# Why is Uranus's axis of rotation tilted?

The poles of Uranus are 'in the wrong place', why is this?

• historically, do we have any evidence of its past?
• also, do we have an understanding of how its rotational axis might be evolving?
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No direct evidence; whatever happened, happened very long ago. Rotational axes don't "evolve", they tend to stay put, unless they get whacked by something from outside. – Florin Andrei Oct 7 '11 at 20:19
@Florin: They don't evolve in general, but they can precess. – dmckee Oct 8 '11 at 1:33
I don't find the word 'tumbling' appropriate for Uranus. If you follow the planet along a full orbit it only tumbles for less than a quarter of its orbit. – tos Jan 31 '12 at 22:36

The leading theory is that at a distant point in its past, Uranus was struck by a very large object, which knocked it to its side, and current tilt.

Imagine if you took a top, and smacked it with a rock. The top might be turning perfectly alright at first, but after it had been hit, the top would most likely be wobbling significantly. Similarly, after an impact, a planet tends to wobble, and it would even more if the impact occurred from a certain axis.

The particular angle (almost 90 degrees) means that Uranus basically "tumbles" on its orbit around the Sun. Additionally, any given latitude happens to have the Sun in Zenith position once per Uranus year.

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According to this new press release (via Bad Astronomy), a single collision would have left Uranus's moons in retrograde orbits, opposite the direction of the planet's spin. The new theory, based on simulations, is that Uranus was hit by at least two bodies.

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I will do some checking, but I believe there is a model that seems to indicate that Uranus got batted around by the influence of Saturn and Jupiter. After at least three encounters with the other two planets, the orbits of all three settled down to more like what they are now. This would have happened over a period of 100,000 years. It may have much to do with the irregularity of the Jovian moon inclinations.

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I've got to chime in on this one for reasons of discussion only, because I can think of two reasons why this would not happen.

Collisions are usually depicted as a center mass to center mass action. when we think of the earth and moon, it's thought to be slightly off center. yet, there can be a 'collision' when two bodies enter each others gravitational field, orbit around a mutual center of gravity for a time, and finally 'collide' after the orbit decays. like mars' moon phobos that will eventually fall to the surface. Not necessarily in one piece, but more like shumaker-levi 9s impact with jupiter.

If we imagine that 2 objects on the plane of the ecliptic, about half the size of uranus 'collide', like this example gravitational capture, we would imagine that the resulting rotational axis would be nearly vertical. like a spinning figure skater pulling in their arms.

But if we imagine that the 2 objects are the same distance from the sun, one above the plane of the ecliptic, and the second below, we would end up with a rotational axis that is horizontal, more like the axis that we observe.

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I wote this up cause "Not necessarily in one piece, but more like shumaker-levi 9s impact with jupiter." It's a valid point. – JokelaTurbine Feb 29 at 10:05