Winter solstice takes place on December 21 this year, marking the moment when days begin to lengthen, and nights become shorter.

The rotational axis of the Earth is tilted 23.5 degrees away from a perfect right angle to the plane where planets orbit around our Sun, with the north pole pointing toward the North Star, Polaris. This tilt stays the same as our planet moves around our parent star. This brings about seasons, as the northern and southern hemispheres receive changing levels of sunlight. Winter solstice takes place in December in northern latitudes, and during June in southern nations. At the time of the winter solstice, the Sun is seen at its lowest point of the year. In the summer, another solstice takes place, when days reach their greatest length, and the night is shorter than at any other time of year.

Winter solstice occurs in the northern hemisphere at 6:03 p.m. on December 21, when those in the south experience summer solstice.

Many people, including Pagans, Druids, and other nature-lovers celebrate the "return of the light" on the solstice. Thousands of people gather at Stonehenge in England to mark the occasion each year.

The Dongzhi Festival is celebrated in China, marking the winter solstice. The event is marked by family get-togethers and feasting.

Winter months were a time of famine throughout northern Europe until the 16th Century. In order to not have to feed animals when crops were scarce, people in those ancient days would slaughter farm animals. This meant fresh meat was widely available, leading to feasts during a time of hunger.

Ancient Scandinavians celebrated their holiday of Yule for 12 days, celebrating the return of the sun by burning large pieces of wood, known as Yule logs. Celebrations were also common in Ancient Rome.

"A fun fact about the coming solstice is that it occurs within about two-and-a-half hours of a new moon. No matter where you live on Earth's globe, a solstice is your signal to celebrate," Deborah Byrd wrote for EarthSky.org.

Although the shortest night of the year occurs on the solstice, the earliest sunset took place in early December. Due to complex motions of the Earth around the Sun, mornings continue to become darker in the northern hemisphere for a couple weeks after the solstice.

If water were not so common on Earth, and life could somehow still exist, the winter solstice would also mark the coldest time of year. However, waters heats and cools extremely slowly, usually delaying the coldest temperatures for several weeks.