Whenever a body, such as the Moon, revolves around another larger body, such as the Earth, strong gravitational forces cause tides. Those tides slow down the rotation of the Moon, so after eons of time, the Moon keeps the same face towards us. The Moon rotates at the same rate as its orbital motion. It’s called tidal locking.

As an aside, sometimes the definition of revolving or revolution, and rotation gets mixed up. Revolution is the going around another body. The Earth takes about 365 days to revolve around the Sun. Rotation is the spinning of a body on its axis. It takes one day for the Earth to rotate.

 Yes, the Moon rotates, but it does so much more slowly than Earth does. A “moon day” is around 29.5 Earth days.  In other words, whereas Earth completes one rotation every 24 hours, the Moon experiences a rotation in that 29.5 days. If it did not rotate, we would see all sides of the Moon over a period of one month. If you lived at any one place on the Moon, you would have about 14.5 days of daylight followed by 14.5 days of darkness.

In fact, the orbit and rotation aren’t perfectly matched because the Earth actually travels in an oval-like elliptical orbit, allowing us to see an additional 8 degrees of its surface.   

            The tides, water washing up and down the beaches, continues to slow the Earth’s rotation. Every year our planet Earth is spinning slower on its axis, a rate of about 1.4 milliseconds every century. A millisecond is one-thousandth of a second. No need to reset your clock! The result is that the Moon is moving away from Earth 1.48 inches per year.

            How is that possible? It’s called Conservation of Angular Momentum. We’re familiar with the dancer or ice skater spinning. When the rotating person brings their arms in, they spin faster. Move the arms to the outside, away from the body, the performer spins slower. Slower rotation of Earth, the Moon moves away.

            Earth-Moon system of rotation and tidal locking is found elsewhere in the Solar System. The Sun and Mercury are locked into a 3:2 orbital phenomena. It’s a bit complicated. But what happens is that Mercury rotates 3 times on its axis, for every 2 times around the Sun. Pluto and its moon, Charon, are tidally locked.

On early Earth, when the Moon was newly formed, days were five hours long, but with the Moon’s braking effect operating on the Earth for the last 4.5 billion years, days have slowed down to the 24 hours that we are familiar with, and they will continue to slow down in the future.

We can see some evidence of the slowdown in the fossil records of some creatures.

By looking at the daily growth bands of corals we can calculate the numbers of days that occurred per year in past periods, and from this we can see that days are getting longer, at a rate of 19 hours every 4.5 billion years.