For the past six months, the days have grown shorter and the nights have grown longer in the Northern Hemisphere. But that's about to reverse itself. Winter solstice 2021, the shortest day of year and the official first day of winter, is on Tuesday, December 21. How it all works has fascinated people for thousands of years. First we'll look at the science and precise timing behind the solstice. Then we'll explore some ancient traditions and celebrations around the world.
The science and timing behind a winter solstice The winter solstice marks the shortest day of the year in the Northern Hemisphere, when the sun appears at its most southerly position, directly overhead at the faraway Tropic of Capricorn. The situation is the reverse in the Southern Hemisphere. There, the December solstice marks the longest day of the year -- and the beginning of summer in places such as Australia, Chile and South Africa.