The December (or Southern) Solstice this year falls on Tuesday 21 December,
when the Sun is at its southernmost point in the sky. This is the
shortest day of the year in the Northern Hemisphere, where it is known
as the Winter Solstice and the longest day in the Southern Hemisphere,
where it is known as the Summer Solstice. At very high latitudes the sun
may not rise (Northern Hemisphere) or set (Southern Hemisphere) for
several weeks on either side of the Southern Solstice.
The solstices are entirely a product of variation in the Earth's
rotation on its axis, which is at an angle of 23.5° to the plain of the
Earth's orbit about the Sun. This means that in December the Earth's
Southern Pole is tilted towards the Sun, while the Northern Pole is
tilted away from it. This means that around the Southern Solstice the
Southern Hemisphere is receiving radiation from the Sun over a longer
part of the than the Northern, and at a steeper angle (so that it to
pass through less atmosphere to reach the planet), creating the southern
summer and northern winter.
The tilt of the Earth during the December Solstice. Wikimedia Commons.
The solstices are fairly noticeable astronomical events, and tied to the
seasons which govern the life cycles of life on Earth, and they have
been celebrated under different names by cultures across the globe, but
most notably by those at higher latitudes, who are more profoundly
affected by the changes of the seasons.
See also...
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