Winter Solstice: The shortest day of 2013 is December 21
Usually, the winter solstice occurs on December 21.
Zee Media Bureau
New Delhi: The official onset of winter, when the sun reaches a point where it will appear to shine farthest to the south of the equator, over the Tropic of Capricorn is marked by the winter solstice, which this year is on Saturday, 21 December.
Usually, the winter solstice occurs on December 21, 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.
This year on Saturday, the sun will stop moving southward, pause, and then start moving northward. This pause is called the "solstice," from the Latin words "sol" for "sun" and "sisto" for "stop."
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.
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 was stationary.