The winter solstice gives us the shortest day and longest night of the year.
After it passes, the days will begin to get longer and longer until June when the summer solstice happens.
Every year we have two solstices and two equinoxes which mark the change in meteorological seasons caused by the tilt of the Earth's axis.
Each December, the Earth's axis tilts away from the sun causing the winter solstice.
While the solstices mark summer and winter, the equinoxes are seen as the beginning of spring and autumn in March and September.
The exact date of the winter solstice varies because the Gregorian calendar does not match the length of the tropical year.
But when is the winter solstice this year?
When is the shortest day of the year?
The winter solstice happens this year on Tuesday, December 21.
For everyone in the Northern Hemisphere, it will take place at 3:48pm GMT.
The winter solstice usually happens between December 20 and 23 , and though the date changes every year the most common date is the 21st.
On the date of the solstice, London will see only seven hours, 49 minutes and 42 seconds of daylight, with the sun rising at 8.03am and setting as early as 3.53pm.
Whatever happens in the Northern Hemisphere, the opposite will happen in Southern, where they will be experiencing the longest day of the year.
How is the winter solstice celebrated?
Representing the passing of the seasons and new beginnings, winter solstice has been considered an important date for many cultures throughout human history.
In fact many of the traditions which are now associated with Christmas began as ways to observe the astronomical phenomenon.
For instance, Christmas trees and wreaths originated from the 12-day pagan holiday of Yule, which is centred around the winter solstice.
Even today, thousands of pagans gather at Stonehenge every year on the solstice to mark the day and watch the sun rise.
It's believed that the "Heel Stone" behind which the sun rises was built especially for the solstices, aligning the boulders with the movements of the sun, allowing light to pour into the heart of Stonehenge during sunrise.
