The same forces causing ocean tides tidally lock the Moon so that the shy lunar farside perpetually snubs Earth and remains hidden from earthbound stargazers.
Same Side of Moon Always Faces Earth
It is easy to verify that as the Moon orbits Earth, the same side of the Moon always faces Earth. Any amateur astronomer with binoculars or a telescope can examine lunar surface features, as the Moon cycles through its phases, and see that they do not change.
As the Moon orbits Earth, it also rotates on its axis in the same time period. To see this effect, have two people act out the roles of Earth and Moon. A tennis ball and globe also work. The person playing the Moon can orbit Earth by walking sideways in a circle around the stationary person playing Earth. If the Moon actor continues to face the Earth actor, The Moon actor is both rotating on a central axis and orbiting the Earth actor in the same time period. (Credit: This demonstration came from either Ms Wiseman or Ms Pearson, my second and third grade teachers. After 50 years, I really can't remember which one.)
Similarly, the Moon continually faces its near side towards Earth by orbiting and rotating in the same time period.
What Are Tidal Forces?
It is not a coincidence that the Moon's orbital period exactly matches its rotation period. The same tidal forces that cause ocean tides have locked the same face of the Moon towards Earth.
The lunar tidal force is the difference between the Moon's gravitational force acting on the sides of Earth closest to and most distant from the Moon. The tidal force causes a double high tide bulge in Earth's oceans. Tidal forces act on rock as well as water, but rock does not flow as easily and has a much smaller bulge. Constantly moving tidal bulges generate significant friction.
What is Tidal Locking?
Earth also exerts tidal forces on the Moon. The Moon has no oceans, but the rocky Moon has a small tidal bulge. When the Earth and Moon first formed, the Moon's orbital and rotational periods were unequal. As the Moon rotated on its axis, the moving rock tides generated friction in the lunar interior. This friction slowed the Moon's rotation rate until it equaled its orbital period. The Moon became tidally locked to Earth.
The tidal bulge stopped moving and the friction in the lunar interior stopped. The Moon's orbital and rotational periods remain equal and the Moon remains tidally locked to Earth.
Future Effects of Tidal Locking on Earth
Lunar tidal forces on Earth also generate friction which slows Earth's rotation rate. The day is getting about 0.002 seconds longer per century. With time, very much time, Earth's rotation rate will slow until Earth's rotational period equals the Moon's orbital period, which will have increased to about 50 current days (Zeilik and Gregory). Earth will become tidally locked to the Moon and the Moon will be perpetually visible only from one half of Earth.
The law of conservation of angular momentum requires the Moon to gradually spiral to a larger orbit, so that the total angular momentum, or spinning motion, of the Earth Moon system remains constant. The Moon is slowly becoming more distant.
The elegance of physics resides in its economy. A single principle often explains a wide range of seemingly unrelated phenomena. The same effect that causes Earth's ocean tides keeps the same face of the Moon towards Earth, triggers volcanoes on Jupiter's moon Io, and is slowly lengthening Earth's day.
Further Reading
Zeilik and Gregory, Introductory Astronomy & Astrophysics, Saunders, 1998. Pages 45 to 47 provide mathematical details on effects of tidal forces.
Paul A. Heckert - I have a Ph.D. in astrophysics, over 30 years experience teaching physics and astronomy, and over 60 published research articles.
