Social Sciences: Winter Solstice

  Winter solstice is  an astronomical phenomenon which for the Northern Hemisphere occurs in December and which for the Southern Hemisphere occurs in June.
For the Northern Hemisphere, the moment of winter solstice is when the sun's elevation with respect to the North Pole is at its most negative value since the previous December. (The elevation with respect to the South Pole is at its greatest since the previous December.) The hemisphere has its longest night and shortest day around the moment of solstice with the night within the Arctic being 24 hours long.
Analogous remarks hold for the Southern Hemisphere. Thus, for the Southern Hemisphere, at the moment of its winter solstice in June, the sun is at its greatest height as observed from the North Pole

Depending on one's position on the globe, the December solstice usually occurs on the 21st and the 22nd and the June solstice usually occurs on June the 20th, 21st, or 22nd. Thus the December 2016 solstice coincides with 20th of the month in American Samoa, with the 21st in London and with the 22nd at Kiritimati.

The axial tilt of Earth and gyroscopic effects of the planet's daily rotation keep the axis of rotation pointed at the same point in the sky. As the Earth follows its orbit around the Sun, the same hemisphere that faced away from the Sun, experiencing winter, will, in half a year, face towards the Sun and experience summer. Since the two hemispheres face opposite directions along the planetary pole, as one polar hemisphere experiences winter, the other experiences summer.
More evident from high latitudes, a hemisphere's winter solstice occurs on the shortest day and longest night of the year, when the sun's daily maximum elevation in the sky is the lowest.

 The winter solstice itself lasts only a moment in time, so other terms are used for the day on which it occurs, such as "midwinter", or "the shortest day". For the same reason, it should not be confused with "the first day of winter" or "the start of winter" (Lidong in the East Asian calendars). The seasonal significance of the winter solstice is in the reversal of the gradual lengthening of nights and shortening of days. 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).
Worldwide, interpretation of the event has varied from culture to culture, but many cultures have held a recognition of rebirth, involving holidays, festivals, gatherings, rituals or other celebrations around that time.