Why does the Same Side of the Moon Always Face the Earth?
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The same side of the moon always faces the Earth. The "dark side" of the moon is not actually dark — it gets cycles of day and night just like most places on Earth — it is just so named because we never see it. The "far side of the moon" is a more correct term.
The reason that one side of the moon is never visible from the Earth is because the moon spins once on its axis in precisely the same amount of time it takes to revolve around the Earth. If its rate of rotation were slightly different than its rate of revolution, we would eventually be exposed to the entire surface of the moon. However, these two intervals have been equal for all of recorded history, and probably for millions of years or longer.
This otherwise bizarre phenomenon can be explained in terms of a subtle effect generated by gravitation and friction — tidal locking. Through their mutual gravitational attraction, the Earth and the moon create tidal bulges on each other. One bulge faces in the direction of the other body, and one faces away. These bulges generate heat through the friction of rock rubbing against itself. Over time, they siphon energy away from the rotational momentum of both bodies, producing a breaking effect.
Because the Earth's mass dominates the Earth-Moon system, the moon experiences the greater braking effect. Over time, the moon's rotation has progressively slowed until the rate of rotation matches the rate at which the tidal bulge moves around the body. Today, lunar tidal bulges are located at a constant position with respect to the rotation of the moon, meaning a sort of equilibrium has been reached.
The Earth's rate of rotation slows over time due to moon-created tidal forces, but the braking effect is much smaller -- in order to be synchronized perfectly with the moon, the Earth would need to rotate only once per lunar cycle, or about every 29.5 days. Then the moon would always be in the same place in the sky, and visible from only one side of the Earth, but this is not the case. In certain planetary systems, like that of dwarf planet Pluto and its satellite Charon, both bodies are tidally locked to one another.
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Here's the deal. Earth's gravity is considered constant (i.e. a gravity 'point source' so to speak). This is not true. The Earth's gravity varies at various places on the globe. This is due to land masses, mountains, etc. The actual density of the Earth varies from place to place. The more dense areas actually have a higher gravitational pull than less dense places.
Also, the Earth is not a perfect sphere. Due to its rotation on its axis, the Earth actually bulges out at the equator. This is called oblateness. The gravitational pull at the equator is greater than the gravitational pull at the poles. Since the Moon is actually made of the same stuff as the Earth (with less iron) it is also non-homogeneous and also has varying gravitational characteristics. So, if you could put a pin or axle at the exact center of rotation of the Moon, and let it hang there, it would behave like a compass, and the higher gravitational concentration areas of the Moon's mass would be attracted to another gravitational mass (i.e. such as the Earth). This 'compass like' behavior is why one face of the Moon is always facing Earth. The densest concentration of the Moon's mass is gravitationally locked to face Earth. That is why we always only see one face of the Moon. - anon52006
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Earth has gravity and tries to pull the moon. Moon in turn has gravity and tries to pull the earth. Due to pulling in opposite direction the present distance is maintained. Earth rotates on its axis the this causes the lines of attraction to break and makes moon to revolve around the earth. It is similar to moving one magnet using another without physical contact. As the force of attraction is always constant the path is circular and we see only face of the moon. Now let us imagine a child going around its mother always holding her for support. The face of the child is always towards the mother and the back is never visible to the mother. This is what is happening in case of earth and moon and that is why we see only the same face of the moon. Also the moon only revolves around the earth. If, as is generally explained, had the moon taken 29 days for one rotation also, we should have seen the other face of the moon after 14.5 days. - Vishwanath
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The side of the moon we see has craters.. which inclines it. obviously it has not always been facing inward. Apparently an alien base has been set up on the far side of the moon observing us until we reach planetary globalization. - anon48114
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i believe that the moon which we also believe contains iron has got more iron in the side that faces the earth which then is attracted by the magnetic force of the earth. --keith the confused - anon39501
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I don't understand the reasoning that the moon rotates completely in the time it takes to rotate around the earth-- thereby only making one side continually visable-- can you make this answer more simple enabling this thick old man to understand. - anon35360
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Why can't the moon rotate itself independently of Earth? - anon33263
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Why Does the Same Side of the Moon Always Face the Earth? "...subtle effect generated by gravitation and friction — tidal locking..." and "...they siphon energy away from the rotational momentum of both bodies..." This explanation makes absolutely no sense !! - anon26376
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Which side of the moon is lighted when it is between the sun and the earth?How many rotations did the moon make after one complete revolution around the earth? In what direction does the golden side of the moon face: a.after the first quarter around the earth? b.after the second quarter turn around the earth? c.after the third quarter turn around the earth? d.after the fourth quarter turn around the earth? - anon25372
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No kidding! Why couldn't they just have taught it to us straight-up like this in school! Thanks. - anon17320
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You're welcome! - AnissimovM
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this article have helped me to accomplish all the answers that i wanted. it has also taught me some new facts about the moon and its motion with earth. thank you i really appreciate it - anon11156
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Written by
Michael Anissimov
Last Modified: 10 November 2009
copyright © 2003 - 2009
conjecture corporation
