The winter solstice is usually on 21 December – but occasionally it falls on 22 December
The official onset of winter and the beginning of the end for long, dark nights in the northern hemisphere is marked by the winter solstice, which is this year at 5.30am (GMT) on Thursday.
The winter solstice happens because the Earth does not spin upright, but on an axis 23.5 degrees from vertical. As the Earth orbits the sun, it reaches the moment of winter solstice when the north pole is tilted furthest from the sun, making it the shortest day of the year.
The winter solstice is as far south as the sun ever gets, shining directly overhead along the Tropic of Capricorn, 23.5 degrees south of the equator.
Usually, the winter solstice occurs on the 21 December, but that can shift for the same reasons we have leap years: the Earth takes 365.25 days to orbit the sun, but the Gregorian calendar has only 365 days so each year the solstice is pushed back by around six hours.
“At extreme ends of the four year calendrical cycle the solstice can sometimes get shunted into the early hours of the 22nd, as it does this year. Next year is a leap year, which resets the calendar so that the winter solstice will fall on the 21st again,” – Marek Kukula at the Royal Observatory in Greenwich, London.
In the northern hemisphere, the winter solstice marks the shortest day and the longest night, but the sun continues to rise later for some days afterwards. The reason is that Earth’s orbit around the sun is not circular, but elliptical. As the Earth rotates, it also moves along this curved path, and so sees the sun a little sooner than if it were stationary – source