Why is Earth spinning faster?

Two days in the past two months have been 1.5 milliseconds faster than days in the past few decades
04 August 2022

Interview with 

Matt Bothwell, Institute of Astronomy, Cambridge University


The Earth see from space, with the Sun just beginning to emerge from behind it


Earlier this week, measurements made by the UK’s National Physical Laboratory showed that the Earth was spinning faster; one day in June and another in July were over a millisecond and a half shorter than measurements made during the preceding decades. This was widely reported as “Earth’s shortest day ever”. That’s not true though, is it, Matt Bothwell, Cambridge University’s Public Astronomer: the Earth used to spin much faster in the past and has been slowing down. These new readings just buck the trend of the last 50 years or so…

Matt - Right. Exactly. The earth is about four and a half billion years old. And so in the past, the earth was spinning a lot quicker than it is now. You wouldn't have to go back in time much more than a billion years or so to have days less than 20 hours long. So yes, it's the shortest day for the last couple of hundred years maybe.

Chris - And why hitherto is the Earth's rate of rotation been slowing down?

Matt - Well, it's something called tidal friction. So the moon's gravitational pull on the earth is a bit uneven because the earth is not a perfect sphere. So as earth spins around, the moon's pull causes this phenomena, which physicists call tidal friction, which tugs on the earth and gently acts as a break on its rotation.

Chris - Does that mean then that we are giving some of Earth's rotational energy to the moon?

Matt - Yes, exactly. Yes. The system is losing energy, and so the moon is actually having the same effect. This tidal friction causes the moon to move away from the earth. It's happening very slowly. It's about the same speed that your fingernails grow, but over very long periods, it would be notable.

Chris - And that has had the effect of slowing down rotation and therefore days have been lengthening over time.

Matt - Yes, exactly. So we have a 24 hour day now. In the past, around a billion years ago, our day was about 19 hours. I think go back another billion years. Our day would've been about 14 hours. So yes, the earth is getting slower and lazier with time as we all do, I guess.

Chris - Why then does it appear to have bucked the trend because the headline was the Earth's suddenly speeding up. So where would it get the energy to go from a slower speed to a faster speed again?

Matt - It turns out that the answer is really complicated and I think it's hard to narrow down one single explanation because often when we think of the earth spinning in space, we just think of like a blue marble or something just rotating away. But the earth has loads of complex internal structure. There's magma sloshing around the middle and there's water sloshing around on the top and all that can do funny things to the spin. So one potential explanation is that the melting of the polar ice caps is redistributing the weight away from the poles and towards the center and that might cause a slight increase. But there are lots of different things that can be causing this. So it's hard to narrow it down to one.

Chris - Is it a bit similar then to when one watches ice skating or ballet and you have a dancer or an ice skater doing a pirouette, one of the first things they do is they're turning in a big circle. They've got their arms and legs out all over the place, and then they pull them into the centre of their body and they'll speed up. That's because they're moving the mass instead of in a big circle, they're moving it through a much smaller one. So everything has to speed up. Could the earth be doing a similar sort of thing by redistributing the mass around itself? You're not suddenly finding new energy. It's just that you redistribute the spin and that speeds things up.

Matt - Yeah, exactly. That's one really good way to put it. I had a physics professor in undergrad who used to do this memorable thing a bit like an ice skater, but he did it on a spinning office chair with weights and he would spin himself around with the weight at arm's length, and then bring them into his chest and fly around really fast. And yes, you're exactly right. As you redistribute weight around a system, the rate of rotation can change. And that's what we're seeing with the earth. It's worth saying that the increase in the day, sorry, the increase in the rotation period is very small, right? It's less than two milliseconds faster, uh, than it was this time last year. So it's a very minute increase.

Chris - How did they actually pick that up, Matt?

Matt - The earth is under very careful monitoring, particularly from a series of radio telescopes all the way around the surface of the earth, which talk to satellites, which very accurately map the earth surface to measure the rotation. It's a technique called very long baseline interferometry that provides a very detailed look at what the surface is doing.

Chris - What will be the consequences of us having a day that lasted 1.59, milliseconds, less long than we had been accustomed to then?

Matt - Very little to be honest. There's typically like a millisecond wobble in the earth rotation period anyway. It gets faster and slower over the course of a year by around a millisecond. So I think other than being quite a cool and interesting thing and a reason to talk about the rotation of the earth, I don't think there's gonna be any actual consequences.

Chris - Because we have seen, I think the last time it happened was about five or six years ago, the addition of leap seconds to days, in order to keep our clocks lined up. Does this mean we're gonna start knocking time off again then to even things up?

Matt - Well, some people have proposed that. Yes. So, you could take a second away if there are multiple years in a row that have these millisecond quicker days and ultimately we might have to take a second away to catch up with things. Some people are not very happy about this. I know Facebook wrote an article expressing their frustration at the idea of taking away seconds from the global time system. I think it would irreversibly break the Facebook codes. And so I think they were suggesting just to change the global timekeeping system rather than changing Facebook's code.



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