The winter solstice is Thursday: 7 things to know about the shortest day of the year
The winter solstice is upon us: Thursday, December 21, will be the shortest day of 2017 for anyone living north of the equator. If pagan rituals are your thing, this is probably a big moment for you. If not, the official first day of winter is neat for other reasons too.
Technically speaking, the winter solstice occurs when the sun is directly over the Tropic of Capricorn, or 23.5° south latitude. In 2017, this will occur at exactly 11:28 am Eastern time on December 21.
Below is a short scientific guide to the solstice and the longest night of the year (though not, as we’ll see, the longest night in Earth’s history — that happened back in 1912).
1) Why do we have a winter solstice, anyway?
Most people know this one. Earth orbits around the sun on a tilted axis (likely because our planet collided with some other massive object billions of years ago, back when it was still being formed).
So between September and March, Earth’s Northern Hemisphere gets less exposure to direct sunlight over the course of a day. The rest of the year, the north gets more direct sunlight and the Southern Hemisphere gets less. It’s the reason for the seasons:
In the Northern Hemisphere, "peak" sunlight usually occurs on June 20, 21, or 22 of any given year. That’s the summer solstice. By contrast, the Southern Hemisphere reaches peak sunlight on December 21, 22, or 23 and the north hits peak darkness — that’s our winter solstice.
In the Northern Hemisphere, the winter solstice occurs when the sun is directly overhead the Tropic of Capricorn, or 23.5° south latitude. Like so:
In 2017, the winter solstice will occur at exactly 11:28 am Eastern Time.
2) How many hours of sunlight will I get on Thursday?
That depends on where you live. The farther north from the equator you are, the less sunlight you’ll get during the solstice — and the longer the night will be. Alaska climatologist Brian Brettschneider created this terrific guide for the United States:
Here in Washington, DC, we will get nine hours and 26 minutes of sunlight.
On the off chance you live near the Arctic Circle, you’ll barely get any daylight during the solstice. Fairbanks, Alaska, for instance will get three hours and 41 minutes. (If you live north of the circle, you’ll get none at all.)
The time lapse below shows the eerie scene in Fairbanks — the sun basically skims the horizon for a brief while and then vanishes:
(Brettschneider has more charts on the solstice, including the differences in daylight between winter and summer solstices; his entire post is worth your time.)
Here’s another cool way to visualize the extreme of the winter solstice. In 2013, a resident of Alberta, Canada — several hundred miles south of Fairbanks, but still in a high latitude — took this pinhole camera photograph of the sun’s path throughout the year, and shared it with the astronomy website EarthSky. You can see the dramatic change in the arc of the sun from December to June.
This is a 6 month pinhole photo taken from solstice to solstice, in Medicine Hat, Alberta, Canada. We are one of the sunniest cities in Canada, and this shows it nicely.
Posted by Ian Hennes on Saturday, December 21, 2013
(You can easily make a similar image at home. All you need is a can, photo paper, some tape, and a pin. Instructions here.)
3) Is the winter solstice the earliest sunset of the year?
Not usually. Just because December 21 is the shortest day of the year for the Northern Hemisphere, it doesn’t mean every location has its earliest sunset or latest sunrise on that day.
Brettschneider also created this map showing earliest sunset/latest sunrise for different parts of North America:
If you live in DC, you missed the earliest sunset — it happened back on December 6. But you can still catch the latest sunrises on January 3 and 4. If you like sleeping in, that’s arguably the most exciting day of the winter. No annoying sun bothering you in the morning.
4) Is the winter solstice the coldest day of the year?
Not usually! It’s true that the Northern Hemisphere gets the least direct sunlight on the winter solstice (which is officially the first day of winter). But the coldest months are yet to come — usually in January or February, depending on where you live.
A big reason for this “seasonal lag” is that the Earth’s massive oceans absorb much of the sun’s energy and release it slowly, over time. So there’s a delay between when there’s the least sun and when the air temperatures are actually coldest. The same thing happens in summer — there’s a delay between when solar insolation is at its maximum (the summer solstice in June) and when the hottest months are (usually July or August).
This seasonal lag varies greatly from place to place — during the summer, it’s pretty extreme in San Francisco, which is surrounded by water on three sides and where temperatures don’t typically peak until September. Likewise, places far from large bodies of water, like Iowa, can often see sharper swings in temperature than places like Rome that are surrounded by ocean.
5) What does all this have to do with Stonehenge?
No one knows for sure why Stonehenge was built some 5,000 years ago (at least I don’t, sorry). But one strong possibility is that it was used to mark solstices and equinoxes. That’s because the structure is directly aligned toward the sunset during the winter solstice. (The sun also rises directly over the Heel Stone during the summer solstice.)
Why was the winter solstice a big deal? Here’s Teresa Wilson of the American Astronomical Society to explain: “While the summer solstice draws a larger crowd, the winter solstice may have been more important to the ancient builders. At this time, cattle were slaughtered so the animals did not need to be fed through the winter, and wine and beer made previously had finally fermented.”
Nowadays, humans still gather to pay homage the winter solstice at Stonehenge — they just use modern technology.
At least the winter solstice at Stonehenge seems like a top-rate party:
The Wikipedia entry on Stonehenge is absurdly detailed, so read up on that if you want more. And Vox’s Joss Fong created this video to explain how Stonehenge was (probably) constructed.
6) Is Thursday the longest night in Earth’s entire history?
Probably not, although it's close. And the reason why is quite interesting. My colleague Joseph Stromberg did a fantastic deep dive into this topic a few years back, but here’s the two-minute version.
Ever since the Earth has had liquid oceans and a moon, its rotation has been gradually slowing over time due to tidal friction. That means that over very, very long periods of time, the days have been getting steadily longer. About 4.5 billion years ago, it took the Earth just six hours to complete one rotation. About 350 million years ago, it took 23 hours. Today, of course, it takes about 24 hours. And the days will gradually get longer still.
Given that, you'd think the winter solstice of 2016 would be the longest night in all of history. But while it's certainly up there, it doesn't quite take top honors.
That's because tidal friction isn’t the only thing affecting the Earth’s rotation — there are a few countervailing factors. The melting of glacial ice, which has been occurring since the end of the last ice age 12,000 years ago (and is now ramping up because of global warming) is actually speeding up Earth’s rotation very slightly, shortening the days by a few fractions of a millisecond. Likewise, geologic activity in the Earth’s core, earthquakes, ocean currents, and seasonal wind changes can also speed up or slow down the planet's rotation.
When you put all these factors together, scientists have estimated that the longest day in Earth’s history (so far) likely occurred back in 1912. That year’s summer solstice was the longest period of daylight the Northern Hemisphere has ever seen. And, conversely, the 1912 winter solstice was the longest night we’ve ever seen.
Eventually, the effects of tidal friction should overcome all those other factors, and Earth’s days will get longer and longer as the planet’s rotation keeps slowing (forcing timekeepers to add leap seconds to the calendar periodically). Which means that in the future, there will be plenty of winter solstices that set new records as the "longest night in Earth's history."
7) I clicked this article accidentally and really just want a cool picture of the sun
We got you:
The image above was taken by NASA’s Solar Dynamics Observatory, a spacecraft launched in 2010 to better understand the sun.
In 2018, NASA will launch the Parker Probe Plus, a spacecraft that will come within 4 million miles of the surface of the sun (much closer than any spacecraft has been before). The goal is to study the sun’s atmosphere, weather, and magnetism, and figure out the mystery of why the sun’s corona (its atmosphere) is much hotter than its surface. Still, even several million miles away, the probe will have to withstand temperatures of 2,500 degrees Fahrenheit.
It’s essential to understand the sun: It’s nothing to mess with. Brad Plumer wrote about what happens when the sun erupts and sends space weather our way to wreak havoc on Earth.
Happy solstice!