New Delhi: The official onset of winter has begun as the winter solstice this year falls on December 21 thereby making it the shortest day and the longest night of the year in the northern hemisphere.
On a solstice the sun is at its greatest distance from the celestial equator. The solstice happens when the sun reaches a point where it will appear to shine farthest to the south of the equator, over the Tropic of Capricorn.
The reason behind the occurrence of winter solstice is that the Earth does not spin upright, but on an axis 23.5 degrees from the vertical.
In the southern hemisphere, the winter solstice occurs on June 21 or 22 when the sun is placed in the farthest north.
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.