Why the Same Side of the Moon Always Faces the Earth


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The Moon always has the same side pointing towards the Earth due to its rotation speed matching its orbit around the Earth exactly.   This isn’t by chance and it wasn’t always this way.  When the Moon first formed, its rotation speed was very different than it is now.  Over time, the Earth’s gravitational field gradually slowed the Moon’s rotation until the orbital period and the rotational speed stabilized.  This effect isn’t unique to our moon, but also to many of the moon’s orbiting various planets in our solar system.

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  • Nice try, but a better (I think) explanation is, simply stated, that the earth’s moon is lopsided. Why is it lopsided? Because the earth’s gravitation pull on it while it was forming caused it to have more mass on one side…and that slight difference has caused the spin to adjust over millions of years so that we see only one side now.

    Nice website here though…I like Today I Found Out !

    • Daven Hiskey

      @Bill: That is correct. The technical term for these are “tidal bulges”; the bulges actually travel over the surface of the body, creating something of an oblate spheroid. This is very similar to what we see with the tides of the ocean, only obviously to a much lesser extent due to the rigidity of a solid body, such as the moon and the land on the Earth. Although, in cases like with Jupiter and IO, the difference on the solid surface can actually be hundreds of meters, which ultimatley heats the planet itself in a similar fashion to how something like a paper clip or the like heats up when you bend it back and forth.

      Eventually, the object will become tidally locked and it just then makes for a lopsided body. In the end, as you say, it’s the torque that causes the rotation to exactly match the orbit such that only one side faces the larger body. The smaller body also has a similar effect on the larger body, but obviously to a much lesser extent in cases like our Earth relative to the moon. However, eventually the smaller body will cause the larger body to have the same rotational period, which will match exactly the orbital period. Then, the distance between them will change gradually in step with the rotational period change in order to conserve angular momentum, which is why the Moon is getting further and further away from the Earth every year. So, several billion years from now, the same side of the Earth will always face the moon just as the same side of the moon currently always faces the Earth and, of course, the moon will be much further away from the Earth to conserve angular momentum of the system.

      Since tidal forces increase cubically with respect to distance, most relatively large satellites that are close to their respective planets in our solar system are tidally locked in the same way our moon is with the Earth.

      I should have put a bit of the above in the quick fact, but I try to make them as short as possible to adhere to the “quick” part and on this one I dropped the ball as a few sentences more on the above seems to be needed (now that I read it again). Good catch, Bill.

      Interestingly, because Mercury is relatively close to the Sun, it experiences a near tidal lock with the Sun for this same reason. Though, instead of a 1:1 lock, it’s a 3:2 (three rotations for every two times around the Sun).

  • Lucille Gandionco

    We see one side of the moon because of tidal and gravitational lock between the moon and the Earth. The effect is the moon’s rotational period equals its orbital period. This can further be explained using the physics of rotational and circular motion.

    There is no direct evidence that the moon was different than when it was formed and that its rotational and orbital behavior was in effect different. The moon has been there when God created the universe. Genesis 1.

    • Daven Hiskey

      @Lucille Gandionco: “There is no direct evidence that the moon was different than when it was formed and that its rotational and orbital behavior was in effect different.” I know some geologists and physicists who would argue with you on that point and the oxygen isotopic ratios of the moon rocks seems to argue against the “just there” theory as well.

      As a scientist, I’m always up for a good discussion on the whole “Christianity vs. Science” thing in all its manifestations (though I’ve always thought science should ultimately match up with religion as the same being to create the universe also created the scientific laws, so choosing to purposefully make the science come out wrong is akin to lying, so would necessarily make that being not perfect, by the being’s own laws. So, if you’re a Christian, science and religion should match up 1:1, in the end. Note: that’s speaking of the actual precepts and text of when God’s speaking in the Bible, not necessarily the amazing amount of dogma that has sprung up that isn’t explicitly stated anywhere in text, nor when a human is speaking in the Bible nor, of course, when science itself is wrong, as can be the case from time to time, though ultimately happens less and less often as we progress.)

      In any event, your comment presumes that a perfect God being would create the universe in such away that everything just suddenly is as it “should” be, the instant of creation. In order to do it this way, it would take an amazing amount of effort to put every little photon in place throughout the entire universe and project exactly how the universe would be had it been 14-ish billion years old, including all the stars in their various states, percentages of the various materials in the universe, and the like. Given an all-knowing being would always necessarily do things in the simplest (most perfect) way possible, wouldn’t it be more likely that such a being would create a universe the simplest way, (which would then ascribe to Occam’s razor)? This, in turn, would seem to be exactly how the universe appears to have been created (the Big Bang), as I can’t imagine a simpler way to create everything that is. Can you?

      An “I am” being that lives outside of the bounds of time, and, indeed, before time existed, would also not likely ever describe things in terms of “days”, (time doesn’t even exist at that point, let alone an Earth day). Rather, it would seem logical that a timeless being would describe things as “events”, which is one of the reasons, along with the original Hebrew wording, some Bible scholars feel the “7 days” should have been translated “7 events”. Each event then could take billions of years; what is a few billion years to a being that lives outside of time?

      In any event, that’s not to say such a being couldn’t have done it as needed to make it appear that the universe was in the state it would need to be to be about 14 billion years old once the humans started observing it. But you have to ask yourself then, why do it that way? Why put the universe in that exact state and why provide a wealth of evidence that the universe is about 14 billion years old? Further, why make it appear that the various celestial bodies developed this way or that? Why go to all that effort to make everything, from a scientific standpoint, perfectly match-up in said way, when you could have just created it the way it appears and then done absolutely nothing in the interim. Just create the necessary singularity, set up the scientific laws of the universe…. and step back and let it all happen. Much simpler or more “perfect”, and, because time is not something to be considered for timeless beings, a much better way to do it. And, indeed, the one part of the dawn of the universe that science has yet to explain in a satisfactory fashion is this “first cause”, also sometimes called “the hand of God”.

      In the end, the Big Bang and everything that’s come after as far as the formation of the Universe (including the moon 😉 ) does not seem to contradict anything explicitly said in Genesis, as Pope Pius XII declared in 1951 (and indeed, if you’re intimately familiar with the Big Bang theory and Genesis, you’ll find there is no direct contradiction, at least, speaking of the formation of the Universe). It would seem it’s the dogma that’s being contradicted, not the actual text of the Bible. It’s seemingly the simplest way to create everything, which is how a perfect God-like being would do it, and it provides a first cause that so far can’t be explained except by some sort of “hand of God” scenario. So it would seem, at least in this case, the whole “Science vs. Christianity” thing isn’t really applicable. Science is more or less affirming Christianity’s views, in terms of the explicit text, though most Christians refuse to look into the matter far enough to see it and most scientists are only familiar with the dogma and not the text, so they don’t see it either. So we end up arguing for argument’s sake. 🙂

  • Daven…are my comments correct? I think so. Are they offensive? I think not. I wonder you won’t post them.


    • Daven Hiskey

      @bill: Yep, see my reply above. The reason I didn’t approve it very quickly is I’m currently moving (just bought a house) and any comment that seems to need a reply sort of is getting put on the back-burner at the moment as far as getting approved until I can read it over and give it a properly thought out reply. 🙂

  • That’s not all it means. It also means that Earth maintains a permanent location on the moon sky. Seem from the moon, the location of Earth never changes. There is no such thing as ‘earth rise’ or ‘earth set’.

  • …Daven I like your intellect and passion. Nicely put. However, in my experience, trying to reason rationally with Biblical literalists is frustrating at best. My Mom used to say that God incorporated the apparently million year old fossils in the Earth when he created it in order to “test our faith”.

    Thanks for the site, it’s a good one…I’m an info. ‘junkie’ and you have mostly original stuff, not just the same replication of articles that a lot of other sites have.


  • Also, I just noticed “Names” comment: Name says:
    February 28, 2011 at 1:36 pm

    That’s not all it means. It also means that Earth maintains a permanent location on the moon sky. Seem from the moon, the location of Earth never changes. There is no such thing as ‘earth rise’ or ‘earth set’.

    …that sounds right, very interesting.