On Tuesday, December 21, Northern Hemisphere residents will enjoy the shortest day of 2021. Known as the winter solstice, it also marks the start of the astronomical winter season. This means that while the days following will grow longer, they will also be colder. Conversely, Southern Hemisphere residents will celebrate the summer solstice — the beginning of their astronomical summer — with the longest day and shortest night of the year.

The disparate seasons result from the Earth's rotation around the Sun and its own axis, which is tilted at a 23.5-degree angle. The December solstice marks the time when the Northern Hemisphere is farthest away from the sun. Hence, it receives significantly less sunlight than the Southern Hemisphere, which is tilted towards the star.

The solstice date varies slightly from year to year and can fall anywhere from December 20th to 23rd. That's because the Gregorian calendar has 365 days, with an extra "leap" day every four years. It does not correspond precisely with the solar year, which lasts 365.2422 days. Though the entire day is celebrated, the solstice occurs when the sun reaches its southernmost point in the sky. In 2021, that will be at 10:59 am EST (7:59 am PST).

The winter solstice was particularly significant for ancient cultures. The longer days that followed provided comfort that the Sun had not left them. The star's "rebirth" was marked with elaborate ceremonies performed at structures specially built for the occasion.

Among the most famous is Stonehenge in Wiltshire, England. The prehistoric religious site comprises a modest circle of stones precisely aligned in the direction of the summer solstice's sunrise and the winter solstice's sunset. The ancient monument still attracts hundreds of people during the solstices.

Newgrange, a burial mound in Ireland's Boyne Valley, is also a popular solstice destination. The massive stone structure, which dates back over 5,000 years, is designed to receive a ray of light into its central chamber at the dawn of the winter solstice. While the event is closed to visitors due to the pandemic, the breathtaking sunrise will be live-streamed from the chamber from December 20th to 22nd, 2021.

Many cities and towns have started their own winter solstice traditions. In Japan, residents, and even some zoo animals, soak in hot baths filled with yuzu – a citrus fruit reputed to provide the body immunity from colds. In Korea, the winter solstice is called Dongzhi, or "Little Lunar New Year." It celebrates the symbolic return of the Sun. Many families observe the special day by eating patjuk — a red bean porridge made using adzuki beans and rice flour dumplings known as saealshim. Residents also sprinkle the red beans around the house to keep evil spirits at bay.

The Secret Lantern Society in Vancouver, Canada, marks the winter solstice with processions, music, and artistic performances in four locations around the city. Two venues also feature the Labyrinths of Light. The stunning displays, each comprising 700 lit candles, are designed to provide visitors with a calming experience.

Happy Winter Solstice!

Resources: earthsky.org, theseoulguide.com, wikipedia.org.