What is it?
The winter solstice is the shortest day of the year in the Northern Hemisphere. From June to December, the days shorten and shorten until the solstice. After the winter solstice, days gradually grow longer again (yay!), which brings warmer temperatures. On the actual solstice, the North Pole gets zero energy from the sun
— that is, no sunlight at all.
In the summer, we celebrate the summer solstice, the longest day of the year. (In the Southern Hemisphere, everything is flip-flopped — they're celebrating the summer solstice in December.)
When is it?
Each year, the winter solstice falls on either December 21 or 22. This year, it takes place Tuesday, December 22 at 4:48 UTC (December 21 at 11:48 p.m. EST).
But I noticed the sun started setting later before the solstice...
An astute observation! Depending on where you live, the shortest day of the year doesn't necessarily
fall on the day with the earliest sunset or the latest sunrise. This has to do with what's called "true solar noon
," the time when the sun is at its highest point in the sky. In early December, solar noon is about ten minutes earlier than it is when we hit the solstice. Thus, depending on the latitude where you live, the sunset may actually be slightly later on the solstice than it was earlier in the month. The closer you live to the Arctic, the more closely the earliest sunset and the winter solstice will match up.
What about the latest sunrise?
Unless you live in the Arctic Circle, the latest sunrise usually arrives in early January, which makes sense, knowing that solar noon moves later in the day starting in early December. There's a variation in solar noon and noon on the clock, because of the tilt of the earth's axis and the earth's not-quite-circular orbit around the sun.
How long have we known about the solstice?
Our earliest ancestors tracked the seasons and years by changes in the sky: the movement of the sun, stars, and moon. Stonehenge is one of the most famous monuments in the world built to observe and celebrate our trek around the sun. Nowadays, 3,000 to 5,000 people visit Stonehenge to watch the sunrise on the winter solstice and up to 30,000
visit for the summer solstice.