Web Desk(June 20, 2018): The summer solstice is upon us: June 21 will be the longest day of 2018 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 over the Tropic of Cancer or 23.5° north latitude. In 2018, this will occur at exactly 6:07 am Eastern on Thursday the 21st. So set your alarm, or feel free to sleep right through it.
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 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.
2) How many hours of sunlight will I get on Thursday?
That depends on where you live. The further north you are, the more sunlight you’ll see during the solstice.Note that the solstice also gives us the longest twilight of the year, usually about 1 to 1.5 extra hours after sunset.
3) Is the solstice the latest sunset of the year?
Not necessarily. Just because June 21 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, the latest sunset will be on the day after the Solstice, the 22nd. 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 centre.
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 2018 would be the longest day in all of history. But while it’s certainly up there, it doesn’t quite take top honours.
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.