Moon is the Earth’s natural satellite that revolves around the Earth and also rotates on its axis. Now for a while imagine yourself as a young astronomer who observes the Moon for many successive nights and find beautiful craters and huge lava plains. But one astonishing feature you would surely come across is that the modest Moon always looks the same. Isn’t it quite peculiar for an object that is both rotating and revolving to seem similar to us on every night, but actually the fact is that it isn’t. The logic behind this stays quite simple.
The Moon always show up only one side called the near side and so it always looks the same from our Earth. The other side that we cannot see from the Earth is called the dark or the far side of the Moon. Only the near side of the Moon is visible from the Earth because the rate with which Moon revolves around the Earth is the same with which it rotates or spins on its axis, a phenomenon that the astronomers call as ‘synchronous rotation’ or ‘tidal locking.’
The concept of tidal locking is based on the concept of gravity. As gravity weakens with distance, the side of the Earth that faces the Moon is closer to the Moon than the rest of the Earth and so is attracted strongly to the Moon. In contrast, the far side of the moon feels a weaker than average attraction towards the Moon. Water on the Earth is free to move in response to these forces, causing tides. This force also stretches the Earth along the line joining the Earth and the Moon and causing bulges known as the ‘tidal bulges.’
Another thing to consider is that the Earth rotates, and so does its tidal bulges, the solid body of our planet is constantly deformed which require a lot of time and energy. Due to Earth’s rotation and a lag in deformation to change the bulge’s position causes the bulge to move out of the perfect alignment with Moon and moves ahead. To this, the moon responds by pulling on the bulge slowing the Earth’s rotation, but this effect is minuscule.
Now lets consider it the other way round, the tidal effect of the Earth on the Moon works in the same way. Obviously, there are no tides due to lack of any water bodies on Moon but the distortion of Moon’s body is similar to Earth, and thus Moon also has its tidal bulges. In fact, because of Earth’s greater mass, the tidal effect of the Earth on the Moon is much more than the reverse which leads to Moon’s deformation close to 20m. Slowing of the Moon’s rotation on its axis due to pull on the tidal bulge of the Moon by the Earth is another reason behind the tidal locking. The tidally distorted Moon orbits about the Earth, always keeping the same face and the long axis of its bulge towards the Earth.
So next time when you look at the Moon appreciate the fact that the Moon is like everyone here on Earth, it has a dark side which it never shows to anybody.