Thursday is the winter solstice -- the shortest day of 2017.
December 21 also marks the first day of astronomical winter -- although meteorological winter began December 1. In the Northern Hemisphere, where most of the world's population lives, the winter solstice marks the shortest day of the year and the longest night.
The good news for sunlight seeking Northern Hemisphere natives is that the days start getting longer beginning Friday -- and they can start counting down to spring.
It's the shortest day of the year because, during the winter solstice, the Northern Hemisphere is tilted 23.5 degrees away from the sun and the sun's position is at its most southerly point, directly over the Tropic of Capricorn -- right across the middle of Australia.
With the sun positioned so far to the south the tilt of the Earth leaves the Arctic Circle entirely in the dark. In the Southern Hemisphere, the opposite is true. They experience their longest day and shortest night of the year.
The Southern Hemisphere refers to this event as the December Solstice because seasonally it signals the beginning of their summer.
Historically the solstice has been celebrated by people around the globe. At Stonehenge in England, many gather to watch the sunrise to signify the start of longer days ahead. In ancient times the Incas would celebrate the solstice at the Temple of the Sun in Machu Picchu, Peru.