The Northern Hemisphere's procession of dwindling days is about to reach its nadir. The winter solstice is the year's shortest day, but the start of winter also launches the sun's steady climb towards the long, warm days of summer.
The solstice occurs on Tuesday, December 22 at 4:48 UTC—that's late Monday night across most of North America. It happens at the same moment no matter where you live, but because we've divided Earth into 24 times zones people around the world observe it at 24 different times of day.
Why does the solstice occur anyway, and how have people observed it through history? Read on for everything you need to know about the December solstice.
The Solstice From Space
Earth's tilt is the reason for the season. Our planet orbits the sun while tilted at an average of 23.5 degrees, so the Northern and Southern Hemispheres receive unequal amounts of sunlight. This causes both the solstices and the seasons.
Each hemisphere's cooler half of the year happens when it's tilted away from the sun, and its winter solstice (December in the north, June in the south) marks the point when that half of the globe is tilted away from the sun at its most extreme angle.
Lack of exposure to the sun's rays makes the winter solstice the darkest day of the year but it's not the coldest. That's still a month or more away, depending on your location, because oceans and landmasses are slow to lose the heat energy they absorbed during the warmer months.
Earliest Sunset? Not on the Solstice
Still, most of us actually see the year's earliest sunset a week or two before the solstice. Why? Because the sun and our human clocks don't keep exactly the same time.
We've organized our days into precise 24-hour segments but the Earth doesn't spin on its axis so precisely. So while the time from noon to noon is always exactly 24 hours the time between solar noons, the moment each day when the sun reaches its highest peak, varies. So as we move through the year the chronological time of the solar noon shifts seasonally—and so do each day's sunrises and sunsets.
During December, solar noons can be some 30 seconds longer than 24 hours apart. That means while the shortest amount of total daylight falls on the solstice the day's sunset is actually a few minutes later on our clocks than it had been earlier in the month—because both sunrise and solar noon have also occurred later in that chronological day than they had in days earlier in December. To see the earliest sunset coincide more closely with the solstice simply head towards the Arctic, where the difference between the two dwindles.
For more than 2 billion Christians the solstice has long been overshadowed by Christmas. But to historian David Gwynn of the University of London, the proximity of the two events may not be an accident.
One theory holds that Christmas was set on December 25 to replace a Roman holiday, which had roots in a pagan cult of Sol Invictus (the unconquered sun), says Gwynn. A second theory surmises that early Christians arrived at December 25 by counting forwards nine months from March 25, the traditional date set for the Annunciation to Mary.