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Homework Help: What if the moon disappeared?

  1. Dec 28, 2013 #1
    What if the moon disappeared??

    1. The problem statement, all variables and given/known data
    I am doing a project answering this question: what if the moon disappeared? I was looking up how the moon was affecting Earth's rotation around its axis and noticed how the moon pulls Earth backward, slowing down Earth's rotation. One of the websites says that one day on Earth used to be six hours before the moon was created, but the moon in its orbit started extending it as it receded away from Earth.

    My question is, does this mean that if the moon were to disappear (destroyed by aliens or something) Earth would start spinning faster? (back to 6 hours days?) The website says its impact on Earth would be insignificant because the days are still extended due to tidal braking of the Sun. But wasn't Sun there with Earth before the moon was created? Why wasn't Earth's spin affected by Sun back then and one day only lasted 6 hours? (It will be really helpful if you could also explain to me about solar tidal braking!)

    2. Relevant equations

    here is the website if you wanted to have a look.

    3. The attempt at a solution
    Last edited: Dec 28, 2013
  2. jcsd
  3. Dec 28, 2013 #2


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    Staff Emeritus
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    Homework Helper

    At least learn the difference between 'break' (to smash or damage) and 'brake' (to slow down or retard).

    Since this is HW, you must at least try to answer your questions. If their is something you don't understand about the answers you develop, you can ask someone to clarify with what you are having problems. These are the rules for PF.
  4. Dec 28, 2013 #3


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    Yes, the Sun also contributes to Earth's tides, but the effect is weaker than the moon's.
    So if the moon were to vanish, the Earth's spin would continue to slow, but more gradually.
    As to how much the tidal braking from the Sun contributes to the current slowing, that's a tricky question. Remember that the two effects are sometimes in phase (spring tides) and sometimes out of phase (neap tides). Can you suggest some suitable equations to model that?
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