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You can complain about having the longest day ever today, and here is the science to prove it!

Hosted by: Olivia Gordon
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[SciShow intro plays]

Olivia: Do you ever feel like you’ve just had the longest day ever? Well, you might be right. I mean, literally! On average, days on Earth are getting longer, as the rotation of our planet keeps slowing down. And in a paper published this week in the Proceedings of the Royal Society A, a group of British researchers came up with the best estimate yet for exactly how much longer our days are getting.

The scientists looked at thousands of years’ worth of records, going all the way back to 720 BCE. Of course, nobody was using telescopes 2700 years ago, but they did notice when the Sun or Moon went dark during an eclipse. And often, they wrote it down. And those records are enough to calculate the length of a day thousands of years ago with pretty good accuracy.

Then, after about 1620, when astronomers started observing the sky through telescopes, they wrote down when stars disappeared behind the Moon from one day to the next. That can be used to calculate the length of a day even more accurately than the eclipse method, based on what we know about when stars should disappear behind the Moon. And after 1962, we were measuring time using atomic clocks, so we have what’s practically a perfect record of how long every day has been for the past 50 years.

After gathering all these kinds of data, the UK researchers put it all together in a graph that mapped out how long a day has been since 720 BCE, and looked for a trend. What they found was that on average, the day has been getting about 1.8 milliseconds longer every century. In a previous paper published in 1995, this difference was calculated to be 1.7 milliseconds every century.

Which, okay, is a tiny difference. But it’s actually super important! Because, we already know why our days are getting longer: the pull of the Moon’s gravity shifts around Earth’s insides, creating bulges. It’s the same thing that causes tides. Then, as Earth rotates, the bulges move away from the Moon, but the Moon pulls back on them, and hat pull slows Earth’s rotation.

But here’s the thing: Mathematically speaking, the force of the moon’s gravitation on Earth should actually be slowing down our rotation even more -- making the day longer by 2.3 milliseconds every century. But we just said that the day is only slowing by about 1.8 milliseconds! So this means there must be other factors at work that are actually speeding up Earth’s rotation to make up the difference.

One of those factors has to do with the last ice age. The weight of all that ice pushed down on Earth’s poles a little, and now that the ice is mostly gone, the poles are bouncing back up. That puts more of Earth’s mass closer to the poles, which makes us spin a little faster. It’s like when a spinning figure skater pulls her arms closer to her body and speeds up.

But that’s still not enough to account for all of the difference. There’s something else speeding up Earth’s spin, which probably has to do with how the core of the planet interacts with another layer, the mantle. Geologists are still figuring out the details of what those interactions really are. So, this research will help scientists learn more about many of the processes that shape our planet over long periods of time. But in the meantime, yes -- you can complain about having the longest day ever today, and no one can tell you you’re wrong!

Now, historical records are also causing us to rethink some other timelines, too — like, how long smallpox has been around. We used to think that people had been getting smallpox for 3 or 4 thousand years, because we’ve found mummies with pockmarks that look like the symptoms of smallpox. But in a study published yesterday in the journal Current Biology, researchers found that smallpox might actually be just a few hundred years old.

The team found the oldest known sample of the virus that causes smallpox on a child mummy from mid-17th century Lithuania. Then, they used that sample to sequence the genome of that strain of the virus. And when they compared the older virus’s genome with more modern strains, they found something weird: they were able to trace both strains to a common ancestor that would’ve been around in the late 1500s to mid-1600s. So, according to this DNA analysis, that’s when smallpox would’ve started spreading around the world — in the 1580s at the very earliest.

So if that’s true, then all those older mummies that we thought had smallpox probably had something else, like chicken pox or the measles. The only way those mummies from thousands of years ago could have had smallpox would be if they had other types of the disease — types that were totally gone by the 1970s, when the virus was wiped out. Which means that virologists are going to have to rethink their timelines for how the smallpox virus evolved and spread through the world, all because of what was found on one child who died a few hundred years ago.

Thanks for watching this episode of SciShow News, and thanks especially to all of our patrons on Patreon who make this show possible. If you want to help us keep making videos like this, just go to And don’t forget to go to and subscribe!