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I'm wondering what kind of astronomical or geological events would cause the reversal of Earth's rotation. For instance, Is a meteorite passing very close to Earth able to reverse its rotation? Can the presence of Earth inside extreme magnetic field cause such event?

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Both of your questions are identical to ideas promoted by Velikovsky. That's convenient, because Velikovsky has been thoroughly debunked (see, e.g., abob.libs.uga.edu/bobk/velidelu.html ), so the answer to both questions is no. –  Ben Crowell Mar 31 '13 at 17:58
Why is the downvote? I'm not aware of Velikovsky's work since I am not a specialist! Also, my main interest is the question in the title and I just mentioned two possibilities. You did not explain why the answer is NO for both questions. –  Mohammad Al-Turkistany Mar 31 '13 at 18:09
Related: physics.stackexchange.com/q/29235/2451 –  Qmechanic Mar 31 '13 at 19:29
If you want to reverse the rotation of the earth, that's a HUGE change in angular momentum. Since the total angular momentum has to be conserved in the universe, you need to find a (reasonable) way to transfer it somewhere. And, er.. I can't think of any. –  Siva Apr 1 '13 at 7:37

4 Answers 4

If you are asking whether the rotation of the Earth could be brought to a stop, then rotated in the other direction, then I know of no way this could be done. I can't think of any way to exert the torque necessary to do this on a short timescale. In principle the tidal forces of the Sun will eventually slow the Earth's rotation so it always shows the same face to the Sun. However I suspect this would be such a slow process that the Sun will have formed a red giant and swallowed the Earth before it's complete.

However it is not unknown for the axis of rotation of planets to change. In fact Venus rotates in a retrograde direction so it's likely that it's axis of rotation has flipped 180º at some point in it's history. The axis of Mars is believed to move in a chaotic way and it too could flip. In this sense the rotation of planets can be reversed.

The slow changes in the axis of Mars are due to interactions with other bodies in the Solar System and take place over multi-million year timescales. However it is in principle possible for the axis of rotation to be changed by collisions with other bodies. The axis of Uranus is at around 90º to the axis of the Solar System, and this may be due to a collision with an Earth size protoplanet in the early stages of the Solar System formation.

But to return to the original question, the rotation of the Earth is stabilised by the presence of the Moon and is unlikely to flip in the way Mars could. A collision with a sufficiently large object could change the axis of rotation, as probably happened to Uranus, but none of the asteroids Earth could possible collide with are big enough to do this. Boring though it seems, we are stuck with our North and South poles (approximately) where they are.

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"In principle the tidal forces of the Sun will eventually slow the Earth's rotation so it always shows the same face to the Sun." -- Only given certain carefully chosen principles which ignore the existence of the Moon. The Moon's much stronger tides will eventually cause the Earth's rotation to become tidally locked to the Moon, not to the Sun. (Assuming our descendants or successors don't do something about it in the meantime.) –  Keith Thompson Mar 31 '13 at 19:52
Fair enough. In any case the Sun's transition to a red giant is likely to frustrate the process :-) –  John Rennie Mar 31 '13 at 20:05
Now, we don't need to stop the rotation and rotate it in other direction. We can always flip the axis. That would require the same energy but has more chances of human survival. –  Cheeku Mar 31 '13 at 22:50
"the rotation of the Earth is stabilised by the presence of the Moon." Why? As long as the Earth's rotation is not locked to the moon's orbit, how does the Moon affect its stability? –  akrasia Sep 1 '14 at 20:38
@akrasia: you should post that as a question. Broadly, the Earth bulges out at the equator and the Moon pulls on the bulge and stops it moving too far out of the plane of the Moon's orbit. However I'm not sure I have ever seen a really clear description of the effect. –  John Rennie Sep 2 '14 at 16:02

An asteroid passing close by would result in less angular momentum being transferred than one colliding with the earth.

XKCD did a similar calculation here for preventing the slowing of the earth's rotation in order to avoid leap seconds. The short answer is:

Here’s the bad news: To get enough spin, we have to hit the Earth with on the order of a billion liters of rock per second (several times the volumetric discharge rate of the Amazon). This adds up to about one six-mile dinosaur-killing asteroid every couple days. Humanity—and life—wouldn’t survive the bombardment long.

The volume of colliding rock needed to actually reverse the rotation of the earth would be sufficient to melt the whole surface. A collision like that hasn't happened for a few billion years.

An extremely strong magnetic field would likely just reverse the polarity of the earth's magnetic dynamo and have very little effect on it's rotation.

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A collision sufficient to reverse the Earth's rotation wouldn't just melt the surface it would disrupt the planet and create a new asteroid belt. (Quite possibly I'm wrong; feel free to do the math.) –  Keith Thompson Mar 31 '13 at 19:53
At the very least we could get a second moon! –  Chris White Apr 1 '13 at 0:33

Is a meteorite passing very close to Earth able to reverse its rotation?

A meteorite is an asteroid (or other object) that enters the Earth's atmosphere.

Rotational Energy of planet Earth

As the Earth has a period of about 23.93 hours, it has an angular velocity of $7.29×10^{−5} rad/s$. The Earth has a moment of inertia, $I = 8.04×10^{37} kg·m2$.[1] Therefore, it has a rotational kinetic energy of $2.138×10^{29} J$.

Tunguska event was probably around $5×10^{15} J$

The Chicxulub impact that is thought to have killed the dinosaurs is estmated to have had an energy of $4.2×10^{23} J$

So we are still at least six orders of magnitude short.

To achieve this without an actual impact seems far less possible.

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Since the liquid and solid core of the earth already rotate at different speeds as the crust, wouldn't this mean that only the rotational speed of the crust should be altered? –  fibonatic Dec 13 '13 at 16:38
@fibonatic: interesting point but the interfaces between crust, mantle, liquid and solid cores are not entirely frictionless. The crust is obviously thin and light (relatively) so my answer ought to take your observation into account. –  RedGrittyBrick Dec 13 '13 at 16:55
I agree that the crust wil lose some of its clockwise angular momentum, however due to tidal forces this will always happen. But I wonder if it would be possible to change the direction of the crust? It would probably tear and possibly melt at several places due to the stesses of an asteroid impact. –  fibonatic Dec 13 '13 at 17:48

The universe now is expanding but its momentum will end and it will stop expanding and a reverse action will occur due to the force of gravitation . It is then when all the universe will shift gear backwards . Thus reversing the motion of the constellations , their stars and their planets . No body knows the speed of the expansion of universe or when it will stop or the speed of its recoil . It is then when the sun rises from the West . That is the simple truth and if some one disagrees with this I will be more than grateful for his input . Thank you all .

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Thanks to dark energy, the universe's rate of expansion is, actually, accelerating and not slowing. So by current cosmology, it will never stop expanding. –  ACuriousJim Sep 1 '14 at 18:06
I the universe were to stop expanding, this would not cause time to go backwards, nor would it cause the angular momentum or anything else to reverse. Earth would keep spinning in exactly the same way until a black hole came and swallowed it up as the contracting brought everything together –  ACuriousJim Sep 1 '14 at 18:08
However, none of that matters because the Sun will die far before any of that happens, but before it does, it will envelope Earth and vaporize most if not all of it. Then when it does die, whatever is left of Earth will be destroyed. So it will never spin the other way –  ACuriousJim Sep 1 '14 at 18:08
One last thing, the "speed" of the expansion of the universe is right now measured to be 67.8km/s per megaparsec. Please do not make claims to what other's don't know or what the truth is without doing research into it first –  ACuriousJim Sep 1 '14 at 18:12
This is simply wrong. If you need justification for that, the comments by Jim put it nicely. –  HDE 226868 Sep 30 '14 at 0:45

protected by Qmechanic Sep 1 '14 at 18:04

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