Winter Solstice

Winter solstice
Lawrence Hall of Science visitors observe sunset on the day of the winter solstice using the Sunstones II.
Also called	Midwinter, Yule, the Longest Night, Jól
Observed by	Various cultures
Type	Cultural, astronomical
Significance	Astronomically marks the beginning of shortening nights and lengthening days
Celebrations	Festivals, spending time with loved ones, feasting, singing, dancing, fires
Date	Between December 21 and December 23 (NH) Between June 20 and June 21 (SH)
Frequency	Twice a year (once in the northern hemisphere, once in the southern hemisphere, six months apart)
Related to	Winter festivals and the solstice

The winter solstice (or hibernal solstice), also known as midwinter, is an astronomical phenomenon marking the day with the shortest period of daylight and the longest night of the year. In the Northern Hemisphere this is the December solstice and in the Southern Hemisphere this is the June solstice.

The axial tilt of Earth and gyroscopic effects of its daily rotation mean that the two opposite points in the sky to which the Earth's axis of rotation points (axial precession) change very slowly (making a complete circle approximately every 26,000 years). As the Earth follows its orbit around the Sun, the polar hemisphere that faced away from the Sun, experiencing winter, will, in half a year, face towards the Sun and experience summer. This is because the two hemispheres face opposite directions along Earth's axis, and so as one polar hemisphere experiences winter, the other experiences summer.

More evident from high latitudes, a hemisphere's winter solstice occurs on the day with the shortest period of daylight and longest night of the year, when the sun's daily maximum elevation in the sky is at its lowest. Although the winter solstice itself lasts only a moment in time, the term sometimes refers to the day on which it occurs. Other names are "midwinter", the "extreme of winter" (Dongzhi), or the "shortest day". In some cultures it is seen as the middle of winter, while in others it is seen as the beginning of winter. In meteorology, winter in the Northern Hemisphere spans the entire period of December through February. The seasonal significance of the winter solstice is in the reversal of the gradual lengthening of nights and shortening hours of daylight during the day. The earliest sunset and latest sunrise dates differ from winter solstice, however, and these depend on latitude, due to the variation in the solar day throughout the year caused by the Earth's elliptical orbit (see earliest and latest sunrise and sunset).