The summer solstice is upon us: June 20th and the 21st will be the longest days 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 solstice is still pretty neat.
Technically speaking, the summer solstice occurs when the sun is directly overhead the Tropic of Cancer, or 23.5° north latitude. In 2017, this will occur at exactly 12:24 am (Eastern) on the 21st. (But we can celebrate on either day.)
Below is a short scientific guide to the longest day of the year (though not, as we’ll see, the longest day in Earth’s history — that happened back in 1912).
1) Why do we have a summer solstice, anyway?
Okay, most people know this one. Earth orbits around the sun on a tilted axis (probably because our planet collided with some other massive object billions of years ago, back when it was still being formed).
So between March and September, Earth’s Northern Hemisphere gets more exposure to direct sunlight over the course of a day. The rest of the year, the Southern Hemisphere gets more. 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.
2) How many hours of sunlight will I get on Tuesday?
On the off chance you live near the Arctic Circle, the sun never really sets during the solstice.
(By contrast, during the winter solstice, Fairbanks only gets about three hours of sunlight.)
Here’s another cool way to visualize the extreme of the summer 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. (You can easily make a similar image at home. All you need is a can, photo paper, some tape, and a pin. Instructions here.)
Side note: This year, the Muslim holy month of Ramadan coincides with the solstice. (Ramadan’s dates vary each year, but in 2017 it runs from May 26 to June 24.) Which makes for a grueling challenge in some places: Muslims are supposed to fast until sunset during Ramadan, but for those living in Norway, Sweden, or Iceland, daylight can last up to 20 hours. “In these cases,” Vox’s Jennifer Williams explains, “Muslim religious authorities have decreed that Muslims can either fast along with the closest Muslim country or fast along with Mecca, Saudi Arabia.”
3) Is the solstice the latest sunset of the year?
Not necessarily. Just because June 20 is the longest day of the year for the Northern Hemisphere doesn’t mean every location has its earliest sunrise or latest sunset on that day.
If you live in Washington, DC, you missed the earliest sunrise — it happened back on June 13. But you can still catch the latest sunset on June 27. If you like sleeping in, that’s arguably the most exciting day of the summer.
4) What does all this have to do with Stonehenge?
No one really knows why Stonehenge was built some 5,000 years ago (at least I don’t, sorry). But one possibility is that it was used to mark solstices and equinoxes. That’s because during the summer solstice, the sun rises just over the structure’s Heel Stone and hits the Altar Stone dead center.
Here’s a graphic from NASA imagining what a summer solstice sunrise might’ve looked like back when Stonehenge was fully intact:
Nowadays, humans still gather to pay homage the summer solstice at Stonehenge — they just use modern technology, like so:
The Wikipedia entry on Stonehenge is absurdly detailed, so read up on that if you want more.
5) Is this the longest day in Earth’s entire history?
Probably not, although it’s close. And the reason why is quite interesting. Joseph Stromberg did a fantastic deep dive into this topic for Vox 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 — 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 2017 would be the longest day 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 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 planet’s core, earthquakes, ocean currents, and seasonal wind changes can also speed up or slow down Earth’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 its rotation keeps slowing (forcing timekeepers to add leap seconds to the calendar periodically). Which means that in the future, there will be plenty of summer solstices that set new records as the “longest day in Earth’s history.”
6) Isn’t there going to be a solar eclipse?
No, not on the solstice.
But there will be a rare solar eclipse across the entire continental US a bit later in the summer, on August 21.
On that day, the Earth, moon, and sun will be in perfect alignment to cast a 60-mile-wide shadow that will trace itself across the country like a dark laser pointer on a whiteboard.
In the bull’s eye center of the moon’s shadow known as the totality, the sky will go dark for a few minutes in the middle of the day, stars will appear, and birds will become confused and start chirping their nighttime songs. And it’s all because of a cosmic coincidence: From the Earth, both the moon and sun appear to be roughly the same size.
7) I clicked this article accidentally and really just want a cool picture of the sun
The image above was taken by NASA’s Solar Dynamics Observatory, a spacecraft launched in 2010 to better understand the sun. Caption: “A full-disk multiwavelength extreme ultraviolet image of the sun taken March 30, 2010. False colors trace different gas temperatures. Reds are relatively cool (about 60,000 Kelvin, or 107,540 F); blues and greens are hotter (greater than 1 million Kelvin, or 1,799,540 F).”
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 (i.e., 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!