The summer season solstice is nearly upon us: June 21 would be the longest day of 2019 for anybody residing north of the equator. If pagan rituals are your factor, that is in all probability an enormous second for you. If not, the solstice continues to be fairly neat.

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Technically talking, the summer season solstice happens when the solar is instantly over the Tropic of Most cancers, or 23.5 levels north latitude. This can happen at precisely 11:54 am Japanese on Friday the 21st.

Under is a brief scientific information to the longest day of the yr. (Although not, as we’ll see, the longest day in Earth’s historical past — that occurred again in 1912.)

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1) Why do we’ve got a summer season solstice, anyway?

Okay, most individuals know this one. Earth orbits across the solar on a tilted axis. (Most likely as a result of our planet collided with another huge object billions of years in the past, again when it was nonetheless being fashioned.)

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So between March and September, Earth’s Northern Hemisphere will get extra publicity to direct daylight over the course of a day. The remainder of the yr, the Southern Hemisphere will get extra. It’s why we’ve got seasons.


Within the Northern Hemisphere, “peak” daylight normally happens on June 20, 21, or 22 of any given yr. That’s the summer season solstice. In contrast, the Southern Hemisphere reaches peak daylight on December 21, 22, or 23 and the Northern hits peak darkness — that’s our winter solstice.


2) What number of hours of daylight will I get on the 21st?

That depends upon the place you reside. The farther north you’re, the extra daylight you’ll see through the solstice. Alaska-based climatologist Brian Brettschneider created this terrific information:

On the possibility you reside close to the Arctic Circle, the solar by no means actually units through the solstice.

Right here’s one other cool option to visualize the intense of the summer season solstice. In 2013, a resident of Alberta, Canada — a number of hundred miles south of Fairbanks however nonetheless in a excessive latitude — took this pinhole digicam {photograph} of the solar’s path all year long and shared it with the astronomy web site EarthSky. You’ll be able to see the dramatic change within the arc of the solar from December to June. (You’ll be able to simply make an identical picture at house. All you want is a can, photograph paper, some tape, and a pin. Directions right here.)

It is a 6 month pinhole photograph taken from solstice to solstice, in Drugs Hat, Alberta, Canada. We’re one of many sunniest cities in Canada, and this reveals it properly.

Posted by Ian Hennes on Saturday, December 21, 2013

Word that the solstice additionally offers us the longest twilight of the yr, normally about one to at least one and a half additional hours after sundown. (Brettschneider has extra charts on that; his whole submit is value your time.)

3) Is the solstice the most recent sundown of the yr?

Not essentially. Simply because June 21 is the longest day of the yr for the Northern Hemisphere doesn’t imply each location has its earliest dawn or newest sundown on that day.

When you reside in Washington, DC, the most recent sunsets will begin on the day after the solstice, the 22nd. When you like sleeping in, that’s arguably probably the most thrilling day of the summer season. can inform you when the most recent sundown will happen in your space.

4) What does all this should do with Stonehenge?

Nobody actually is aware of why Stonehenge was constructed some 5,000 years in the past (at the very least we don’t, sorry). However one chance is that it was used to mark solstices and equinoxes. That’s as a result of through the summer season solstice, the solar rises simply over the construction’s Heel Stone and hits the Altar Stone lifeless heart.

Right here’s a graphic from NASA imagining what a summer season solstice dawn may’ve regarded like again when Stonehenge was absolutely intact:


These days, people nonetheless collect to pay homage to the summer season solstice at Stonehenge — they only use fashionable expertise, like so:

Thousands Gather To Celebrate Summer Solstice At Stonehenge

Tim Eire/Getty Photos

Individuals at Stonehenge on the solstices know the way to throw a celebration. Right here’s a picture from a latest winter solstice on the website.

Matt Cardy/Getty Photos

5) Is that this the longest day in Earth’s whole historical past?

Most likely not, though it’s shut. And the rationale why is sort of attention-grabbing. Joseph Stromberg did a improbable deep dive into this subject for Vox a number of years again, however right here’s the two-minute model.

Ever for the reason that Earth has had liquid oceans and a moon, its rotation has been step by step slowing over time on account of tidal friction. That implies that over very, very lengthy intervals of time, the times have been getting steadily longer. About 4.5 billion years in the past, it took Earth simply six hours to finish one rotation. About 350 million years in the past, it took 23 hours. Right this moment, after all, it takes about 24 hours. And the times will step by step get longer nonetheless.

Provided that, you’d suppose 2018 can be the longest day in all of historical past. However whereas it’s definitely up there, it doesn’t fairly take prime honors.

That’s as a result of tidal friction isn’t the one factor affecting Earth’s rotation; there are a number of countervailing components. The melting of glacial ice, which has been occurring for the reason that finish of the final ice age 12,000 years in the past (and is now ramping up due to world warming), is definitely dashing up Earth’s rotation very barely, shortening the times by a number of fractions of a millisecond. Likewise, geologic exercise within the planet’s core, earthquakes, ocean currents, and seasonal wind modifications also can velocity up or decelerate Earth’s rotation.

While you put all these components collectively, scientists have estimated that the longest day in Earth’s historical past (thus far) possible occurred again in 1912. That yr’s summer season solstice was the longest interval of daylight the Northern Hemisphere has ever seen (and, conversely, the 1912 winter solstice was the longest evening we’ve ever seen).

Ultimately, the consequences of tidal friction ought to overcome all these different components, and Earth’s days will get longer and longer as its rotation retains slowing (forcing timekeepers so as to add leap seconds to the calendar periodically). Which implies that sooner or later, there might be loads of summer season solstices that set new data because the “longest day in Earth’s historical past.”

6) Do I have to put on sunscreen?

Sure, it’s best to, although, as Vox’s Julia Belluz has reported, the analysis on whether or not sunscreen truly helps stop the extra aggressive type of pores and skin most cancers is missing. As she writes:

The US Preventive Companies Job Drive summed up the proof on the well being results of sunscreen use … [and] discovered that sunscreen diminished the incidence of squamous cell most cancers, however that it had no impact on basal cell most cancers. What’s extra, “There aren’t any direct knowledge in regards to the impact of sunscreen on melanoma incidence.”

Nonetheless, analysis is at all times evolving and newer research are rising that present sunscreen can curb melanoma danger, reminiscent of this long-term trial from Australia.

That mentioned, it does, positively assist stop sunburn, which is disagreeable. For extra on the science of sunscreen, learn Belluz’s explainer.

7) Are there solstices on different planets?

Sure! All of the planets in our photo voltaic system rotate on a tilted axis and due to this fact have seasons, solstices, and equinoxes. A few of these tilts are minor (like Mercury, which is tilted at 2.11 levels). However others are extra just like the Earth (23.5 levels) or are much more excessive (Uranus is tilted 98 levels!).

Under, see a good looking composite picture of Saturn on its equinox captured by the Cassini spacecraft (RIP) in 2009. The fuel large is tilted 27 levels relative to the solar, and equinoxes on the planet are much less frequent than on Earth. Saturn solely sees an equinox about as soon as each 15 years (as a result of it takes Saturn 29 years to finish one orbit across the solar).

Cassini Imaging Workforce/NASA

8) I heard there was a photo voltaic eclipse taking place this summer season

You heard right! There might be a photo voltaic eclipse on July 2. And when you’re in Chile or Argentina, (or within the South Pacific), you’ll have the ability to see it.

Right here’s the trail that the totality (the world the place you possibly can see the solar utterly lined up by the moon) will take throughout the globe:

9) I clicked this text by chance and actually simply desire a cool image of the solar

We acquired you:

The solar blew out a coronal mass ejection together with a part of a photo voltaic filament over a three-hour interval (February 24, 2015). Among the strands fell again into the solar.
Photo voltaic Dynamics Observatory/NASA

The picture above was taken by NASA’s Photo voltaic Dynamics Observatory, a spacecraft launched in 2010 to higher perceive the solar.

Final yr, NASA launched the Parker Photo voltaic Probe, a spacecraft that may come inside Four million miles of the floor of the solar (a lot nearer than any spacecraft has been earlier than). The purpose is to review the solar’s ambiance, climate, and magnetism and determine the thriller of why the solar’s corona (its ambiance) is far hotter than its floor. Nonetheless, even a number of million miles away, the probe must face up to temperatures of two,500 levels Fahrenheit.

It’s important to grasp the solar: It’s nothing to mess with. At Vox, Brad Plumer wrote about what occurs when the solar erupts and sends house climate our option to wreak havoc on Earth.

Completely happy solstice!


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