Google celebrates the winter solstice with a doodle.

The winter solstice has been celebrated for thousands of years and is probably the astronomical event most often associated with a festival. Let's see what the solstice is and some of the celebrations that surround it.

The Earth tilts
The Earth turns on its axis to give us alternating day and night. The axis isn’t straight up and down, but tilted 23.5 degrees with respect to our path around the Sun. As Earth orbits the Sun, the axis continues to point in the same direction. This is why we have seasons, and the number of daylight hours changes during the year.

There are two annual solstices. They occur when one pole’s full 23.5-degree tilt is towards the Sun. In December it’s the south pole, giving the southern hemisphere its longest day. Since the north pole is then tilted the full amount away from the Sun, the northern hemisphere has its shortest day. In June the opposite occurs. The northern winter solstice is usually on the 21st or 22nd, which is the first day of winter.

Why celebrate winter?
No one celebrated winter itself, a harsh season, cold and dark. Game was scarce and so was food to forage. Crops weren’t growing and survival depended on what was preserved and stored. Starvation before spring came was always a threat.

It’s easy to imagine why the Sun was considered a god in so many cultures, for life on Earth depends on it. By December, for six months each day had been shorter than the last, and the Sun hadn't risen as high. Would this continue until the life-giving Sun disappeared altogether?

A formal ritual might either encourage the Sun’s return or thank him for returning. It would also bring the community together and strengthen it for the lean months to follow. If spring is the renewal of the natural world, the winter solstice is the renewal of the Sun. In many cultures it’s the New Year.

There is archaeological evidence for the importance of the winter solstice to ancient peoples. For example, over four thousand years ago a Neolithic culture in Ireland built a stone structure at Newgrange that is aligned to the rising winter solstice Sun. And another such culture in England built Stonehenge. Stonehenge is popularly associated with the rising midsummer Sun, but it seems more likely to have been oriented to the winter solstice sunset. These structures are also evidence of a well-developed observational astronomy.

Saturnalia
The Roman Saturnalia – honoring their god Saturn – was observed in the days leading up to the solstice. People exchanged gifts, with candles being a particularly popular one. There was feasting and drinking, games, gambling and riotous behavior. Social reversals, such as slaves and masters exchanging places, were another feature. When Christmas was a 12-day celebration in the Middle Ages, many of these aspects of Saturnalia were associated with Twelfth Night.

Yule
Yule was a pre-Christian 12-day winter celebration in northern Europe. Elements of the festivities were later incorporated into Christmas. This is why Christmas is sometimes called Yule in English. However Neo-Pagan and Wiccan Yule is celebrated as a winter solstice festival. It emphasizes older elements of Yule and the rebirth of the Sun. Although Christmas is observed on the same dates in both northern and southern hemispheres, Yule is celebrated in June in the southern hemisphere.

Dongzhi
Dongzhi is a major festival in China and other East Asian countries. Most Chinese festivals are observed according to the lunar calendar, making them movable in terms of a solar date. However Dongzhi is a winter solstice festival whose origins go back over more than two thousand years. It has a religious element and is also a time for families to come together.

Soyalangwul
The Hopi (“Peaceful people”) are Native Americans of the southwestern USA. Their winter solstice Soyal ceremony is one of renewal. It helps to return the Sun to its upward summer path and prepare the Hopi spiritually for the new year. Soyalangwul (“Establishing Life Anew for the World”) is a serious part of the right ordering of the Hopi world. Unlike the revelry of Saturnalia, tranquility and spiritual focus is important, involving at various times silence, fasting and eating sacred foods.

Inti Raymi
Inti Raymi was the winter solstice festival of the Incas of Peru, where the winter solstice is in June. Without the Sun the people would starve, so they sacrificed to the Sun God and asked him to return. The Spanish colonial authorities banned the festival in the sixteenth century. The modern one, which dates from 1944, is a reconstruction of the original based on old documents. But no actual sacrifices are included. Inti Raymi is the second largest festival in South America.

Maruaroa o Takurua
Another southern hemisphere winter solstice (and new year) festival is Maruaroa o Takurua. The Maori people of New Zealand celebrate it. They use a lunar calendar and don’t determine the time of the solstice through solar observation. They watch for the rising of the Pleiades, the star cluster that the Maoris call Matariki. It first reappears in the Eastern sky just before sunrise around the winter solstice. The solstice celebration takes place at the first new moon after its rising.