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

B Earths Rotation Is Slowing

  1. Jun 3, 2016 #1
    So I THINK I understand that the tidal bulge is resting slightly in front of the moon because of the Earths rotation and the moons gravity, it pulls the moon 'forward' a little adding energy to the moons orbit and expanding it, what I do not understand is why the Earths rotation is slowing. I think I understand that it the energy of the tides (And also tidal electricity generation) has to come from somewhere. Could somebody please explain to me why mechanically the earths orbit slows down? Also do these tidal interactions occur with the atmosphere and mantle? Can these tidal interactions occur between hypothetical 100% solid bodies, if so to what extent? Also if my original assumptions are wrong please let me know, as I know you will. Thank you for your time. :)
  2. jcsd
  3. Jun 3, 2016 #2
    Last I heard, the Earth's rotation was speeding up. This was due to dams holding large bodies of water at high latitudes, thus bringing more of the Earth's mass towards the center.
  4. Jun 4, 2016 #3


    User Avatar
    Science Advisor
    Gold Member

    The moon has a braking effect on earth's rotation due to tidal friction. This also causes the moon to slowly recede from the earth. It will eventually become tidally locked with earth, but, probably not before the sun incinerates it.
  5. Jun 4, 2016 #4


    User Avatar
    Science Advisor
    Gold Member
    2017 Award

    dunno where you heard that, but it is incorrect

    if what you said was true, we would be taking away leap seconds, not adding them periodically

  6. Jun 4, 2016 #5


    User Avatar

    Staff: Mentor

    The Moon's gravity pulls on the bulges, and the bulges pull on the rest of the Earth. So as the Moon is pulled forward by the leading bulge, the Earth is pulled in the opposite direction, which slows its rotation down.

    I'm sure the atmosphere is pulled just like the tides are, it's just that any effect would be much smaller than the tides. As for the mantle, I don't know.

    If you mean 100% rigid bodies, then I don't believe so.
  7. Jun 4, 2016 #6


    User Avatar
    2017 Award

    Staff: Mentor

    There are also tides in the solid part of Earth. The LHC at CERN has to adjust their magnets frequently because the ring geometry changes due to those tides. On a global scale, the Earth is not that solid.
    LHC and tides
  8. Jun 4, 2016 #7


    User Avatar
    Science Advisor
    Gold Member

    Here's a pic from a random website showing the interaction Drakkith described:

    Regarding effects of rigidity on tidal deformations, you might want to look at 'Love numbers'. Googling for the phrase is slightly problematic, for obvious reasons - just make sure you add the keyword 'tides' to your search query.
    In short, you don't get tidal braking from a 100% rigid solid, as the tidal potential can't produce a tidal bulge. Since such a thing doesn't exists in reality, you end up with varying degrees of tidal interactions depending on composition - the atmosphere responds differently than the oceans, which differ from the crust, which differ from deeper, denser regions of the planet.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted