- The December solstice of 2018 happens on Friday at 5:23 p.m. ET, marking the first day of winter for the northern hemisphere.
- Winter solstice in the northern hemisphere also marks the arrival of summer in the southern hemisphere.
- A solstice is either the shortest or longest day of the year, depending on where you live.
- Earth's tilted axis and orbit around the sun - not the planet's rotation - is the driving force behind a solstice.
The December solstice of 2018 happens at 5:23 p.m. ET on Friday.
To people who live in Earth's 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, whose beginning 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. However, 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
Earth orbits the sun once every 365 days and six hours, and our planet rotates once per day around a tilted axis.
That tilt is about 23.45 degrees (for now), and it bathes different parts of the world with various intensities of light over the course of a year. Meanwhile, Earth's rotation around its axis keeps the sun's heat even, sort of like a 7,917-mile-wide rotisserie chicken made of rock and a little water.
From the view on the ground in the northern hemisphere, the December solstice is when the sun's high point in the sky, called a zenith, reaches its minimum or low point close to the horizon. From space, it's when the sun's most direct rays creep the farthest south, to a line called the Tropic of Capricorn:
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How Earth's axial tilt and the sun work to create the December solstice, which marks the start of winter in the northern hemisphere.
If you stand on the Tropic of Capricorn at midday on December 21, the sun will appear more or less directly overhead. Your shadow will also be at its absolute minimum. (Solstice literally means "sun-stopping," according to TimeAndDate.com.)
The length of daylight will be at its longest, too. This gets more extreme the more south you go, since there's more of Earth's atmosphere to refract sunlight the farther you are from the equator.
But this moment won't last, since the Earth makes its way around the sun at a speed of roughly 66,600 mph.
How Earth's axis and orbit drive the seasons
Our planet's orbit is elliptical and its center of gravity slightly offset from the sun.
This means the time it takes to cycle through the seasons isn't perfectly divvied up:
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How Earth, its axial tilt, and the sun work to create solstices, equinoxes, and seasons.
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 around Earth in a geosynchronous orbit, which means they move 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 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 story was adapted from a similar post about the June solstice.