1.2.6 Synchronous rotation
Because most moons in the Solar System are in close orbit around much larger planetary bodies, over time the speed of a moon’s rotation is decreased as the tidal, or gravitational, pull between the two bodies drags on the moon’s spin. Eventually, this slows down a moon’s rotation so much that it completes only one rotation about its axis per orbit, resulting in the same side of the moon facing its planet at all times (known as captured rotation, or synchronous rotation). With the Earth and the Moon, these competing tidal forces have also exerted drag on the Earth’s rotation, slowing its rate of spin about its axis and thus lengthening a day by almost two milliseconds per century.
Tidal forces affect you too. While standing on the Earth’s surface, your head is nearly two metres further away from the centre of the Earth than your feet are. The Earth’s gravitational field can be treated as if all the mass were concentrated at the planet’s centre. Since the force of gravity decreases as the distance increases, your feet are pulled down slightly more strongly than your head. The anatomical consequences of this stretching are negligible for you, but for a much more extended body such as a moon, the physical consequences can be quite noticeable and in one case, as you will see later, very severe.