 p.m. ET on Wednesday.

For people who live in the northern hemisphere, it will be the shortest day of the year. It also signals the arrival of winter and a gradual advance toward the spring season, the start of which is marked by an equinox.

For those in the southern hemisphere, it’s exactly the opposite: The December solstice marks the start of summer when days have reached their longest and brightest. This means daylight hours will start to shrink and sunlight will weaken through the March equinox and up until the June solstice.

Two things drive this all-important seasonal shuffle: Earth’s tilted axis and the planet’s orbit around the sun.

How the December solstice works

The winter solstice is the point when the sun will stay closest to the horizon all day. 

The earth rotates around its axis once a day and orbits the sun once every 365 days, approximately.

The earth’s axis is not perfectly perpendicular to its orbit around the sun. It’s tilted, by about 23.45 degrees, which means different parts of the world receive more or less sunlight, depending on the time of year.

earth during december solstice summer winter tropic cancer capricorn tilt axis sunlight graphic insider shayanne galThe earth’s axis is tilted by comparison with its orbit around the sun.

Shayanne Gal/Business Insider

On the winter solstice, the northern hemisphere is tilted away from the sun, so the sun rises later and sets earlier, stays very close to the horizon, and countries in the north get the least sunlight they will get all year. 

For areas above the Arctic Circle, like in northern Canada and Scandinavian countries, the sun will not even rise. 

earth during december solstice summer winter tropic cancer capricorn tilt axis sunlight graphic insider shayanne galThe earth’s axis is tilted by comparison with its orbit around the sun.

Shayanne Gal/Business Insider

At the same time, the earth’s southern hemisphere is tilted towards the sun.

That means the sun rises earlier, southern countries get the most light of the year, and the summer begins. If you stand on the Tropic of Capricorn at midday on Wednesday, a person standing outside will have practically no shadow as the sun is almost overhead. 

In countries below the Antarctic Circle, the sun doesn’t set. 

How Earth’s axis and orbit drive the seasons

Our planet’s orbit is elliptical, and its center of gravity is slightly offset from the sun.

This means the time it takes to cycle through the seasons isn’t perfectly divvied up:

earth equinoxes solstices sun light axial tilt seasons diagram graphics insider shayanne galHow Earth, its axial tilt, and the sun work to create solstices, equinoxes, and seasons.

Shayanne Gal/Business Insider

As the graphic above shows, it takes 89 days after the December solstice for Earth to reach the March equinox — that’s when the most direct rays of the sun have slipped back up to the equator. Another 92 days and 19 hours later, it will be the June solstice. At that point, the sun’s most direct rays reach the Tropic of Cancer, summer starts for the northern hemisphere, and winter begins for those south of the equator.

Then it takes 93 days and 14 hours for the sun’s zenith to get back to the equator and kick off the September equinox, followed by 89 days and 19 hours to complete the cycle with the December solstice.

During each of these phases, certain regions of Earth’s surface get more sunlight, and energy gets stored or sapped from water sources, leading to the creation of seasonal temperatures and weather variations.

What the seasons look like from space

Some satellites fly in a geosynchronous orbit, which means they orbit the Earth fast enough to hover above one spot on the planet.

This creates a great opportunity to photograph the Earth over the course of the year and see how the angle of sun changes.

NASA’s Goddard Space Flight Center created the animation below using geosynchronous-satellite images taken over Africa, and it clearly shows the seasonal progression.

This article was originally published on December 21, 2022. Dave Mosher contributed to the previous version of this article. 

Read the original article on Business Insider