(CNN) — For 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 2020, the shortest day of year and the official start of winter, is on Monday, December 21. How it all works has fascinated people for thousands of years.
The science and timing behind a winter solstice
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 Argentina, Australia 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 and the day itself shifts 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.
— Tokyo: 7:02 p.m. Monday
— Bangkok: 5:02 p.m. Monday
— Dubai: 2:02 p.m. Monday
— Rome: 11:02 a.m. Monday
— Casablanca, Morocco: 10:02 a.m. Monday (same as UTC)
— Boston: 5:02 a.m. Monday
— Vancouver: 2:02 a.m. Monday
— Honolulu: 12:02 a.m. Monday
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.
The House of the Books (Singer House) on Nevsky Avenue in St. Petersburg, Russia, is lit up electrically. Sunshine is in scarce supply here in the days leading up to winter solstice.
Alexander Demianchuk/TASS/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.
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.”
Cancellations and modified celebrations
Many places around the world hold festivals that honor the winter solstice. But because of the Covid-19 pandemic, they have either been canceled for 2020 or altered to allow for a safe, socially-distanced occasion.
A choir sings at Stonehenge to mark the winter solstice. In-person visits are canceled for 2020, however.
Ben Birchall /PA Images/Getty Images
For 2020, they’re taking the festival to Zoom, with people encouraged to share their lanterns and other efforts online.
The ‘Christmas Star’
Saturn, top, and Jupiter, below, are seen after sunset on December 13 from Shenandoah National Park in Luray, Virginia.
While the pandemic is raining on our parades on Earth, its reach cannot extend into the solar system.
The solstice and the “Christmas Star’ will give a coronavirus-weary world two potent symbols of hope and reminders of a universe that marches ever onward to its own beat no virus can stop.
CNN’s Katia Hetter, CNN’s Ashley Strickland and Autumn Spanne contributed to this article.