Why do we see the same side of the moon always?

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We all know that earth revolves around the sun while the moon revolves around the earth, while rotating on their own axis. The moon has different phases because it orbits the earth. Though the moon takes only 27.3 days to orbit the earth, the lunar phase cycle which is new moon to new moon is 29.5 days. This is beacause the moon spends the extra 2.2 days  trying to catch up, as Earth travels about 45 million miles around the Sun during the time the Moon completes a single orbit around the Earth. The reason, the same side of moon, is seen, is due to the Earth's gravity slowing down the rotational speed of the moon.

 

Why do we see only one side of the moon?

The moon takes 29.5 earth days to complete its revolution and it is about the same time to complete one rotation too. The moon is pretty slow when it comes to rotation. And this is not a coincidence! Moon, when it was still an infant, rotated at a much faster speed than now. But over the million of years, the moon’s rotation has slowed down and got gravitationally locked to earth. The phenomenon is known as tidal locking - generated by the two universal forces if gravitation and friction. If the rate of rotation was even slightly different, we would eventually see the whole surface of the moon. But, as the rotation time is same as that of the revolution, we see the same area always.

 

What is Tidal Locking and how does it affect the moon and earth?

The phenomenon of tidal locking is mutual. Thus, it affects both the earth and the moon. The moon and the earth create ‘tidal bulges’ on each other. These bulges in turn generate heat due to friction between the rocks. Over time the energy from the rotational momentum of both the bodies is lost as energy in these bulges, causing a Braking effect. Now as the earth has a far greater mass than the moon, the braking effect more prominently affects the moon. Over the million of years, earth and moon has formed a system, the rotation of the moon has slowed down until it now matches the rate of movement of the tidal bulge over the body. a sort of equilibrium has been reached as the tidal bulges at the moon are located at constant position relative to its rotation.

 

How much of the moon surface can we actually see?

The moon is sphere and thus it is logically correct that we ought to see only 41% of its surface. But we get to see 59% of the moon’s surface. This is due to the liberations, which are moon’s irregular motion in its elliptical orbit it describes around the earth. Liberations are measured using the latitudinal and longitudinal coordinates, computed from the Sinus Medii, which is a small plain to the immediate right side just below the Copernicus crater. From this point the Central meridian (north-south) and the Lunar Equator (East-west) emanate. There are many different liberations affecting different sides of the moon and contributing to the added surface area we see. Thus, the area we see changes from day to day, which has the maximum score of 59%. Thus the mutual effect of the tidal locking and the liberations makes the moon face the earth in the same way always while showing the maximum possible area.

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