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 last 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 long: 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.
That 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 our 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!
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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.
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.
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