21st December will the shortest day of 2017 and marks the winter solstice, which occurs on the latest dawn of the year at the moment when the sun is at its lowest point in the sky.
The annual event celebrates the point where the North Pole is tilted exactly 23.5 degrees away from the sun and happens at the same moment around world.
As a result of the north of Earth leaning away from the Sun, December 21st sees the fewest hours of sunlight in one day. The sun will rise in the UK at around 08.04 GMT and set at 15.54 GMT.
What is the Winter Solstice?
The winter solstice happens every year when the Sun reaches its most southerly declination of -23.5 degrees. In other words, when the North Pole is tilted furthest – 23.5 degrees – away from the Sun, delivering the fewest hours of sunlight of the year.
The Sun is directly overhead of the Tropic of Capricorn in the Southern Hemisphere during the December solstice and is closer to the horizon than at any other time in the year, meaning shorter days and longer nights.
The day after the winter solstice marks the beginning of lengthening days, leading up to the summer solstice in June.
In the Southern Hemisphere, the opposite is true. Dawn comes early, and dusk comes late. The sun is high and the shortest noontime shadow of the year happens there. In the Southern Hemisphere, people will experience their longest day and shortest night.
It’s always on the 21st December then?
While it more often than not falls on December 21st, the exact time of the solstice varies each year.
In the Northern hemisphere the winter solstice is the shortest day of the year, because it is tilted away from the sun, and receives the least amount of sunlight on that day.
However, the earliest sunset does not occur on the solstice, because of the slight discrepancy between ‘solar time’ and the clocks we use.
The shortest day of the year often falls on December 21st, but the modern calendar of 365 days a year – with an extra day every four years – does not correspond exactly to the solar year of 365.2422 days.
The solstice can happen on December 20th, 21st, 22nd or 23rd, though December 20th or 23rd solstices are rare.
The last December 23rd solstice was in 1903 and will not happen again until 2303.