Why Do We Only See One Side of the Moon?

Posted by Ethan on April 29, 2008
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And if the band you’re in starts playing different tunes
I’ll see you on the dark side of the Moon.

One of my favorite readers, Zrinka, asks us why we’re only able to see one side of the Moon from Earth. Seriously, look at the different phases; we always see the same side of the Moon:

How does this happen? Well, the Moon makes one revolution around the Earth about every 29 days, and that’s what causes the Moon phases. But the Moon also rotates once every 29 days also. Because of this, the same side of the Moon always faces the Earth:

We call this “being locked.” We’re not the only system like this, by the way. Both of Mars’ moons, Phobos and Deimos, always have the same side facing Mars. All of Jupiter’s, Saturn’s, Uranus’, and Neptune’s moons are locked to those planets as well. And in a really weird case, Pluto and its largest moon, Charon, are locked to each other, so that both always show one another the same face:

But what made this happen? Why do Moons wind up locked to the planets they orbit? Like everything else in the solar system, the culprit is gravity. When you stand up, the Earth pulls down on you. But your feet are just a little bit closer to the center of the Earth than your head. This slight difference means that the Earth pulls with a slightly larger gravitational force on the lower half of you than the upper half; this slight difference is called a tidal force. When something like a Moon is close enough to a planet, these gravitational tidal forces cause the Moon to spin at the same rate that it revolves around the planet. When this happens, we call it tidal locking, and that’s why we always see the same side of the Moon!

This is true of all the planets’ moons that we know of, and is even true for some asteroids that are bound to each other. In fact, based on what we know, we think that an extra-solar planet, Gliese 581 c, is tidally locked to its Sun, just like our Moon is locked to us. Pretty neat!

Update: Here are a couple of images for you to compare. First up is the “near side” of the Moon, or the side that always faces us:

and next is the side you’ll never see from Earth, the “far side,” or the side always facing directly away from Earth:

These pictures were taken by the Clementine spacecraft, which was the first astronomical data I ever worked with as an undergraduate (back in 1998). Isn’t it neat how different these sides are? And now you know a little more about the side of the Moon you’ll never see!

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  1. #1 Zrinka
    April 29, 2008

    Thank you Ethan, I am happy with your answer. I learned something new! The way you explain makes it easy to memorize the subject. In my primary and high school most of us students weren’t interested in Physics much, but now I know why. Because of the teachers, with no enthusiasm. This time readers like Matt won’t find details (btw. I would really like to be reader at that level and I hope I will reach it one day and enjoy in all important details), but for now I’m still learning basics and this kind of answer is enough for me. Ethan, you are definitely my favorite teacher!

  2. #2 ethan
    April 29, 2008

    We’re still learning about tidal locking, too. For instance, the Moon revolves one time for every one time it rotates, and so it’s locked to the Earth. The Earth rotates 365 times for every one time it revolves, and so it isn’t locked. But the planet Mercury rotates 3 times for every 2 times it revolves around the Sun. Why? Nobody knows! Also, if you haven’t been back in a while, check out the updated pictures of the near and far side of the Moon, above!

  3. #3 Brian
    April 29, 2008

    Ethan, that’s not true. We know why Mercury rotates 3 times for every 2 times it revolves. Mercury is trapped in a 3:2 spin-orbit resonance with the Sun, rather than the 1:1 synchronous state like the Moon is with the Earth. There are many other examples of interesting resonances in the solar system, especially due to Jupiter’s gravity. The textbook “Solar System Dynamics” by Murray and Dermott is a good, although technical, reference for this kind of stuff.

  4. #4 ethan
    April 29, 2008

    Yes, it’s in a 3:2 spin-orbit resonance. But giving it a name doesn’t explain how it got there. It’s very unusual, and we don’t know why it should have happened. The leading theory was that Mercury had a huge crater on its unimaged side; that theory is running out of room now that Messenger is photographing the parts that Mariner 10 missed.

    But nearly every celestial body that is in a tidal lock or resonance is in a 1:1 state. We lack the ability to predict what body will wind up in which resonance, and that’s why I say we don’t understand it.

  5. #5 Brian
    April 29, 2008

    It’s not that we don’t understand the dynamics. We do. The problem is not knowing the initial conditions, including events like large impacts. In fact, we can even make inferences about the initial conditions based on the fact that certain ‘odd’ resonances exists. You cited one such inference with the Mercury impact theory, which I don’t think is debunked at all since Mercury’s Caloris impactor could possibly serve as the culprit.

  6. #6 Gary
    April 29, 2008

    Why are the two sides of the moon so different in appearance?

  7. #7 ethan
    April 29, 2008

    Gary, that is a good question! It turns out that the far side of the Moon has more (and larger) craters on it than the near side, which is what you’d expect, since the near side has the Earth to shield a little bit of it. But the big differences in color that you see are what we call albedo, which is a special type of reflectivity. I’m not sure why the near side has a lower reflectivity than the far side on average; I think we just got lucky. Brian, do you know?

  8. #8 Brian
    April 29, 2008

    Gary/Ethan, the near side and far side look different because much of the near side has been resurfaced by mare basalts. These “lunar maria” are the dark areas that fill in the low-lying areas within large impact basins on the near side. 31.2% of the near side is covered by these volcanic deposits compared to only 2.5% on the far side. This is why the far side looks different; it hasn’t been covered by as much dark (low albedo) volcanic material. While you might initially think this difference could be due to the gravitational forces between the tidally locked Earth-Moon system or the varying crustal thickness of the Moon, both ideas aren’t the answer. The current best theory to explain the why the near side was more volcanically active than the far side is that more of the Moon’s heat-producing radioactive elements (known as “KREEP”) seem to be concentrated there. Why that is the case is anyone’s guess at this point.

  9. #9 Tamara
    April 29, 2008

    What’s the moon like below its surface, moving into the interior? And what’s the current thought on its formation? Was the study that came out in December that the moon is 30 million years younger and that the composition seems a lot more like Earth than previously thought, something that’s still in an early testing stage and unconfirmed? Or, has it has changed the way scientists are viewing the moon? If it’s the latter, then what types of questions has that raised for scientists?

  10. #10 ethan
    April 30, 2008

    Maybe the most interesting part of your question, Tamara, is asking where the Moon came from. That one is probably worth a whole extra post on its own. I’ll get working on it. ;-)

  11. #11 Brian
    April 30, 2008

    Tamara, almost everything we know about the lunar subsurface comes from the Apollo Seismic Experiment. By analyzing data from moonquakes, lunar seismologists figured out what the interior of the Moon is like. The uppermost layer is called “regolith”, which is busted up rock and dust from all of the impacts on the lunar surface. Below that is the rest of the lunar crust followed by the mantle and core. We know from chemical analyses of lunar rock samples that the Moon is made up of pretty much the same stuff as the Earth. The main difference in composition is that the Moon is missing heavier elements like metals compared to the Earth. These are two of many strong pieces of evidence that the Moon and Earth formed from the same material (see upcoming post by Ethan). Another method that have proven useful for characterizing the lunar core is measuring the distance to the moon using laser reflectors left there by Apollo to determine the Moon’s “love number”, which relates to how much it deforms due to the pull of Earth’s gravity. That data told us that the Moon has a molten inner core. Since the Moon is so much smaller than the Earth, it cooled much more quickly, so the lunar mantle and core are much more solid compared to the Earth’s, though.

  12. #12 Brian
    April 30, 2008

    I’d like to add that several current and upcoming lunar missions include radar sounding experiments that will probe the upper few kilometers of the lunar subsurface to look for water or other useful materials, as well as characterize the layering of the lunar regolith and upper crust.

  13. #13 Anna K.
    April 30, 2008

    It is a sad thing to be so challenged at picturing motion . . . I can’t do contra dancing, which is a betrayal of my heritage, and I cannot figure out how the moon rotates.

    Does it do a furtive little circle on Day 29 when it is sure that no one is looking? Or does it slowly rotate somehow over 29 days but as everything is all moving in circles, we end up seeing the same side of the moon even though everything is moving?

    The whole thing makes me dizzy, and yet, I really appreciate this post.

    This is something I have wondered about for years; why the moon always looked the same.

  14. #14 Brian
    April 30, 2008

    Anna, think about the Earth and Moon like two gears spinning together. Maybe that’ll help you visualize how they could always show the same face to each other.

  15. #15 Anna K.
    April 30, 2008

    Hmmmmm. I’ll take your word for it, but still cannot picture how it would work without animation. I played around with some pot lids just now, but apparently spinning pot lids is not enough, since to me it still looks like one should eventually see all sides.

    It’s all in the timing, I suppose.

  16. #16 ethan
    April 30, 2008

    Anna,

    Try picturing it like this: stand up, take a pot lid and hold it at arms length with two hands, so that the inside of the pot lid is facing you and the outside is facing away. Then, using your feet, spin slowly in a circle. Notice how you always see the inside of the pot lid, never the outside. The Moon is locked to us in the same way. (You could do it with a beach ball if you have one, instead of a pot lid.)

    Now, have someone else hold the pot lid and move it around you the same way you moved it around yourself, but you spin around 29 times during the time it takes the lid to go around you once; now you’re spinning like the Earth, and the pot lid is facing you like the Moon! Hope that helps.

    Ethan

  17. #17 Anna K.
    April 30, 2008

    I am supposed to be finishing up the monthly household budget tonight, so naturally I was spinning slowly around the dining room with a pot lid. What didn’t make sense to me is why it was supposed to be rotating at the same time it was revolving, since the pot lid looked like it was only making one big circle to me, rather than two.

    So my husband and I just spent several minutes not budgeting, while he insisted that the pot lid was both rotating and revolving, and I said it looked like it was just revolving. Then he said, “Think of it from the perspective of the sun.” And of course, as it makes one revolution around the earth, it also rotates and shows all of its sides to the sun.

    So now, at last, I get it! (I hope!)

  18. #18 ethan
    April 30, 2008

    Anna, that’s right! You and your husband figured it out, and I’m glad I could help. Congratulations!

  19. #19 ethan
    April 30, 2008

    Tamara,

    Your question is answered, hopefully to your satisfaction:

    http://startswithabang.com/?p=409

    Ethan

  20. #20 Tamara
    April 30, 2008

    Thank you Brian for such a comprehensive response. It really helped me visualize the composition of the interior, and understand why the lunar mantle and core are more solid. Plus it was interesting to read the information you gave about the current and upcoming lunar missions and the laser reflector measurements relating to the deformity of the Moon. I’m looking forward to your post expanding on this Ethan. It was a great original post and discussion to read :)

  21. #21 lena
    November 28, 2011

    Anna is so helpful.

  22. #22 Makoena
    May 21, 2012

    Thanks for a crystal clear and interesting explanation keep it up as we will also be learning from day to day from your site cheers!!

  23. #23 Jennifer Lawrence
    tooele utah
    September 20, 2012

    thanks for the answer and the help but im just wondering about the earths effect on the other planets?

  24. #24 Jennifer Lawrence
    September 20, 2012

    ;)

  25. #25 Mike Porritt
    Melbourne Australia
    December 7, 2012

    After about 60 years of thinking it was amazing that the moon rotated and revolved at the same rate I finally looked it up and found your explanation. The explanation is cool but there seems to be a small bit missing .. it seems to me that the tidal locking would only occur if the moon was egg shaped(and I guess it probably is) or at least denser on the earth facing side.. if it were spherical and uniform i struggle to see why the difference in gravity frome core to surface would have any nett influence ? Have I missed a critical part of the explanation ?? Am I off the planet ?? :)

  26. #26 Cleon Teunissen
    December 8, 2012

    @Mike,

    Yes, the tidal effect of the Earth’s gravitation on the Moon deforms it. In it’s current tidally locked state that deformation is static.

    Tidal effect deformation occurs anyway, both when tidally locked or not tidally locked. For example, besides the tides of the Earth’s oceans there is also a solid Earth tide. It’s very small, something like centimeters, but it’s not zero.

    Initially, when the Moon had formed, it probably rotated on its own axis faster than once per orbit. So back then the Earth’s tidal effect was “kneading” the Moon, like kneading dough.

    That kneading effect has consequences, and among them was that the Moon’s rotation rate was getting slower. Eventually the Moon reached a state of tidal lock. A that point the kneading stopped. The tidal deformation is still there, but there’s no kneading anymore as the the same side of the Moon keeps facing the Earth.

    From the onset of tidal lock on the Moon has remained in that rotation rate of once per orbit.

    Conversely, the Moon’s tidal effect (on the Earth’s oceans) is slowing down the Earth. Geologists look for sedimentation patterns. There is daily cycle, monthly cycle and yearly cycle in rate of deposition. Sometimes the layering is so well defined that ratio’s of these cycles can be determined. Combining data like that it has been inferred that something like 600 million years ago there were 400 days per year. That is, the days were shorter back then.

  27. #27 bhavika
    sirsa India
    January 21, 2013

    Thanks Ethan it helped me a lot but I read in a book that we can also see back side of the moon but now I came to know its not true thanks

  28. #28 Steve
    April 3, 2013

    No…this is wrong. If the moon did rotate we would see the other side. I did the pot lid excercise and I can see both sides. So, unless you have a clearer explanation, the moon does not rotate. And all one must do to verify it would be to follow it’s orbital path in a SR71 I suppose.

  29. #29 Wow
    April 4, 2013

    “No…this is wrong. If the moon did rotate we would see the other side.”

    No, if the moon did not rotate, then as it moves round us, what was the left side toward us and therefore visible becomes the left side away from us and therefore not visible.

    If I were to run around you but keep facing the same direction, you’d see all sides of me.

  30. #30 Steve
    April 4, 2013

    Well, if scientists fail to understand that the moon is facing us all the, time with a singleside as it orbits us cannot possibly be spinning, how are we supposed to believe them on anything else astrophysical

  31. #31 Wow
    April 4, 2013

    “Well, if scientists fail to understand that the moon is facing us all the, time”

    Since the moon doesn’t, the problem isn’t the scientists, is it.

    It’s you that is the problem.

  32. #32 Wow
    April 4, 2013

    Cut the wrong bit.

    ““Well, if scientists fail to understand that the moon is facing us all the, time… it … cannot possibly be spinning”

    Since the moon doesn’t, the problem isn’t the scientists, is it.

    It’s you that is the problem.”

    Because you suck at English, the sentence needed an emergency operation to make it well again.

  33. #33 Sean T
    April 4, 2013

    Steve,

    Do an experiment. Put a poster or something else on which you can focus on a wall in a room. Have someone stand in the middle of the room. Now, walk in a circle around that person, taking care to always be looking at the object you’ve placed on the wall. That ensures that you are not rotating your body as you walk around your partner. Now, ask your partner if he/she can see all sides of your body, or just your front as you walk around him/her. (Obviously your partner should keep turning so that he/she can always see you.)

    If you think hard about this, you’d realize that your partner can see all sides of you when you do as I’ve described. You would have to make one full rotation as you walk around in your “orbit” in order for your partner to only see your front side.

  34. #34 Sinisa Lazarek
    April 4, 2013

    What is most interesting to me is how the side facing us has much more darker regions (guessing these are cooled lava spots) then the opposite side. Any reason for this or just coincidence?

  35. #35 Wow
    April 4, 2013

    Asteroids tend to come from somewhere other than the earth.

    When battered by the huge number of asteroids ~4Bn years ago, this caused the marias we see all over the moon (and the earth, but that’s been surfaced with a green/blue carpet in more recent times).

    Since then, there’s been a much lower rate of impact and most that have been close enough to the moon’s face toward us to be seen have been deflected by us being nearby.

    The arse-end of the moon hasn’t had any such impediment to being cratered.

    And the small craters will make the maria bright and pockmarked.

  36. #36 Sinisa Lazarek
    April 4, 2013

    @Wow

    ahh.. so you’re saying the side facing us is darker because it’s been hit less and thus moon soil hasn’t covered them as much as on the side opposite of earth. Cool didn’t think of that :)

  37. #37 Sinisa Lazarek
    April 4, 2013

    I misinterpreted the darker areas as being hit more, which was odd since the earth is blocking the path. Thanx again

  38. #38 Wow
    April 4, 2013

    I seem to remember something about this on either Cosmos or Wonders of the Solar System.

  39. #39 CB
    April 4, 2013

    Can’t believe this got necroed just so someone could go “The moon doesn’t rotate, derp, therefore all of astronomy is wrong”.