018.4 Tidal Friction

Friction between the tides that the Earth raises on the Moon and the Moon's rotation have already produced a tidal lock. The same side of the Moon always faces the Earth.

Note: Because the Moon goes around the Earth once a month, it really is rotating: It rotates on its axis once a month.

Newton's Law of Action and Reaction insists that if the tides raised by the Moon are slowing the Earth's rotation, there must be a reaction force on the Moon. That force is pushing the Moon into an orbit of ever-increasing distance from the Earth.

Pluto and its Moon Charon are orbiting only a small distance apart and are thought to both be in tidal lock so that they face each other at all times.