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 toward the long, warm days of summer.
This year, the northern winter solstice occurs on Wednesday, December 21, at 5:44 a.m. ET (10:44 UTC). 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 will 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.
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.
Most of us see the year's earliest sunset a week or two before the solstice. That’s 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 Earth doesn't spin on its axis that 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. As we move through the year, the chronological time of 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.
To see the earliest sunset coincide more closely with the solstice, simply head toward the Arctic, where the difference between the two dwindles.
Can I See the Solstice?
It is possible to see the effects of the solstice by noting what happens in the skies overhead, as well as the changes in sunlight over time.
The sun's arc across the sky has been steadily dropping lower and becoming shorter since June. At the northern winter solstice, it reaches its lowest possible arc—so low that in the few days surrounding the solstice, it appears to rise and set in the same place. That phenomenon produced the Latin phrasing from which the word “solstice” was derived, which means “sun stands still.”
The sun's low angle also means that your noontime shadow is the longest of the entire year during the winter solstice.
Ancient Solstice Sites
In the ancient world, people built a number of monuments to commemorate the solstice. One example is Newgrange, a huge Stone Age tomb mound built in the Irish countryside around 3200 B.C., or about a thousand years before Stonehenge. A tunnel facing the solstice sunrise runs to a main chamber, where the dead may have once been placed. A small window bathes the chamber in solstice light for 17 minutes.
The Paracas people of Peru, who lived around 800 to 100 B.C., crisscrossed the desert with lines of earth and rock called geoglyphs that connect ceremonial mounds with the place where the winter solstice sun sets on the horizon. The famed Nazca Lines—awe-inspiring monkeys, lizards, and other figures etched into the earth by a subsequent Peruvian culture around A.D. 1 to 700—also feature alignments with the winter solstice.
Ancient Egypt's sprawling Temple of Karnak was constructed in alignment with the winter solstice at Luxor more than 4,000 years ago. Similar alignments can be seen at Angkor Wat in Cambodia and Machu Picchu in Peru.
For more than two 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.
According to Gwynn, one theory holds that Christmas was set on December 25 to replace a Roman holiday, which had roots in the pagan cult of Sol Invictus (“the unconquered sun”).
Other solstice traditions color today's winter holiday celebrations. Scandinavians once celebrated Juul, or Yule, a multiday feast marking the sun god's return. In Britain, Druids observed the solstice by cutting mistletoe.
And some ancient solstice celebrations continue in the present day. Iran's Yalda festival marks the day when Mithra, an angel of light, was thought to have been born. The tradition was adopted into Zoroastrianism and is still observed by staying up late and savoring treats like watermelon and pomegranate.
China's Dōngzhì festival marks the time when winter's darkness begins to give way to light. Families observe this time by enjoying special foods, such as glutinous rice balls known as tang yuan.