July 5, 2018
The night sky is often dominated by the Moon. Sometimes bright, other times only a sliver reveals itself. It orbits (goes around) the Earth every 27 days. It also rotates about its axis every 27 days, a long time compared to Earth which does so every 24 hours (one day). This property of orbiting at the same rate of rotation is called tidal locking. The moon is tidally locked to the Earth.
To visualise this, place a round object on a table to represent Earth. Now grab a ball (the Moon) and move it in a circle around your Earth. Finally, move your Moon so that only one side of it faces your Earth. You will notice that you have to rotate your Moon as you orbit the Earth. When you have done one full circle around your Earth, you will notice that your moon has also rotated once on its axis. This is how tidal locking works, every orbit around the Earth is equal to one rotation of the Moon.
Because of tidal locking, we only ever see the one side of the moon. The dark side of the moon refers to the side of the moon that faces away from Earth. That does not mean that the dark side of the moon is always dark, it is just that we never see it. When there is a full moon, then the dark side is completely dark, but when we see a half moon, the `dark side' is half covered in light as well. Light is always shining on one side of the Earth, just like it is on the Moon. That's why it is more appropriate to call the `dark side' of the Moon the far side of the Moon.
HOw it works
When two bodies are gravitationally bound (like the Moon and the Earth), tidal forces occur. These are the forces that produce tides in the ocean. The closer the object is to something, the stronger the gravitational force. This means that the Moon pulls slightly harder on the near face of the Earth. Because the Earth is larger than the Moon, the Moon also pulls the sides of the Earth inwards. These forces are represented by the blue arrows in the diagram below.
The Earth is therefore, squished and stretched by the Moon. You can replicate this by pushing in the sides of a water balloon. This not only affects the water on Earth, but the Earth itself. The Moon deforms the Earth by around 10cm, while the Earth deforms the Moon via the same forces by 20m (this is because the Earth is bigger and therefore, pulls harder on the Moon).
The Earth spins once every 24 hours, while the Moon orbits once ever 27 days. This means that the Earth is spinning the bulge away from the Moon. As such, the Moon is always pulling back on the bulge (shown below). This in turn, slows the rotation of the Earth. For every 100 years, the Earth day becomes 0.0016 seconds longer because of the Moon.
Again, all of these forces occur on the Moon as well. And again, because the Earth is so much larger than the Moon, it's pull is much stronger and has acted a lot quicker. The Earth has slowed the rotation of the Moon so much so that it now it is tidally locked. Consider a ball on the end of a string. The string is the gravity between the Earth and the Moon. As you wing the string over your head, the ball orbits around you. Only one side of the ball faces you as it goes around. This is similar to the Moon's behaviour around the Earth.
Eventually the Earth will slow down such that it will be tidally locked to the Moon. That means that only one side of the Earth will see the Moon. Tidal locking does not just occur on the Earth and Moon. Pluto and it's largest moon Charon are both tidally locked with each other. Only one side of Pluto is exposed to Charon, and vice versa. Other examples of tidal locking are seen with some of the moons around Jupiter and Mars.
If you wish to learn about the mathematics behind tidal locking, the following is a good place to start:
Goldreich, P., & Soter, S. (1966). Q in the Solar System. Icarus, 5(1-6), 375-389.