(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.
If you want to be super-precise in your observations (and who doesn't want to be?), the exact time of the 2021 winter solstice will be 15:59 Coordinated Universal Time (UTC) on Tuesday, according to EarthSky.org and Farmers' Almanac. That's almost six hours later than last year's time.
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 in balmy Singapore, just 137 kilometers or 85 miles north of the equator, barely notice the difference, with just nine less minutes of daylight than they have during the summer solstice. Much higher in latitude, Madrid, Spain, still logs in a respectable nine hours and 17 minutes of daylight during the winter solstice.
People take advantage of the brief daylight hours in Helsinki, Finland, on December 20, 2020.
Alessandro Rampazzo/Anadolu Agency/Getty Images
The difference is more stark in frigid Helsinki, Finland, where the sun will rise at 9:23 a.m. and set at 3:12 p.m., resulting in less than six hours of anemic daylight. It's a whopping 13 hours and seven minutes less daylight than these Finns get during the June summer solstice. Residents of Nome, Alaska, will be even more sunlight deprived with just three hours, 54 minutes and 31 seconds of very weak daylight on Tuesday. But that's downright generous compared with Prudhoe Bay, Alaska. It sits inside the Arctic Circle and won't see a single ray of sunshine.
What causes the winter solstice to even happen?
Because our planet is tilted on its rotational axis, we experience seasons here on Earth. As the planet 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:
In the Welsh language, "Alban Arthan" means for "Light of Winter," according to the Farmers' Almanac. It might be the oldest seasonal festival of humankind. Part of Druidic traditions, the winter solstice is considered a time of death and rebirth.
In Ancient Rome, Saturnalia began on December 17 and lasted for seven days. It honored Saturn, the Roman god of agriculture. The people enjoyed carnival-like festivities resembling modern Mardi Gras celebrations and even delayed their war-making. Saturnalia continued into the third and fourth centuries AD.
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."
They thought this was the apex of yin (from Chinese medicine theory). Yin represents darkness and cold and stillness, thus the longest day of winter. Dongzhi marks the return yang -- and the slow ascendance of light and warmth. Dumplings are usually eaten to celebrate in some East Asian cultures.
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.
Better known for pirates than the solstice, the town of Penzance on the southwest coast of England revived the delightful tradition of a Cornish processional. Shut down in 2020, it's back on in 2021. Event organizers ask people to get tested before attending, according to the festival's Facebook page.
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
The UK's most famous site for solstice celebrations is Stonehenge. On the winter solstice, visitors traditionally have had opportunity to enter the towering, mysterious stone circle for a sunrise ceremony run by local pagan and druid groups. The in-person sunrise gathering was canceled in 2020, but it is being held this year. The English Heritage Society cautions that could be subject to change depending on the most recent health guidelines as the Omicron variant continues to rapidly spread. Face masks are mandatory, and testing is encouraged. The society has set it up so you can livestream the sunrise at Stonehenge from the toasty comfort of home.
In Canada, Vancouver's Winter Solstice Lantern Festival is a sparkling celebration of solstice traditions from around the world. Traditionally, the Secret Lantern Society assembles a wide array of music, dance, food and spectacular lantern-lit processions.
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.