Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Gravitation in the solar system

  1. Oct 27, 2006 #1
    Let's say, in an hypothetic situation, that the moon would explode to tiny bits.

    What would happen to the earth?

    I was said it would actually change:

    The orbit of the earth *duh!*
    Water levels all around the world *duh!*

    And the point I have a problem with is this last one...

    It will change the earth's rotational speed, this derived from the fact that the moon goes around the earth at the same speed than the earth revolves upon it's own axis... Is this true, or totally fallacious?

    The person in question also mentioned studies regarding this possible fallacy...
  2. jcsd
  3. Oct 27, 2006 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    First we have the sun-exploding thread. Now we have the moon-exploding thread. What's with all this all of the sudden? Is someone running a crackpot website and posting all of these questions, while you guys bite the bait?

  4. Oct 27, 2006 #3


    User Avatar
    Gold Member

    "...the moon goes around the earth at the same speed than the earth revolves upon it's own axis..."

    Think about this for a second. If the Moon overhead were orbiting at the same speed as the Earth rotates below, then the Moon would always hover over the same spot on Earth.

    Does it? No. The Moon is in a different place overhead (and sometimes not at all) every day.

    You should tell your friend to shut up until he knows what he's talking about.
  5. Oct 27, 2006 #4
    *chuckle* Aye, that was about what I was going to tell him. hehe
  6. Oct 27, 2006 #5
    Hold on, doesn't the Moon affect the Earth's rotation rate anyway?
    Isn't it slowing down as the Moon moves further away from us?
    (Fossils show the Earth's day was much shorter in prehistoric times when the Moon was closer.)
  7. Oct 27, 2006 #6
    Earth's rotation rate, and Moon's orbit are gradually synchronising.
    It's called 'tidal locking'
    The Moon is already tidally locked to the earth (we always see the same face of the Moon, all but a slight rocking - called 'libration')

    I suppose if the Moon were suddenly vapourised, then the Earth's rotation would stop slowing.

    But it ain't gonna happen.........
  8. Oct 30, 2006 #7
    So the moon DOES affect the earth's rotational speed?
  9. Oct 30, 2006 #8


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, it slows it down very slowly, but if you remove the moon, the rotation speed will not suddenly change, it will just stop slowing down.
  10. Oct 30, 2006 #9


    User Avatar
    Science Advisor

    Just for perspective; the Moon's gravitational "drag" slows the Earth's rotation by about 1.5 milliseconds per century. it takes a hundred million years for the Earth's "day" to get a second-and-a-half longer. If this deceleration were to cease, then a hundred million years from now a day will be just as long as it is today, rather than 1.5 seconds longer.

    We live with this deceleration every day of our lives. However, if it were to suddenly dissapear, I don't think anyone would fall over! Of course, here in Michigan, we just moved our clocks back one hour yesterday (Sunday); I wonder if that would get messed up?
  11. Oct 30, 2006 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    If the moon exploded, by conservation of momentum, the center of mass of the moon would remain in the same place. So there would be no net effect


    (For a little while at any rate...)
  12. Oct 30, 2006 #11
    Well, if the earth rotation slows down for about 1.5 milliseconds(1.5x10^-3 sec; per day) in one century it would take 100.000 years(1000 centuries) until the day would be 1.5 seconds longer.
  13. Oct 30, 2006 #12


    User Avatar
    Science Advisor

    Ooops! You're absolutely right, sstone. I missplaced my decimal, did microseconds instead of milliseconds; one hundred thousand years is the correct figure not one hundred million.

    Thanks for catching that.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook