Is Michelson Describing Earth Tides or Ocean Tides?

[cragar]

I am reading George Gamow's book on gravity and in his book he says:
The American physicists AA Michelson found from his experiments that every 12 hours
the the surface of the Earth is deformed by one foot, as compared with a 4-5 foot
deformation of the ocean's surface.

This seems weird to me, does this have to do with the centripetal force because we are rotating. Or is he's just talking about the ocean tides,

 

[tiny-tim]

hi cragar!

the clue is the "12 hours" …

so yes, he's talking about tidal forces (mostly from the Moon, partly from the Sun)

(see http://en.wikipedia.org/wiki/Tidal_force for details)

 

[D H]

This seems weird to me, does this have to do with the centripetal force because we are rotating. Or is he's just talking about the ocean tides,

He's talking about Earth tides (wiki article: http://en.wikipedia.org/wiki/Earth_tide). Just as the oceans are subject to tidal forces, so is the Earth as a whole. Those tidal strains on the Earth would have no effect if the Earth was a perfect rigid body. It isn't. The Earth is better described by the theory of elastoplasticity. It deforms elastically to small stresses, but above some limit it starts to behave more like a plastic.