Wobbles in the Earth's rotation

Changes deep in the Earth's core are changing the Earth's rotation and affecting how long days are.
11 July 2013


Dominic: - I've been looking at a story which has to do with how long days are Star trailson our planet, the Earth, and of course, there's a really obvious answer to that, we all know that days are 24 hours long. But at a more deep and fundamental level of course, the sun rises and sets each day because our planet is rotating and the 24-hour period of the day is the period of rotation of our planet. Now, if the Earth were rotating at a steady rate, days would always be at the same length. But this is a spinning ball in space and things can exert rotational pulls on that planet and change the speed at which it rotates. There are quite a lot of different processes that can affect that rotation and a paper in Nature this week written by Richard Holme from the University of Liverpool and his colleagues tries to disentangle what some of these processes are by looking at historical data taken over the last 40 years of how the Earth had been rotating.

Chris - How much of a difference are we talking about in terms of the length of day?

Dominic - We're talking thousandths of a second at most.

Chris - So, a long time then.

Dominic - Well, it actually adds up so that over a couple of years, that can add up to a second and that becomes something you can measure with a clock. If you're looking at where the sun is in the sky, you can start to notice, if you've got a very good telescope that the sun is slightly behind where it ought to be.

Chris - So, what do these guys predict or say is going on?

Dominic - There are all sorts of processes going on on different timescales. On very short timescales, there's weather, there's ocean currents that affects the Earth's rotation on a day to day basis. On longer timescales, the Earth is made up of different radial layers. You have a liquid core and then you have a solid mantle, and those are rotating at different speeds. As they transfer rotation between them, that affects how fast the outer crust of the Earth is rotating. On the very longest timescales, the moon, the Earth's companion in space is gradually draining the Earth's rotational energy, and that means that days are getting longer over periods of tens of millions of years.

But what's really fascinating, three times in the last 40 years, the Earth has suddenly changed its speed of rotation in 1969, '72 and '78. That was at exactly the same time as the Earth's magnetic field shifted. The Earth's magnetic field is produced by the Earth's core and so, they predict that this is presumably also a phenomenon coming from the centre of the Earth. So, what they think is that perhaps there are bits of solid material in amongst this molten material at the centre of the Earth - if that is catching on the solid mantle above it, then when that catches, you have a sort of earthquake very close to the Earth's core. That's affecting both the Earth's rotation and its magnetic field. And so, that might be what caused these blips in the Earth's rotation.

Chris - So, when you say there was a blip, what actually do you mean by that?

Dominic - Well, the Earth has this magnetic field which is very closely aligned to its rotation axis. If you get a compass out, it points to something which is close to the north pole, but it's offset by a couple degrees. And if you have a very sensitive magnetic field measuring device, you can measure how strong that field is. But from time to time, that field is gradually changing and sometimes it shows a very dramatic shift in either its direction or in its magnitude. And that suggests something is going on in the Earth's core where this field is being generated.

Chris - So, by looking at the rotation of the Earth, you are slowly - I suppose - getting a proxy measure for how this field may be generated and what's going on right in the centre of the Earth.

Dominic - Yes. I mean obviously, we can't explore the centre of the Earth and the magnetic field is actually one of the very few things we can measure that's coming out of that centre of the Earth. So, this is telling us something about a part of the planet we don't understand terribly well.


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