Is The Earth's Rotation Changing?

Roland Piquepaille writes "We all know about the current controversies associated with the ozone layer or the global warming phenomenon. Now, the NASA's Earth Science Enterprise (ESE) is warning us that atmospheric changes or El Niño events can affect the Earth's rotation. During El Niño years, for example, the rotation of the Earth may slow ever so slightly because of stronger winds, increasing the length of a day by a fraction of a millisecond. David A. Salstein, an atmospheric scientist from Atmospheric and Environmental Research, Inc., led a recent study about this possible effect. Salstein looked at meteorological and astronomical measurements from different sources and found they were in good agreement. Check this column for a synthesis. For technical explanations, images and animations, please read this NASA paper, Changes in the Earth's rotation are in the wind."

Comments

Mountains do the same thing

[silvaran]

When snow collects on mountains, it increases the earth's radius ever so slightly... so the actual day span increases by a fraction of a second. It's a small fraction though, but it still exists. This happens more during the winter when the earth is farther away from the sun. Anyways, it's nothing to get worried about. We've been dealing with rotational inconsistencies for awhile.

What's the average length of a day? Something like 23 hours, 59 minutes and 56 seconds or something like that. Which is why we have a leap year:

If the year is divisible by 4
Unless it's divisible by 100
But always if it's divisible by 400

So hey... leapYear = ((year%400)==0)||(((year%4)==0)&&((year%100)!=0));

Can someone answer this though: Do we manually synchronize our clocks every once and awhile (say every few years anyways) just to make sure? I heard a rumor about it (most people have to reset their clocks after the power goes out anyways, and PC clocks are horribly inaccurate), so is this true?

Re:Mountains do the same thing

[TheShadow]

I hope you live in the southern hemisphere because the earth is actually closer to the sun during the winter in the nothern hemisphere.

Re:Mountains do the same thing

[pclminion]

When snow collects on mountains, it increases the earth's radius ever so slightly... so the actual day span increases by a fraction of a second.

True, but when the snow melts in spring the rotation will speed back up again (rotational inertia decreasing as mass moves downward).

This is fundamentally different from wind friction, which is a non-conservative force which *irreversibly* slows the Earth's rotation. The only way it might speed up again is if the wind started blowing the opposite direction with equal force.

This happens more during the winter when the earth is farther away from the sun.

The Earth is *nearer* the sun in the Northern Winter. It is the tilt of the rotational axis which produces winter, not distance from the sun. The moment of closest approach (perihelion) actually precesses very slowly (arcseconds per year). In short, there is utterly no relationship between distance to the sun and the seasons we experience on Earth.

Do we manually synchronize our clocks every once and awhile (say every few years anyways) just to make sure?

It depends what kind of clock. The cesium clock is the scientific *definition* of a second, therefore it doesn't need to be calibrated since everything else is calibrated to *it*. How often you need to synchronize your clock depends on how accurate it is (usually measured in parts per million, or parts per trillion for accurate clocks).

Earth rotation is slowing continually...

[little1973]

...due to the Moon and the Sun. On one day the rotation of the Earth will stop and as we only see one side of the Moon, only one side of the Earth will face the Sun. Once I calculated the time when the rotation will stop and I got about 5 billion years (assuming a linear slowdown). It's quite strange because that's about the remaining life of the Sun, too.

Title and teaser misrepresent article

[nniillss]

1. The rotation of the Earth decays slowly because of interaction with the moon, i.e. the friction of the tides.

2. The interaction between Earth (solid ground plus oceans) and atmosphere can only exchange each participant's orbital momentum; it does not change the total orbital momentum.

3. Therefore, large-scale atmospheric phenomena can accelerate/decelerate the rotation of the earth on slow timescales (months/years). They have no influence on the long-scale deceleration (cf. point 1). The main point of the article is that one can use this short-time correlation as a test of measurements of the atmosphere and numerics: The fact that the two vastly different systems, namely the meteorological and the astronomical, are in good agreement according to the conservation of angular momentum gives us assurance that both these types of measurements must be accurate.

Re:OK, there's only one way to solve this ...

[theycallmeB]

Well, actually we all need to run WEST if we want to speed the Earth up a little. And it would work, just not as well as some giant rocket engine (think cartoon villian plots sized giant rocket). You work on running, I'll build the rocket.

Re:But what about the moon?

[tbmaddux]

whatever changes strong winds make in the earth's rotation must be temporary because of the conservation of angular momentum.

This would be true if you neglected dissipation/friction, which you shouldn't.

If you really want to get agitated about the earth's rotation slowing down, consider the moon.

Most studies of this sort of thing do have to account for the moon and its tidal coupling to the earth as a leading-order effect on the earth's rotation. The linked article is exploring atmospherics as a second-order effect. Another important second-order effect on the earth's rotation is glacial isostatic adjustment, the viscoelastic response of the earth to loading/unloading from the different mass distributions of glaciers and oceans on the earth's surface. As the earth changes shape, its spin rate changes.

This adjustment also important to us because it is of the same order at many locations as the change in sea level due to the temperature of the ocean.

Re:But what about the moon?

[pclminion]

It seems to me that whatever changes strong winds make in the earth's rotation must be temporary because of the conservation of angular momentum. When the wind pick up, the earth slows down. Wehn the winds die down, the earth speeds up again.

No. Friction is a non-conservative force. The energy is irreversibly transformed into heat. *Total energy* is conserved, but there is no physical law saying that kinetic energy must remain kinetic, or rotational must remain rotational.

Imagine a bathtub full of water, with the water sloshing around in the bathtub. As the sloshing water rubs against the sides of the tub, it transfers energy to the tub in the form of heat. Eventually the sloshing ceases, and all the kinetic energy the water had is now converted to heat. The process is irreversible -- you don't suddenly see the bathtub *cooling down* as the water spontaneously starts sloshing again.

I mean, this is basic thermodynamics.

leap seconds keep noon at noon

[at10u8]

The fact that the earth's rotation is slowing down has been known for most of a century. That its speed varies seasonally has been known since the 1930s. That the speed varies daily under the influence of the winds and tides has been known since the 1980s. That its speed varies daily due to the oblateness of the solid inner core has been known since the 1990s. That its speed varies on a timetable of decades under the influence of core/mantle currents is still being measured.

All of these measurements are made under the purview of the International Earth Rotation Service. There are models for all manner of astrophysical and geophysical effects considered in the Conventions that are used when reducing the data.

The way that solar noon is kept at civil time noon is by inserting leap seconds. In most places civil time is offset directly from UTC. When a leap second is inserted the day is 86401 seconds long.

This irregularity upsets some kinds of timekeeping systems, and as a result there has been discussion that leap seconds should be abolished. That would cause noon to drift away from noon. That may not be a good thing.

Re:OK, there's only one way to solve this ...

[pclminion]

Sorry, I'm going through this article trying to correct as much bad physics as I can, even though I know you meant this as a joke. So here's my pedantic comment:

Everybody run east as fast as you can, to speed the Earth up again!

Disregarding the honest mistake (you need to run West, not East)... This would actually work, as long as everyone *keeps running*. As soon as they stop running, the angular momentum which was transferred to the Earth will be transferred back to the runners. You can't change the total angular momentum of the system.

In order to speed up the Earth you would have to use a rocket or some kind of cannon which is capable of flinging material *clear off* Earth's surface, never to return. Even then, the amount of energy contained in the rotation of the Earth is *astonishingly huge*. It's doubtful we'll ever come up with anything that could make even the slightest impact on it.

Dont't worry, these guys are in charge.

[mangu]

Have you ever worried about who is responsible for making sure the Earth is rotating? Check the International Earth Rotation Service website.