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It is known as a fact that Earth's rotation is slowing every year, that's why we sometimes have 61 seconds long minutes.

In the Java documentation of Time : http://docs.oracle.com/javase/7/docs/api/java/util/Date.html , it is said :

Some computer standards are defined in terms of Greenwich mean time (GMT), which is equivalent to universal time (UT). GMT is the "civil" name for the standard; UT is the "scientific" name for the same standard. The distinction between UTC and UT is that UTC is based on an atomic clock and UT is based on astronomical observations, which for all practical purposes is an invisibly fine hair to split. Because the earth's rotation is not uniform (it slows down and speeds up in complicated ways), UT does not always flow uniformly. Leap seconds are introduced as needed into UTC so as to keep UTC within 0.9 seconds of UT1, which is a version of UT with certain corrections applied. There are other time and date systems as well; for example, the time scale used by the satellite-based global positioning system (GPS) is synchronized to UTC but is not adjusted for leap seconds.

Why is that so? How can this occur?

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closed as off-topic by Undo, called2voyage Dec 12 '13 at 14:55

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions about Earth science, unless directly related to phenomena observable on other celestials, Solar system in general of which Earth is a part, or as an origin of observational astronomy where its movement, local/global phenomena might affect observations and measurements, is off-topic. For more information, see the meta discussion." – called2voyage
If this question can be reworded to fit the rules in the help center, please edit the question.

    
Fabinout, unfortunately we don't cover questions of Earth science here. If you feel the answers you have gotten so far have been incomplete in any way, you are welcome to ask this question on Physics.SE or wait for the new Geoscience proposal to come out to beta! –  called2voyage Dec 12 '13 at 14:56
    
@called2voyage It's ok, I just thought the answer was astronomical, and it wasn't, my bad. –  Fabinout Dec 12 '13 at 15:06
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3 Answers 3

up vote 3 down vote accepted

Any body with an irregular mass distribution will experience those wobblings. And more if it is not just irregular but fluid.

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I didn't even know that bodies with an irregular mass wouldn't have a constant rotation speed. But I didn't understand your second sentence, care to explain? –  Fabinout Dec 11 '13 at 11:11
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@Fabinout fluid bodies like Earth or a balloon filled with a mixture of water and oil will undergo changes in their mass distribution, and any mass redistribution will change rotational speed due to constant angular momentum. –  Envite Dec 11 '13 at 11:19
    
Oh yeah, thanks that makes sense, even more since you edited your second sentence. –  Fabinout Dec 11 '13 at 11:21
    
Yes, I realized the typo –  Envite Dec 11 '13 at 11:22
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Wikipedia's article on Earth's Rotation proposes an example:

By affecting Earths moment of inertia, some large scale events, such as the 2004 Indian Ocean earthquake, have caused the rotation to speed up.

Another one is seen in Shifting ocean currents can (and do) actually speed up Earth's rotation where it goes on to say:

Back in November 2009, something strange happened the Southern Ocean. The Antarctic Circumpolar Current suddenly slowed down. The change in current was extreme enough to throw off the planet's angular momentum, and so to compensate, the Earth sped up its rotation. For about two weeks, the days were a tenth of a millisecond shorter than they should be. Then, with just as little warning, the current returned to its normal speed, and the Earth slowed back down.

This is understandable since currents affect the amount of water on any given place and thus the distribution of mass on earth.

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It is not the amount of water, but the fact that water is moving around the Antarctica and has its own angular momentum. Total angular momentum of the Earth is conserved, so if you speed the water flowing in the same direction as the planet rotates, the planet slows down. –  Envite Dec 11 '13 at 15:02
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To complete Envite's answer, I would point that you can easily experience that irregularity yourself.

Take a fresh egg. Rotate it on a table. Stop it briefly and release it.

The egg will continue to rotate irregularly because of the fluids moving inside.

Since the egg is not a mathematic ball, and the fluids inside have several phases : yellow part, white part and gaz, the forces inside are complex, so the rotation is irregular.

Moreover I think the Moon is not helping with the regularity of fluids moving (cf sea tides).

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