What’s the winter solstice?

Brr! For people in the Northern Hemisphere (above the Equator, like the United States), winter is a season of cold, dark days—and if you’re lucky, lots of snow to sled on! (Check out Weird But True! facts about winter.)

The winter solstice marks the exact moment when half of Earth is tilted the farthest away from the sun. It usually happens on December 21 or 22, at the exact same second around the world.

Because less sunlight reaches Earth, the winter solstice is also the day of the year with the least amount of daytime, known as the shortest day of the year. But the good news is that every day after the winter solstice will be a little longer, until we reach the day with the most hours of daylight. Called the summer solstice, it usually occurs between June 20 and June 22.

For people who live in the Southern Hemisphere, though, the winter solstice takes place in June. That’s because the seasons are reversed below the Equator. For instance, in New Zealand, it can actually snow in July!

Seasons vs. solstices

For many people, the winter solstice marks the changing of fall to winter. But there’s a difference between the winter solstice—called the astronomical first day of winter—and the first day of the winter season, which is called the meteorological first day of winter. Each year, meteorologists—people who study weather and climate—determine the first day of the winter season based on temperature records. In the United States, winter lasts about 90 days. Scientists often tie it to the calendar, so that winter falls during the latter part of December, January, February, and the early part of March.

One myth is that the winter solstice is the coldest day of the year. But the coldest temperatures are often still at least a month away, depending on where you live. That’s because the Earth’s land and water takes time to cool down.

Can you see the solstice?

Yes—kind of. On the winter solstice, if you stand outside at noon and look at your shadow, it will be the longest shadow you cast the entire year. Here’s why:

Every day, the sun rises in the east and sets in the west, tracing an arc across the sky. The height of that arc changes during Earth’s yearly orbit around the sun. As our planet orbits, one pole is tilted toward the sun, and one pole is tilted away from it. During winter in the Northern Hemisphere, the North Pole is tilted away from the sun, so the height of the arc is low—and your shadow looks long.

In fact, during the days around the winter solstice, the sun is so low on the horizon that it appears to rise and set in the same place. That’s why the word solstice can be translated to "sun stands still' in Latin.

Ancient solstice sites

Ancient people built many monuments to celebrate the winter solstice. One example is Newgrange, a huge tomb mound built in Ireland around 3200 B.C., about a thousand years before Stonehenge. A tunnel facing the solstice sunrise runs to a main chamber, and a small window bathes the chamber in solstice light for 17 minutes.

The famous Nasca Lines—which are giant outlines of monkeys, lizards, and other figures etched into the earth by an ancient culture in Peru around A.D. 1 to 700—match up with the winter solstice. On the solstice, some of these lines touch the spot on the horizon where the sun sets.

Ancient Egypt's temple of Karnak, located in Luxor, was built to align with the winter solstice more than 4,000 years ago. Similar ancient structures can be seen in archaeological sites around the world, including Angkor Wat in Cambodia, Machu Picchu in Peru, and Petra in Jordan.

Still celebrating the sun

Many cultures still mark the winter solstice with fun gatherings.

However you celebrate the winter solstice, if you live in the Northern Hemisphere, make sure to bundle up!

Adapted for Nat Geo Kids by Christine Dell'Amore. Original article by Brian Handwerk.