Friday, January 1, 2016

We all live for the Sun: Everything You Need to Know About the Winter Solstice

Everything You Need to Know About the Winter Solstice

From astronomy to religion, the year's shortest day brings enduring mysteries.

Egypt's Karnak Temple was built in alignment with the solstice, in order to focus light on a shrine to the sun god.

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.  

"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.

"What we're here for is to celebrate the fact that the cycle of the world turns," senior druid King Arthur Pendragon said at the 2014 Stonehenge solstice celebration. "It's a time [when] change and hope is renewed."