The Winter Solstice 2013, which falls on 21 December, marks the first day of winter as the sun rises on this shortest day of the year.

Thousands of revellers turn up every year at Stonehenge in southern England to see the sunrise and sunset on the day at the 4,000-year-old solar monument on Salisbury plain.

What is Winter Solstice?

Winter Solstice is the day in a year when the Northern Hemisphere is the most inclined away from the sun, and hence, the shortest day is observed.

Winter Solstice traditionally marks a change of seasons - from fall to winter in the earth's northern hemisphere and from spring to summer in the southern hemisphere.

Winter Solstice is also the longest night of the year in the northern hemisphere. In contrast, for the southern hemisphere, the Winter Solstice actually marks the longest day of the year.

On Winter Solstice, lesser sunlight is reflected back into space in the northern hemisphere, making it the shortest day there, while most sunlight is reflected out of the southern hemisphere, marking the day as the longest day of the year.

Nasa explains: "On Winter Solstice, the polar north receives no energy from the sun. In contrast, the amount of incoming solar energy the Earth receives on June 21, Summer Solstice, is 30 percent higher at the North Pole than at the equator."

Winter Solstice: Traditions and celebrations

In the Neolithic and Bronze Age, Winter Solstice was a way for people to identify the actual time for harvests and sowing of new crops.

However, the most common modern-day tradition of winter or summer solstice observed around the world is to view sunrise and sunset.

Besides the UK's Stonehenge, which is known for seeing the summer solstice sunrise, the prehistoric monument of Newgrange in Ireland is well known for viewing sunrise on a Winter Solstice. The monument is aligned with the rising sun on the day.

Other ancient monuments built with solstice observing markers include the Kokino megalithic observatory, an archaeological site in the north-eastern Republic of Macedonia, which has stone-carved markers to track the movements of the sun and the moon.

As well as viewing the sunrise or sunset, revellers dance, meditate and chant incantations at these places on a solstice.

The day is welcomed around the world in different ways. In many cultures, a holiday is celebrated around Winter Solstice, including Christmas, Hanukkah, and Kwanzaa.

In Latvia, people follow the ancient Yule Log dragging tradition during Winter Solstice as part of the Yuletide, or holiday season, or Christmas celebrations.