(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.

First we’ll look at the science and precise timing behind the solstice. Then we’ll discover some ancient traditions and celebrations around the world (though many of 2020’s celebrations are being canceled or modified for the pandemic). Finally, we’ll learn about a delightful show in the heavens special for winter solstice 2020.

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 faraway 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 Argentina, Australia and South Africa.

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.

If you want to be super-precise in your observations, the exact time of the 2020 winter solstice will be 10:02 Coordinated Universal Time (UTC) on Monday. Here are some examples of when that will be for local times around the world:

— 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

To check the timing where you live, the website EarthSky has a handy conversion table for your time zone. You might also try the conversion tools at Timezoneconverter.com or WorldTimeServer.com.

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.

The difference is more stark in frigid St. Petersburg, Russia, where the sun will rise at 10 a.m. and set at 3:53 p.m. resulting in less than six hours of anemic daylight.

Residents of Nome, Alaska, will be even more sunlight deprived with just three hours, 54 minutes and 33 seconds of very weak daylight. But that’s downright generous compared with Prudhoe Bay, Alaska. It sits inside the Arctic Circle and won’t see a single ray of daylight.

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.

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.

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.

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.

Newgrange, a prehistoric monument built in Ireland around 3200 BC, is associated with the Alban Arthan festival.

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.

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.

Better known for pirates than the solstice, the town of Penzance on the southwest coast of England revived a delightful array of Cornish solstice events leading up to winter solstice. They’ve canceled events for 2020 but hope to resume in 2021, according to the festival’s Facebook page.

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.

Because of the pandemic, in-person celebrations have been canceled this year. But the English Heritage Society has set it up so you can livestream the sunrise from Stonehenge.

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, they’re taking the festival to Zoom, with people encouraged to share their lanterns and other efforts online.

While the pandemic is raining on our parades on Earth, its reach cannot extend into the solar system.

On the night of December 21, Jupiter and Saturn will appear so closely aligned in our sky that they will look like a double planet. This close approach is called a conjunction.