Tides

(1) Because the Earth's comparatively rapid rotation is continually dragging the tidal bulges away from their ideal locations, the Moon-facing bulge is always slightly ahead of the Moon in the direction of its orbit. (The Moon orbits in the same sense as the Earth spins.) The gravitational pull of the bulge thus tends to accelerate the Moon slightly, which causes it to orbit slightly higher above the Earth. The Moon thus moves about 1.2 inches (3 cm) farther from the Earth every year. As it moves away, the magnitude of the tides slowly decreases. (2) Tidal friction is slowing the Earth's axial spin. During the Jurassic period, for example, approximately 180 million years ago, the day was less than 23 hours long; when life first appeared on Earth, it was only about 20 hours long. Tidal friction long ago caused the Moon to slow its own axial spin to the point where it now always keeps the same side facing Earth. This is a common effect in bodies orbiting more massive ones, as for example the inner moons of Saturn and Jupiter. (3) Stresses in Earth's crust caused by the tides tend to trigger earthquakes. Frequency analysis of all recorded earthquakes shows that they have a strong tendency to occur on a semidiurnal basis, in accord with tidal stress.