Winter Solstice 2018: 10 Facts About the Shortest Day of the Year

Matt Cardy/Getty Images
Matt Cardy/Getty Images

Amid the whirl of the holiday season, many are vaguely aware of the approach of the winter solstice, but how much do you really know about it? Whether you're a fan of winter or just wish it would go away, here are 10 things to note—or even celebrate—about the solstice.

1. IT HAPPENS ON DECEMBER 21 UTC THIS YEAR.

Sun setting behind a tree in the winter
iStock.com/buxtree

The date of the winter solstice varies from year to year, and can fall anywhere between December 20 and December 23, with the 21st or 22nd being the most common dates. The reason for this is because the tropical year—the time it takes for the sun to return to the same spot relative to Earth—is different from the calendar year. The next solstice occurring on December 20 will not happen until 2080, and the next December 23 solstice will not occur until 2303.

2. IT HAPPENS AT A SPECIFIC, BRIEF MOMENT.

sun setting through the trees
iStock.com/yanikap

Not only does the solstice occur on a specific day, but it also occurs at a specific time of day, corresponding to the instant the North Pole is aimed furthest away from the sun on the 23.5 degree tilt of the Earth's axis. This is also the time when the sun shines directly over the Tropic of Capricorn. In 2017, this moment occurs at 4:28 p.m. UTC (Coordinated Universal Time). For those of us on Eastern Standard Time, the solstice will occur at 11:28 a.m. on December 21. And regardless of where you live, the solstice happens at the same moment for everyone on the planet (you can check what time it will occur for you here).

3. IT MARKS THE LONGEST NIGHT AND SHORTEST DAY OF THE YEAR FOR THE NORTHERN HEMISPHERE.

sun setting over Central Park
iStock.com/rmbarricarte

As most are keenly aware, daylight hours grow shorter and shorter as the solstice approaches, and begin to slowly lengthen afterward. It's no wonder that the day of the solstice is referred to in some cultures as the "shortest day" or "extreme of winter." New York City will experience 9 hours and 15 minutes of sunlight, compared to 15 hours and 5 minutes on the summer solstice. Helsinki, Finland, will get 5 hours and 49 minutes of light. Barrow, Alaska, will not have a sunrise at all (and hasn't since mid-November; its next sunrise will be on January 22), while the North Pole has had no sunrise since October. The South Pole, though, will be basking in the glow of the midnight sun, which won't set until March.

4. ANCIENT CULTURES VIEWED THE WINTER SOLSTICE AS A TIME OF DEATH AND REBIRTH.

snow on tree branches
iStock.com/Eerik

The seeming death of the light and very real threat of starvation over the winter months would have weighed heavily on early societies, who held varied solstice celebrations and rites meant to herald the return of the Sun and hope for new life. Scandinavian and Germanic pagans lit fires and may have burned Yule logs as a symbolic means of welcoming back the light. Cattle and other animals were slaughtered around midwinter, followed by feasting on what was the last fresh meat for several months. The modern Druidic celebration Alban Arthan reveres the death of the Old Sun and birth of the New Sun.

5. THE DAY MARKS THE DISCOVERY OF NEW AND STRANGE WORLDS.

Pilgrims landing at Plymouth Rock
Hulton Archive/Getty Images

The Pilgrims arrived at Plymouth on December 21, 1620, to found a society that would allow them to worship freely. On the same day in 1898, Pierre and Marie Curie discovered radium, ushering in an atomic age. And on December 21, 1968, the Apollo 8 spacecraft launched, becoming the first manned moon mission.

6. THE WORD SOLSTICE TRANSLATES ROUGHLY TO "SUN STANDS STILL."

colorful sunset
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Solstice derives from the Latin scientific term solstitium, containing sol, which means "sun," and the past participle stem of sistere, meaning "to make stand." This comes from the fact that the sun’s position in the sky relative to the horizon at noon, which increases and decreases throughout the year, appears to pause in the days surrounding the solstice. In modern times, we view the phenomenon of the solstice from the position of space, and of the Earth relative to the Sun. Earlier people, however, were thinking about the Sun's trajectory, how long it stayed in the sky and what sort of light it cast.

7. STONEHENGE IS ALIGNED TO THE SUNSET ON WINTER SOLSTICE.

Stonehenge sunset
iStock.com/jessicaphoto

The primary axis of the megalithic monument is oriented to the setting sun, while Newgrange, another structure built around the same time as Stonehenge, lines up with the winter solstice sunrise. Some have theorized that the position of the Sun was of religious significance to the people who built Stonehenge, while other theories hold that the monument is constructed along natural features that happen to align with it. The purpose of Stonehenge is still subject to debate, but its importance on the winter solstice continues into the modern era, as thousands of hippies, pagans, and other types of enthusiasts gather there every year to celebrate the occasion.

8. ANCIENT ROMANS CELEBRATED REVERSALS AT THE MIDWINTER FESTIVAL OF SATURNALIA.

Saturnalia parade
A Saturnalia celebration in England in 2012.
Christopher Furlong/Getty Images

The holiday, which began as a festival to honor the agricultural god Saturn, was held to commemorate the dedication of his temple in 497 BCE. It quickly became a time of widespread revelry and debauchery in which societal roles were overturned, with masters serving their slaves and servants being allowed to insult their masters. Mask-wearing and play-acting were also part of Saturnalia's reversals, with each household electing a King of Misrule. Saturnalia was gradually replaced by Christmas throughout the Roman Empire, but many of its customs survive as Christmas traditions.

9. SOME TRADITIONS HOLD THAT DARK SPIRITS WALK THE EARTH ON THE WINTER SOLSTICE.

Snowy woods
iStock.com/Serjio74

The Iranian festival of Yalda is celebrated on the longest night of the year. In pre-Islamic times, it heralded the birth of Mithra, the ancient sun god, and his triumph over darkness. Zoroastrian lore holds that evil spirits wander the earth and the forces of the destructive spirit Ahriman are strongest on this long night. People are encouraged to stay up most of the night in the company of one another, eating, talking, and sharing poetry and stories, in order to avoid any brushes with dark entities. Beliefs about the presence of evil on the longest night are also echoed in Celtic and Germanic folklore.

10. SOME THOUGHT THE WORLD WOULD END ON THE 2012 WINTER SOLSTICE.

snowy woods with sun through the trees
iStock.com/Delpixart

December 21, 2012 corresponds to the date 13.0.0.0.0 in the Mesoamerican Long Count calendar used by the ancient Mayans, marking the end of a 5126-year cycle. Some people feared this juncture would bring about the end of the world or some other cataclysmic event. Others took a more New Age-y view (literally) and believed it heralded the birth of a new era of deep transformation for Earth and its inhabitants. In the end, neither of these things appeared to occur, leaving the world to turn through winter solstices indefinitely, or at least as long as the Sun lasts.

A version of this story originally ran in 2015.

What Caused Pangea to Break Apart?

iStock.com/alfimimnill
iStock.com/alfimimnill

Emily Devenport:

There's another way to look at this question. People tend to think in terms of supercontinents forming and then breaking up again due to convection currents in the mantle, hot material rising and causing rifts in weaker spots, possibly in old sutures where the continents were shoved together—but what is really happening is that ocean basins are opening and closing, and the ocean has an active role in subduction.

The opening and closing of an ocean basin is called a Wilson Cycle. It begins when hot material rising from the mantle stretches the overlying crust. As molten material rises, a rift is formed. The rift is widened as material continues to squeeze into it. If that rifting goes on long enough, through a broad enough swath of a continent, ocean water will eventually flow into it, and an ocean basin begins to form. The upwelling of hot material will continue to rise through that thinner area of crust, pushing the plates apart. The Atlantic Ocean is an example of a basin that is well along in the Wilson Cycle; eventually subduction is going to begin at its margins, and the whole shebang will pivot.

This will happen because at the edge of continents, sediments accumulate. The weight of those sediments, combined with the weight of the water, drives the heavier, denser edge of the oceanic plate under the continental crust, which is fatter and lighter. Eventually subduction begins, and the basin begins to close again. The Pacific Ocean is an example of a basin that's closing.

If you look at a map of the oceanic rift zones, you'll notice that the one in the Atlantic is pretty much in the middle of that ocean, but the Pacific rift zone has been pulled all the way over to North America above Central America. Subduction is actively occurring on all margins of that plate.

The simple picture is that the continents are moving toward each other across the Pacific Ocean while the Atlantic Basin continues to widen. The truth is more complicated. When plates subduct, the water in the crust lowers the melting point of those rocks, so partial melting occurs. The partially melted material begins to rise through the overlying rocks, because it's less dense, and decompression melting occurs. Eventually, the upwelling of hot material forms plutons and volcanoes above the subduction zones. Fore-arc and Back-arc [PDF] basins can form. As the oceanic crust is pulled under the continental plate, island chains and other chunky bits get sutured to the edge of the continent along with sediments, making it larger. Our world is ~4.6 billion years old, so our continents are really large, now. They're unlikely to rift through the ancient cratons that formed their hearts.

What will happen if subduction begins on the eastern side of North America before the Pacific Basin closes? The margin next to California is a transform fault; it's not subducting. Will it eventually push itself under that part of North America again, or will the transform zone get bigger? The hot spot that was driving the ancient Farallon Plate under North America was eventually overridden by the southwestern states (Arizona, New Mexico, etc.) forming a rift zone. Will it continue to rift or poop out?

There are computer models predicting what supercontinent may form next. They will continue to change as our understanding of tectonic processes gets more accurate.

This post originally appeared on Quora. Click here to view.

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