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Planet/moon rotation

  1. Sep 13, 2007 #1
    Question asked by 5th grade student today - "Do all planets rotate like the earth?" This teacher's answer: "Don't know but will find out." Can someone give me a brief answer at a fairly elementary level? And while I'm asking, do planet moons also rotate? What is the cause of the rotation?

    Thanks in advance for any help.
  2. jcsd
  3. Sep 13, 2007 #2

    D H

    Staff: Mentor

    From http://cseligman.com/text/sky/rotationvsday.htm:
    All of the planets orbit, or revolve, around the Sun in the same eastward direction. Most of them also rotate around their axes in that same direction. Venus, Uranus and Pluto, however, rotate in the opposite direction, ... Keep in mind that even though some planets have retrograde ROTATION, they ALL orbit, or REVOLVE around the Sun, in the same direction.
    The planets all orbit in the same direction because the interstellar gas cloud from which our solar system formed had some initial rotation. The planet's kept this orbital rotation momentum when the gas cloud condensed into the sun and the planets. This is an example of the law of conservation of angular momentum. The same applies to most of the planets' rotation about their axes. The oddballs: Something big may have hit Uranus to knock it on its side. Pluto is small; it may even have been a moon of Uranus. Venus is an oddball for unknown reasons.

    Our moon's rotation rate is nearly equal to its orbital rate around the Earth. That's why we only see one side of the moon. This is not coincidental. The Moon is an example of a "tidally locked" body. Most moons do rotate, but this rotation is much more random than planetary rotation. Many moons are captured asteroids; the orbits and rotations of these captured baodies is quite random.
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