# Why does the Earth rotate? [duplicate]

Possible Duplicate:
Why does every thing spin?

So why would the Earth, or any planet for that matter, rotate along an axis? I know of no force which could come into play here, so i assume it just started off with an initial rotation. But where did this rotational energy come from? And are there any known planets which do not rotate?

Assuming no force accounts for earth's rotation, would it be true that the angular velocity of the earth would decrease(by a very very small amount), when bombarded by meteors(Due to an increase in the moment of inertia by the increase in mass of the earth i.e. the additional mass of the meteors)?

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## marked as duplicate by Qmechanic♦, dmckee♦Nov 26 '11 at 19:37

@Qmechanic The question is very much alike, but the answers in that old thread are rather outlandish. – Georg Nov 22 '11 at 11:33
imgs.xkcd.com/comics/angular_momentum.jpg for xkcd's answer to ur meteor question.. – Vineet Menon Nov 23 '11 at 7:01
@VineetMenon I've always wondered what the caption was to that drawing, but I could never find it ... – RBarryYoung Jan 19 at 16:08
@RBarryYoung, caption? you mean explanation? explainxkcd.com/wiki/index.php/162:_Angular_Momentum – Vineet Menon Jan 20 at 5:15
@VineetMenon No the caption. That cartoon is part of the footer/banner for the XKCD website, but it has no captions, and I've never been able to find it before. – RBarryYoung Jan 20 at 23:12

Almost all astrophysical objects up to the size of galaxy clusters rotate. Their angular momentum just has to have some value and why should that be zero? Even when you start of with a big cloud of gas without any angular momentum (the universe on large scales cannot rotate), it may fragment into smaller clouds which torque each other and each obtain some angular momentum, such that their sum is still zero.

Another important aspect is that angular momentum cannot be simply radiated away, unlike energy. This means that an object cooling (by radiating photons) will loose energy but not angular momentum. Usually this implies it will shrink (unless the radiation losses are balanced by some other source of energy, as for the Sun which harnesses fusion energy) and thus obtain larger angular frequencies. The total angular momentum of the Solar system, for instance, is dominated by the orbital angular momentum of Jupiter even though its rotation frequency is 1/12year while the spin rate of the Sun (which contains less angular momentum) is about 1/12hours.

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"And are there any known planets which do not rotate?"

Not in our solar system. Our moon is tidally locked to Earth, which is why we always see the same side. We do suspect that quite a few extrasolar planets are tidally locked to their stars.

You could however argue that such planets do rotate, at a speed of one revolution per orbit.

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The only way it could not rotate would be if the initial cloud from which the solar system formed were all moving at the same speed and in the same direction as the gravity well that became the sun, and it were all pulled radially into it, making no planets at all.

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Doesn't the fact that the earth is heated (by the sun) on one side only play a role in initiating rotation? – Andersi2 Nov 22 '11 at 23:24
@AndersI: I really don't think so. – Mike Dunlavey Nov 23 '11 at 1:13

For a finite mass to rotate with exactly zero angular velocity about its centre of mass is one state out of an infinite number of angular velocities and therefore impossible. So it's far more likely to be in a state of rotation.

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Great answer - very similar reason to why orbit is an ellipse than a circle - there are more ways to be an ellipse than a circle - you should wonder only when it actually is a circle ... – Fakrudeen Nov 22 '11 at 20:57
I know that this sort of anthropic principle reasoning is often used to justify physical phenomena, but I find it deeply unsatisfying. The exact angular velocity with which the Earth actually rotates is also only one state out of an infinite number and yet it is the state we find. – AdamRedwine Nov 23 '11 at 11:00
Why can you assume that all possible angular velocities are equally likely? – bdesham Nov 24 '11 at 0:17
@Adam a body not rotating is a unique state that is easily identified, whereas the Earth's particular rotation isn't. It's equivalent to asking why everything in the universe doesn't rotate at the same angular velocity as the Earth. – Physiks lover Nov 24 '11 at 3:37
@bedsham the probability of an angular velocity being within a range of values isn't distributed uniformly, but the probability of it being a certain value is still zero. – Physiks lover Nov 24 '11 at 3:42

And yes, the angular velocity of the Earth does change with meteorites. Conversely, it also changes when spaceshuttles take off. All systems obey the Law of Conservation of Angular momentum. Defining the boundaries of the system can be difficult, but the Earth is no exception.

Changes in the angular velocity can also occur from shifts of weight on the planet. Earthquakes and volcanoes are large enough events that popular news media often reports how much the rotation has been effected (as either a lengthening or shortening of the day).

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Current theories have it that the Earth formed from a rotating disc of gas and dust, hence it has simply maintained angular momentum.

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This begs the question: why was the original disc of gas and dust rotating? – recipriversexclusion Nov 22 '11 at 20:01
@recipriversexclusion Right! Of course all dressaged physicists will rechurn "angular momentum conservation", thus the gas cloud which collapsed to form our planetary system has had a momentum already. But what happens to a cloud without any momentum? Lets imagine some gas just after the cosmos became transparent? BTW looking for an asymmetry of rotation distribution of galaxies is a topic recently. – Georg Nov 23 '11 at 10:20