Why Do We See Only One Side of the Moon?

Moon, which is relatively smooth. In the upper right image, the moon has rotated on its axis 60 degrees. Middle left, the moon has rotated 120 degrees and the deeply cratered surface of the far side comes into view. The middle right image shows the far side of the moon, 180 degrees from the upper left image. On the lower left, the rotation is 240 degrees. The lower right is 300 degrees, and you can begin to see the familiar near side of the moon come into view.

From Earth, you can only see one side of the Moon. A long time ago the Earth's gravity slowed the Moon's spin. The Moon now rotates once as it orbits the Earth, so that the same side (the same half of a sphere) always faces the Earth. So we on Earth see the near side, and the far side always faces away from us.

Many people thought that there might be strange mysteries on the far side of the Moon (aliens maybe?). In 1959, Russia's Luna 3 reached the Moon. It returned the first picture of the far side of the Moon. Then in 1968 on Christmas eve, three men, Borman, Lovell, and Anders saw the far side of the moon with their own eyes as their Apollo 8 spacecraft orbited the Moon.</br>

Anders described the far side as being the color of dirty beach sand. He also saw a land that was full of craters, but no aliens! (https://www.windows2universe.org/kids_space/darkside.html)

One Moon “day” is approximately 29 1/2 Earth days. This rotation coincides with its orbit around the Earth so that we only see about 59% of the surface of the Moon from Earth. When the Moon first formed, its rotational speed and orbit were very different than they are now. Over time, the Earth’s gravitational field gradually slowed the Moon’s rotation until the orbital period and the rotational speed stabilized, making one side of the Moon always face the Earth.

To start, think of how the Moon causes major tides on the Earth due to the Moon pulling at the Earth via its gravitational field. The Earth has this same effect on the Moon and, being 81.28 times more massive, the effect is much more powerful.

So, as the mass of the Moon is attempting to go one way (in a straight line), the Earth is simultaneously pulling it another way (towards the Earth). Further, the effect of the Earth’s gravitational field is stronger on the side of the Moon closest to the Earth than on the far side (and the same with the Moon’s gravitational field’s effect on the different parts of the surface of the Earth).

This combination essentially stretches the Earth and Moon, creating tidal bulges on both celestial bodies. This occurs on both sides of each, with the bulge on the sides closest together from gravity and on the sides farthest away from inertia. In the latter case, the matter is less affected by the gravitational force with inertia dominating in this instance. To put it another way, the matter is trying to move in a straight line away from the Earth and the gravitational forces here aren’t as strongly able to overcome this, which creates the bulge on that side.

So back before the Moon was tidally locked with the Earth, the bulge on the side of the Moon nearest to Earth ended up slightly leading thanks to friction and the fact that the Moon rotated faster than its orbital period around the Earth. So with this slightly leading bulge being offset from the line of gravitational pull between the Moon and Earth, this created a torque, which overtime resulted in the Moon’s rotation slowing until it became tidally locked with the Earth; thus, only one side faces the Earth. (Note: the bulge on the far side of the Moon had the opposite effect, but the bulge closest to the Earth dominated the interaction.) (http://www.todayifoundout.com/index.php/2014/02/side-moon-always-faces-earth/)


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