Reason for same side of the Moon facing Earth

 Tidal Locking or Same side of the Moon facing Earth    

You’ve only ever seen half of the Moon in the sky. Why does the same side of the Moon always face Earth? This is called tidal locking, but what is the underlying mechanism for this? Earth’s Moon rotates, but it takes precisely as long for the Moon to spin on its axis as it does to complete its monthly orbit around Earth. As a result, the Moon never turns its back to us. This phenomenon, called “synchronous tidal locking,” sounds like a weird coincidence, but it’s actually quite common. All the solar system’s large moons are tidally locked with their planets. The bigger moons synchronize early in their existence, within hundreds of thousands of orbits. Some binary stars are tidally locked to one another, and evidence is building that many planets beyond our solar system are tidally locked with their stars. Earth's gravity pulling on the Moon's tidal bulges created a force called torque, which slowed the Moon's rotation over time. Over time, Earth’s gravity has reduced the Moon’s rotational rate until our satellite rotates on its axis at the same rate it revolves around Earth. 

When the Moon first formed, it was a sea of molten lava. The immense gravity of Earth stretched this molten sea, raising tides on both the near and far sides. Each time that the Moon completes a turn, it also orbits the Earth once, keeping its far side perpetually hidden. Sizes and distances are not to scale. Tidal locking is common, but its dynamics are complex. In the Moon’s case, it started at birth. Earth’s Moon is thought to have formed when a massive object collided with Earth early in its history, splattering some of our planet into space. The hot, molten object which coalesced from the ejected material would have been spinning wildly, with its shape changing as it was pulled at by Earth’s gravity. Earth’s gravitational pull distorts the Moon into a slight football shape even today, but this distortion would have been much more dramatic when the Moon was both closer to Earth and less solid.

But the Moon was spinning, and that rotation carried the tides away from a direct line pointing toward Earth. So, from Earth’s perspective, there was an extra lump of material sitting slightly on either side of a line connecting the centre of Earth with the centre of the Moon. The gravity of Earth tugged on these lumps, trying to bring them back into alignment (i.e., so that the nearer bulge pointed directly at Earth, rather than slightly away). This produced torque, or an additional rotational force, which slowed down the Moon’s rotation. At first this tugging wasn’t successful because the Moon had more than enough rotational energy to overcome the torque from Earth’s pull and keep spinning. But over time, Earth slowly won and the Moon’s rotation slowed down. The part of the Moon which was pulled toward Earth would have shifted as the Moon spun, but always at a delay, since it takes time for so much material to rise and then later fall. This means the Moon’s bulge was always a little out of alignment with Earth, yet always being pulled toward alignment by gravity. As the Moon bent and fluxed in this tug of war, energy was released in the form of heat. As the energy dissipated, the Moon’s rotation slowed until a single spin on its axis took the same amount of time as one trip around Earth. In this state, the bulge on the Moon was no longer shifting relative to the Earth, therefore no more energy needed to be dissipated by this particular process, and the spin rate stopped changing.

This process continued for millions of years until the lumps, the tides raised by Earth’s gravity, sat permanently on a direct line facing Earth. To achieve this, the rotation of the Moon had to synchronize with its orbit, so that it always presents the same face to Earth. At the same time as energy was being burned off within the Moon and slowing its rotation, energy was being added to the Moon’s orbit by similar tidal forces, causing the Moon to slowly drift away from Earth. This story doesn’t end in the past. The same process is happening now. The Moon continues to move away from Earth at a rate of about an inch-and-a-half (4 cm)/year, its drift slowing as it goes. The energy propelling it away comes primarily from Earth’s oceans, which both bulge out in response to the Moon’s gravity and exert a gravitational pull of their own on the Moon. Earth’s bulging oceans don’t exactly match up with the position of the Moon, they’re always a little out of sync because it takes time for all that water to shift and pile up. This interaction does two things: it creates friction which slows Earth’s own rotation, and creates forces which change the Moon’s orbital speed, causing it to fall farther away into space. Other, weaker forces, related to such factors as the Moon’s tilt, its elliptical orbit, Earth’s deep interior, the influence of the Sun, and more, also affect the Moon’s motion, but these cause only very subtle changes, and sizes and distances are not to scale.

It wasn’t until 07 Oct, 1959, that the Soviet Luna 3 spacecraft gave humanity its first-ever look at the farside of the Moon. About 50 billion years from now, if the Moon and Earth could somehow avoid the eventual death of the Sun, the Moon would be so far away, and its orbit so large, that Earth would also tidally lock to the Moon. Only the population of one lucky half of our world would ever see the Moon. In our own solar system, Pluto and its moon Charon (grey) have already become tidally locked to one another, just as Earth and the Moon would someday sync up if given enough time. Scientists think that Charon’s large size and mass may be keeping Pluto’s smaller moons from also syncing up. Humanity may never get to witness that from Earth, but perhaps our relocated descendents will someday gaze up at the tidally locked Moon of another planet in the universe.