The winter solstice is the time when the Sun has reached its lowest altitude in the Northern or the Southern Hemispheres. The winter solstice occurs on December 21-22 of each year in the north, while in the south it occurs in June 20-21 every year. Each Hemisphere receives six months of sunlight due to the Earth´s orbit around the Sun, and the tilt of the Earth´s axis with respect to the ecliptic. At the time of the winter solstice, the Sun´s elevation is the lowest in the sky, making the day on that date the shortest day, and the night the longest night.
Winter solstice in both hemispheres
The winter solstice most commonly marks the start of winter in both Hemispheres. While there is a winter solstice in the northern Hemisphere, there is a summer solstice in the Southern Hemisphere (the day with the most sunlight), and while there is a summer solstice in the northern Hemisphere, there is a winter solstice (the day of the longest night) in the southern hemisphere. This occurs because the Earth´s axial tilt has reached 23° 26´at the time of the solstices. As the Earth continues its rotation around the Sun this tilt decreases until it reaches a 0° tilt with respect to the ecliptic at the time of the equinoxes.
Solstices and equinoxes
The solstices and equinoxes mark the start of the seasons. The solstices occur in June and December, and the equinoxes in March and September. The solstices occur six months apart from one another, as well as the equinoxes. There is a winter solstice and a summer solstice and there is a vernal (spring) equinox and an autumnal (fall) equinox. When a summer solstice occurs in the northern Hemisphere, a winter solstice occurs in the Southern Hemisphere. And the reverse is true when the Sun illuminates the other part of the hemisphere.
Winter solstice in the Northern Hemisphere
At the time of the winter solstice in the Northern hemisphere, the Sun has reached its lowest altitude, looking south, and at the time of the winter solstice in the Southern Hemisphere, the Sun has reached its lowest point in the sky, looking north. This is due because the Earth´s orbit around the Sun does not maintain an upright position, but instead, it sustains an orbit that is tilted by 23° 26´; therefore each Hemisphere receives the light of the Sun during six months every year.
The path of the Sun in the sky changes every day. At the day of the winter solstice in the Northern sky, the Sun has traveled its furthest point on the horizon towards the south, after which it reverses direction, and six months later, a summer solstice is experienced in the Northern Hemisphere, while in the southern Hemisphere, a winter solstice occurs. Over the course of one year, the Sun travels back and forth along the horizon, and we experience the seasons on Earth. Halfway between the solstices are the equinoxes.
Ancient cultures had observed the winter solstices, and other astronomical phenomena in the sky. This allowed them to relate the astronomical events to their traditions, such as harvesting, mating, marrying, etc. Many structures from ancient civilizations seem to have been aligned to the winter solstice, such as Stonehenge in Britain, and some Maya pyramids in Yucatan, Mexico. According to wolfram.com, at the day of the winter solstice, the time elapsed between sunrise and setting of the sun is the shortest, making that day the one with the less sunlight and the longest night.