The summer solstice is upon us: 7 things to know about the longest day of the year

The summer solstice is upon us: Tuesday, June 20, was the longest day of 2017 for anyone living north of the equator.

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.

Vox.com suggests  a short scientific guide to the longest day of the year

1) Why do we have a summer solstice, anyway?

It is common knowledge that 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 reachespeak 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?

That depends on where you live. The further north you are, the more sunlight you’ll see during the solstice.

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.

4) What does all this have to do with Stonehenge?

No one really knows why Stonehenge was built some 5,000 years ago . 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:

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.

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!