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Why Does the Same Side of the Moon Always Face the Earth?

The close side of the Moon.
Through their mutual gravitational attraction, the Earth and Moon create tidal bulges on the other.
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  • Written By: Michael Anissimov
  • Edited By: Niki Foster
  • Images By: Hugo A. Quintero G., Antony Mcaulay
  • Last Modified Date: 28 September 2014
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The same side of the Moon always faces the Earth. The "dark side" is not actually dark — it gets cycles of day and night just like most places on Earth — the "far side" is a more correct term. The reason that one side is never visible from the Earth is because it 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, those on Earth would eventually be exposed to the entire surface of the Moon. These two intervals have been equal for all of recorded history, however, and probably have been for millions of years or longer.

This otherwise bizarre phenomenon can be explained in terms of a subtle effect generated by gravitation and friction called tidal locking. Through their mutual gravitational attraction, the Earth and the Moon create tidal bulges on each other, with one bulge facing in the direction of the other body, and one facing away. These bulges generate heat through the friction of rock rubbing against itself, and also change into a greater orbital force for the Moon, which means that it is continually getting further away from the Earth. Over time, they siphon energy away from the rotational momentum of both bodies, producing a braking effect.

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Because the Earth's mass dominates the Earth-Moon system, the Moon experiences the greater braking effect. Over time, its rotation has progressively slowed until the rate of rotation matched 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 also slows over time due to the 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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anon352840
Post 44

Thank you for the informative and accessible article.

anon326613
Post 43

With all due respect, the only explanation that makes any sense is that 1) the mass of the moon is lopsided, and the earth's gravity locks the "heavy" side towards it, or 2) the moon contains significantly more iron on its earth-facing side which accounts for its locked position.

anon312217
Post 41

Almost all of it is right. But, that's like saying the moon is perfect. Ether some type of force is preventing the moon to flip or it only takes a very long time to flip (26,000 years?).

anon266571
Post 38

Most comments/answers on here, (not all, but most) are just crazy. Some people make this crap up. One moon side is locked with earth, by chance, your god, some powerful alien race. Everyone on earth always sees this same side. The moon rotates and the earth rotates, but the moon also revolves around the earth (common knowledge, I know).

But anyway, in the description of the person walking around the other person where the one walking, or revolving, is the moon and the one in the middle spinning themselves is earth. In order to keep the moon person's head looking at earth, walking around in a circle around earth, the moon person's head is doing one slow rotation that takes the length of the circle to complete. If you were outside this experiment watching these people and you stood in place, you at some point would see the back of the moon person's head, then slowly the side, then slowly they would be facing you head-on, then the other side, then the back of the head again. That's you on the outside looking in.

Now the earth person, every time they spun around in their own time, they see the moon person looking at them. The earth person never gets to see the side of the moon person's head nor the back. But that same side of the moon sees a 360 degree view of the earth person, because they're just spinning away.

I would guess that from one side of the earth to the other, very small, different angular views of the moon are available, depending on what side of the earth you're on at the time, which I would also guess is at moonrise and moonset.

This will get you all started, but don't make things up. Read.

It is crazy they are locked like this. Coincidence? Until shown otherwise, I guess so.

anon266403
Post 36

So depending on where you're from on earth you'll see a different face of the moon due to their equilibrium. So we all think it never changes. We're all seeing someone else's dark side of the moon!

anon182381
Post 31

How about the stars now? How come every night we see the same stars, in the same places. No-matter what date, even though the moon and planets change place!

anon164062
Post 29

yes this article is correct, thank you for it. One rotation on its axis equals one rotation around the Earth. This is strange phenomenon along the fact that the Moon is 365 times smaller while 365 times closer to Earth than Sun. Maybe not naturally designed?

anon161671
Post 28

I love the kool-aid comment, but your astro-physics are incorrect. Your method of disproving that the moon spins on its axis only disproves that a month and a day are not the same length.

First of all, let's get one thing straight. It is correct that one side of the moon always faces the earth.

It takes the moon 27 days, seven hours and 43 minutes to rotate 360 degrees, which is the exact same amount of time it takes to go around the Earth once. Try this if you don't get it.

1. Get a basketball and a ping pong ball.

2. Place a smiley face on one side of the ping pong ball.

3. Start with the smiley facing the basketball.

4. Move the ping-pong ball around the basketball, a)without spinning it -smiley faces the same wall; b)only spin it enough so that the smile faces the basketball as it moves around it.

If it did not spin, each position on earth would see a different side of the moon. If you still don't get it, then you aren't meant to yet.

anon158026
Post 27

What a ridiculous explanation. The problem with science today is the science fiction element in it. It would be nice to see the establishment lose some of its prestige and show some honesty for goodness sake. What happened to phrases like, "One possible explanation is ..." or "Perhaps ..." Such academic honesty would encourage young people to do science and look for alternative explanations instead of turning them away from it with unconfirmed, but "well established "fables.

anon157214
Post 26

thank you for this article.

anon153945
Post 25

I can't believe the number of people who are drinking the kool-aid. There is no way that the same side of the moon is always exposed to the all area of the earth at all times, while the moon is spinning on its own axis. Physically impossible.

Come on people, the Earth spins on its own axis as it rotates around the Sun. The moon only rotates around the Earth but does not spin on its own axis. Simply do this. Have one person represent the Earth. Have another person represent the Moon.

We will make this simple. Keep the earth's rotation around the Sun because this has no consequence in our theory that the same face of the moon always face the Earth while the moon spins on its axis. Now we know the Earth rotates around its axis, so we will have the Earth person spin around in a circle while standing still.

We will then have the Moon person walk around in circle around the Earth person. I want the moon person to try and walk around the Earth person and spin at the same time to keep face to face with the Earth person.

If the Moon person cannot keep face to face with the Earth person while walking around (rotating around Earth) and spinning (spin rotation on its axis), then this disproves this theory.

anon109498
Post 24

If this is true, why don't we see the same phenomenon on all the planets in our solar system, including earth. Why doesn't the earth have a side that faces the sun all year long?

anon97748
Post 23

'Tidal locking' is a made-up term to describe the conventionally unexplainable fact that our moon is the only known moon in the universe to be synchronized with the planet it revolves around. Our moon has many other unique qualities also: it is hollow and it is older than the earth! How is this at all possible? Can only be explained by non-human intelligence. Also, why did NASA abruptly stop exploring the moon without explanation?

anon74889
Post 20

the moon spins on its own axis. In the opposite way of the earth spinning on its own axis. Both the moon and earth spin speed matches exactly with each other in an inverse way, thus making one side only face the earth. try it with two marbles!

Amazing isn't it?

anon70014
Post 19

One side of the moon faces Earth because when it was still in a more liquid state, the heavier elements that make it up were drawn toward Earth before it solidified. This is like the gondola on a hot air balloon as the heavier side is always pointed toward the ground (Earth).

anon69187
Post 18

thank you so much! I was having the hardest time understanding this in physics, but this is explained in a way that's so easy to understand!

anon65705
Post 17

The sun's mass dominates the sun-earth system. Why has this not led to one side of earth always facing the sun? - Thanks

anon65522
Post 16

From what I understand, the statement "we can only see one side of the moon" only makes sense under the premise that you always look at it from the same place on earth.

So, if you originally had observed the moon from Africa, and then started observing the moon from Australia you'd be seeing a different side of the moon (Otherwise none of this makes sense, seeing as previous statements confirm that all of the moon at some point are lighted up).

So, although I really wanted there to be a "secret hidden" side of the moon with large alien structures on it, I think this is a misconception.

anon64054
Post 15

thanks a lot for this. it helps me a lot with my badge.

anon52006
Post 14

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.

Vishwanath
Post 13

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.

anon48114
Post 12

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.

anon39501
Post 11

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

anon35360
Post 10

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.

anon33263
Post 9

Why can't the moon rotate itself independently of Earth?

anon26376
Post 8

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 !!

anon25372
Post 7

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?

anon17320
Post 6

No kidding! Why couldn't they just have taught it to us straight-up like this in school! Thanks.

AnissimovM
Post 2

You're welcome!

anon11156
Post 1

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

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