Winter Solstice 2021

The winter solstice also called the hiemal solstice or hibernal solstice occurs when either of Earth's poles reaches its maximum tilt away from the Sun. This happens twice yearly, once in each hemisphere (Northern and Southern). For that hemisphere, the winter solstice is the day with the shortest period of daylight and longest night of the year, when the Sun is at its lowest daily maximum elevation in the sky. Either pole experiences continuous darkness or twilight around its winter solstice. The opposite event is the summer solstice. Depending on the hemisphere's winter solstice, at the Tropic of Cancer or Capricorn, the Sun reaches 90° below the observer's horizon at solar midnight, to the nadir.

The winter solstice occurs during the hemisphere's winter. In the Northern Hemisphere, this is the December solstice (usually December 21 or 22) and in the Southern Hemisphere, this is the June solstice (usually June 20 or 21). Although the winter solstice itself lasts only a moment, the term sometimes refers to the day on which it occurs. Other names are the "extreme of winter", or the "shortest day". Since the 18th century, the term "midwinter" has sometimes been used synonymously with the winter solstice, although it carries other meanings as well. Traditionally, in many temperate regions, the winter solstice is seen as the middle of winter, but today in some countries and calendars, it is seen as the beginning of winter.

Since prehistory, the winter solstice has been seen as a significant time of year in many cultures and has been marked by festivals and rituals. It marked the symbolic death and rebirth of the Sun. The seasonal significance of the winter solstice is in the reversal of the gradual lengthening of nights and shortening of days.