(CNN) — For the past six months, the days have grown shorter and the nights have grown longer in the Northern Hemisphere. But that’s about to reverse itself.
Winter solstice 2021, the shortest day of year and the official first day of winter, is on Tuesday, December 21. How it all works has fascinated people for thousands of years.
First we’ll look at the science and precise timing behind the solstice. Then we’ll explore some ancient traditions and celebrations around the world.
The science and timing behind a winter solstice
The winter solstice marks the shortest day of the year in the Northern Hemisphere, when the sun appears at its most southerly position, directly overhead at the Tropic of Capricorn.
The situation is the reverse in the Southern Hemisphere. There, the December solstice marks the longest day of the year — and the beginning of summer in places such as Australia, Chile and South Africa.
These three images from NOAA’s GOES East (GOES-16) satellite show us what Earth looks like from space near the winter solstice. The images were captured about 24 hours before the 2018 winter solstice.
When exactly does it occur?
The solstice usually — but not always — takes place on December 21. The time that the solstice occurs shifts every year because the solar year (the time it takes for the sun to reappear in the same spot as seen from Earth) doesn’t exactly match up to our calendar year.
A worker uses a blower to clear snow from a courtyard at Gyeongbokgung Palace in central Seoul on December 13, 2020.
ED JONES/AFP/AFP via Getty Images
Here are some examples of when 15:59 UTC will be for various local times in places around the world. Because of time zone differences, the solstice will technically fall on Wednesday in parts of East Asia.
— Seoul: 12:59 a.m. Wednesday
— Bangkok: 11:59 p.m. Tuesday
— Qatar: 6:59 p.m. Tuesday
— Milan, Italy: 4:59 p.m. Tuesday
— Orlando, Florida: 10:59 a.m. Tuesday
— Calgary, Canada: 8:59 a.m. Tuesday
— Honolulu: 5:59 a.m. Tuesday
What places see and feel the effects of the winter solstice the most?
Daylight decreases dramatically the closer you are to the North Pole on December 21.
People take advantage of the brief daylight hours in Helsinki, Finland, on December 20, 2020.
Alessandro Rampazzo/Anadolu Agency/Getty Images
What causes the winter solstice to even happen?
Because the Earth is tilted on its rotational axis, we experience seasons here on Earth. As the Earth moves around the sun, each hemisphere experiences winter when it’s tilted away from the sun and summer when it’s tilted toward the sun.
Wait. Why is the Earth tilted?
Scientists are not entirely sure how this occurred, but they think that billions of years ago, as the solar system was taking shape, the Earth was subject to violent collisions that caused the axis to tilt.
What other seasonal transitions do we mark?
The equinoxes, both spring and fall, occur when the sun’s rays are directly over the equator. On those two days, everyone has an equal length of day and night. The summer solstice is when the sun’s rays are farthest north over the Tropic of Cancer, giving us our longest day and the official start of summer in the Northern Hemisphere.
Winter solstice traditions and celebrations
Decorated evergreen trees have roots that go back beyond the beginnings of Christianity to ancient Egypt and Rome. The famous Rockefeller tree in New York was illuminated for the 2021 season on December 1.
Ed Jones/AFP via Getty Images
It’s no surprise many cultures and religions celebrate a holiday — whether it be Christmas, Hanukkah, Kwanzaa or pagan festivals — that coincides with the return of longer days.
Ancient peoples whose survival depended on a precise knowledge of seasonal cycles marked this first day of winter with elaborate ceremonies and celebrations. Spiritually, these celebrations symbolize the opportunity for renewal, a shedding of bad habits and negative feelings and an embracing of hope amid darkness as the days once again begin to grow longer.
Many of the ancient symbols and ceremonies of the winter solstice live on today or have been incorporated into newer traditions. Here are just a few of them:
As the Roman Empire came under Christian influence and eventual rule, some of the festival’s customs were melded into celebrations surrounding Christmas and the New Year.
It’s not just ancient Europeans who marked the annual occasion. The Dongzhi Winter Solstice Festival has its roots in ancient Chinese culture. The name translates roughly as “extreme of winter.”
Celebrations in the age of Covid
Many places around the world traditionally hold festivals that honor the winter solstice. But because of the ongoing pandemic now stretching into a second winter solstice, events may be canceled again or altered to allow for a safe, socially distanced occasion.
Oh, the glories of prepandemic times! A choir sings at Stonehenge to mark the winter solstice before words such as Covid and Omicron joined everyday vocabulary.
Ben Birchall /PA Images/Getty Images
For 2020, the festival was a video-only affair. This year, they’re easing back in with in-person, but scaled-down, neighborhood events. Livestreaming will still be an option in 2021.
CNN’s Katia Hetter and Autumn Spanne contributed to this article.