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."

Old news?

[AuraSeer]

I'm sorry, but isn't this widely known? I learned about this effect in my 9th grade science class. Uneven heating of the surface can cause uneven wind resistance blah blah blah... and several million years from now, the day might be a few seconds longer.

Does simply adding the words "El Nino" makes people think this is a new, important idea? The planet's rotation speed is also affected by the impact of meteors and space dust, but I don't see anyone publishing studies to measure that infinitesimal effect.

But what about the moon?

[OldAndSlow]

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.

If you really want to get agitated about the earth's rotation slowing down, consider the moon. Tides act as a brake on the earth/moon system. So the rotation of the earth slows, and the moon (to conserve angular momentum) moves ever so slowly away from the earth.

What about wobble?

[prestidigital]

Some months ago I saw an episode of NOVA which postulated that the moon has been gradually drifting out of Earth's orbit for many hundreds (thousands? millions?) of years. This causes the Earth's spin to be less uniform, to wobble. The more drastic the wobble, the more extreme are the changes in weather. I haven't seen anything else on this since, so perhaps it is not a theory that holds much credibility with scientists. On the surface, it seems to make sense.

A couple of things...

[JoeRobe]

First, I'm curious (maybe someone out there has a link?) about how solar wind affects affects day lengths. It's known and been imaged that bursts of solar wind cause the earth's atmosphere to swell, and I'm curious what this redustribution of mass does to the moment of inertia and rotational speed of the planet.

Second, I find it kind of interesting the change in the way we percieve time. Centuries ago, the earth made a great clock. 24 hours was defined as a day, and if all of the sudden the day became longer, that longer period of time was defined as 24 hours. Now, we see that the earth makes a pretty bad clock (by today's standards), and rather than relying on the earth as our ultimate timepiece, we rely on atomic clocks. It seems strange: we have all of these time units like hours, days, months, years, etc., all defined first by astronomical methods, but now because of our (technological) ability to be more regular than the cosmos, the hour, day, month, year, etc. have sort of lost their origins.

Oceans

[barakn]

The Earth's oblateness (as measured by changes in the gravity field) has been increasing since about 1997 [nasa.gov]. Speculation points to net movement of water from rapidly melting mountain and subpolar glaciers to the equator. One would suspect this would change the Earth's moment of inertia more than would changes in wind, but it is not mentioned in this most recent article.

Re:Mountains do the same thing

[Ctrl-Z]

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?

Are you referring to leap seconds [navy.mil]?

There's a lot of speculation going around

[Quila]

Earth weighs about 6 x 10^24 kg. We take about 4.5 x 10^12 liters of oil from a distance of 6,376,660, from the center of the Earth to about 6,378,160m from the center of the Earth (average oil well depth is roughly 1500m), that's .02% of the radius.

Somebody please do the math of how that would affect angular momentum.

Re:But what about the moon?

[cperciva]

No. Angular momentum is conserved. Rotational energy is not necessarily conserved -- if you heat up the Earth, the athmosphere will expand, the earth's rotation will slow down, and the total rotational energy will increase -- but the angular momentum remains constant.

That is, it remains constant in a closed system. The only long-term changes to the earth's rotation come from the earth's angular momentum being transferred to the moon.

Re:But what about the moon?

[iabervon]

Momentum isn't energy, and is conserved by itself. In fact, the numeric value of momentum depends entirely on the reference frame, and therefore can't tend towards zero due to non-conservative forces, because there's no zero, since there's no absolute reference frame.

There are two ways the earth's rotation can change: keep the same angular momentum by moving mass away from the axis, and throwing mass out into space with a more easterly velocity than it would have sitting where it started.

The study mentioned in the article isn't talking about a non-conservative change in the Earth's rotation, in any case, but rather a conservative change due to permanent (or, at least, long-term) climate change. If the winds blow harder, the Earth slows down; if the winds blow less hard, the Earth speeds up. If the winds continue to blow hard for the next millenium, it'll be a long millenium.

The main cause is tides from the Moon

[Dr. Zowie]

The day is slowing down because of tidal drag from the Moon. Tides stretch the Earth along the Earth-Moon line; the Earth's rotation drags the axis of stretch around (about 45 degrees away from the Earth-Moon line, if I remember right). The asymmetrical shape pulls the Moon forward a little in its orbit, and the equal-and-opposite reaction (remember Newton's 3rd law?) slows down the Earth's spin by the same amount.

The Moon was certainly closer at one time -- Robin Canup, who works down the hall from me, has done some fabulous simulations of the formation of the Moon (thought to be from a giant impact of two planetoids; the larger fragment evolved into the Earth, while the smaller one became the Moon). She claims that Moon must have formed right around the Roche limit (the distance at which it would just barely not be pulled apart by tides). If that's so, then it would have had an orbital period of about 6 hours. Meanwhile, the Earth would have been rotating faster yet.

The ongoing tidal drag is evident in the "leap seconds" that some international committee periodically adds to atomic time to get coordinated universal time. The leap seconds are becoming more frequent, because (surprise) the day is slowing down a microscopic but measurable amount compared to its speed in 1951. (One leap second per three years corresponds to a proportional change of only 1 in 10^8 [100,000,000], so no wisecracks about sleeping in late, please!

Yeah, well...

[jd]

Tomorrow: NASA discovers Navier-Stokes Equation!

Sadly, the fact that gasses (identically to liquids) can create drag on any body within them is far from new, startling or amazing.

In fact, here are a few other trivial points:

• Off-axis volcanic eruptions will also alter the Earth's spin, by some miniscule amount, by acting as a simple rocket.
  
• The tides alter the centre of mass and centre of gravity, so ergo must continuously vary the Earth's rotation.
  
• In winter, the mean radius is lower than that in summer (because of the loss of a lot of vegetation). Because angular momentum is preserved, winter days must really be shorter than summer days (where the mean radius is greater).
  

None of this stuff is outside the scope of an A-level student taking maths and physics. The chances are, though, they won't get 5-figure paychecks for coming up with such trivia.

Re:since 1900

[pr0f3550r]

The earth's rotation length is never static. Even creating dams at high altitude displace weight and cause an effect on spin (imagine spinning and sticking your arms out) albeit slight amounts. Just be glad that this is rotation we are talking about and not revolution.

What about the Wobble?

[NoSelf]

Something i find even more disturbing is humanity's influence on the degree to which the earth wobbles as it rotates - according to a piece in Scientific American last year (couldn't find the article on theirr site), dams and resevoirs have displaced such a huge mass of water that the degree to which our planet wobbles on its axis has noticeably changed.