Tidal waves, in order to be effective in reducing the

rotational speed of a planet, must be accompanied by internal
friction; and this requires that the planet be to some extent
inelastic
Tidal waves, in order to be effective in reducing the
rotational speed of a planet, must be accompanied by internal
friction; and this requires that the planet be to some extent
inelastic. It was the view of Darwin and others that the
viscous state of the Earth and Moon permitted wave friction to
come into play. Michelson has recently proved that the Earth
has a high degree of elasticity. It deforms in response to
tidal forces, but quickly recovers from the action of these
forces. It therefore seems that the rate of tidal evolution of
the Earth-Moon system at present and in the future must be
extremely slow, and possibly almost negligible. What the
conditions within the Earth and Moon were in the distant past
is uncertain, but these bodies probably passed through viscous
stages which endured through enormously long periods of time.
No one seriously doubts that Jupiter, Saturn, Uranus and
Neptune are now largely gaseous, and that they will evolve,
through various degrees of viscosity, into the solid and
comparatively elastic state. It is natural to assume that the
Earth has already passed through an analogous experience.


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