Moon, the natural satellite of Earth, is also the planet’s closest neighbor in space. About a quarter of Earth’s size, the Moon rotates around Earth with a velocity of 1.03 km/s.
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When we look from Earth, we always see the same side of the moon, about 59% of its surface. We call this side ‘near side’. The other one, which we call the ‘dark side’, is not actually dark, but permanently turned away from us. Moon, just like Earth, also goes through the cycle of day and night.
The reason for the far side of the moon being never visible from Earth is the uniformity between the periods of Moon’s rotation on its axis and around Earth. The phenomenon is known as synchronous rotation or tidal locking. The Moon takes same amount of time, 27.3 ‘Earth days’, to spin about its axis and to complete a revolution around the Earth. If they were even slightly different, we could see the entire surface of the satellite. But, these two periods have been same for all of the known history, and is likely to remain so for millions of years in future.
The reason for this phenomenon is called tidal friction. It can be explained in regard to the effects created by gravitation. The Earth and Moon exert gravitational force on each other. This mutual force creates tidal bulges on each other. One bulge faces in the direction of the other body, and one faces away. Over time, they siphon energy away from the rotational momentum of both bodies, producing a breaking effect.
Since the gravitational force of the Earth on Moon is considerably greater than that of Moon on Earth (about six times), the Moon experiences a greater breaking effect. While it attempts to go on a straight line, the Earth attracts it towards it. Millions of years ago, Moon used to rotate at much faster rate than now. Over time, Moon’s rotation has gradually slowed until the rate of rotation matches the rate at which the tidal bulges move around the body; that is tidally locked. At present, lunar tidal bulges are located at a constant position with respect to the rotation of the Moon, forming a sort of equilibrium. Now the Moon is locked into this period, with the same hemisphere always turned towards Earth.
Even though we should only be able to see half of the surface according to this theory, we get to see 59%, as said earlier. This discrepancy is because the moon orbits around Earth in an elliptical trajectory rather than a perfectly circular one, and it creates differences in angular and linear velocities.
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