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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
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@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

3 Answers 3

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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