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.

Can I See the Solstice?

Ancient people didn't know about Earth's orbit but they still observed the solstice by noting what was happening in the skies overhead.

The sun's arc across the sky has been steadily dropping lower and becoming shorter since June. Now, at the north's winter solstice, it has reached its lowest possible arc—so low in fact 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, meaning “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 a number of monuments were built to commemorate the solstice. One example is

Newgrange, a huge Stone Age tomb mound built in the Irish countryside around 3200 B.C., about 1,000 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.

In Peru the Paracas people (circa 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 circa 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 from Angkor Wat to Machu Picchu.

Christmas Connection?

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.

"It is also true that these explanations are not necessarily mutually exclusive,” says Gwynn.

Still Celebrating the Sun

Some ancient solstice celebrations continue to the present. 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.

Scandinavians celebrated Juul, or Yule, a multi-day feast marking the sun god's return. In Britain, Druids observed the solstice by cutting mistletoe. Today some of these traditions are still observed by modern pagans.