The winter solstice is Friday: 8 things to know about the shortest day of the year

A short scientific guide to the solstice.

The winter solstice is upon us: Friday, December 21, is the shortest day of 2018 for anyone living in the Northern Hemisphere.

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, the solstice occurs when the sun is directly over the Tropic of Capricorn, or 23.5° south latitude. In 2018, this will occur at 5:23 pm Eastern time on Friday.

Below is a short scientific guide to the solstice and the longest night of the year.

1) Why do we have solstices?

The winter and summer solstices, the seasons, and the changing length of daylight hours throughout the year are all due to one fact: Earth spins on a tilted axis.

The tilt — possibly caused by a massive object hitting Earth billions of years ago — means that for half the year, the North Pole is pointed toward the sun (as in the picture below). For the other half of the year, the South Pole gets more light. It’s why we have 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.

2) How many hours of sunlight will I get on the solstice?

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.) You can check out how much light you’ll get on the solstice on TimeAndDate.com.

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 during the week of December 5. But you can still catch the latest sunrises on January 4, 5, and 6. 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 the winter solstice have to do with Stonehenge?

No one knows for sure why Stonehenge was built some 5,000 years ago (at least we 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.

6) Is Friday night the longest night in Earth’s entire history?

Probably not, although it’s close. And the reason is quite interesting. Joseph Stromberg did a fantastic deep dive into this topic for Vox, 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) Are there solstices on other planets?

Yes! All the planets in our solar system rotate on a tilted axis and therefore have seasons, solstices, and equinoxes. Some of these tilts are minor (like Mercury, which is tilted at 2.11 degrees). But others are more like the Earth (23.5 degrees) or are even more extreme (Uranus is tilted 98 degrees!).

Below, see a beautiful composite image of Saturn on its equinox captured by the Cassini spacecraft (RIP) in 2009. The gas giant is tilted 27 degrees relative to the sun, and equinoxes on the planet are less frequent than on Earth. Saturn only sees an equinox about once every 15 years (because it takes Saturn 29 years to complete one orbit around the sun).

8) 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.

This past summer, NASA launched the Parker Solar Probe, 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.

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Happy solstice!