Sciency Words: Libration (An A to Z Challenge Post)

Today’s post is a special A to Z Challenge edition of Sciency Words, an ongoing series here on Planet Pailly where we take a look at some interesting science or science related term so we can all expand our scientific vocabularies together. In today’s post, L is for:


The Moon is tidally locked to the Earth, meaning one side is always facing toward us and the other side is always facing away. Except this tidal locking isn’t perfect. The Moon rocks back and forth just a little bit.

The technical term for this is libration. It comes from a Latin word meaning balance. In the visual simulation above (courtesy of Wikipedia), we can see the phases of the Moon on fast-forward. We can also see that the Moon moves a little closer to us and then a little farther away, due to its elliptical orbit.

And if you watch closely, you can see the Moon rocking or swaying back and forth. If you’re having trouble seeing it, I recommend picking a surface feature—a crater, perhaps—and following it with your eyes.

Of course our Moon isn’t the only moon that librates. I first learned about libration from a paper about Enceladus, a moon of Saturn.

Thanks to the Cassini mission, we were able to get extremely precise measurements of Enceladus’s libration, and we discovered Enceladus librates a lot. Like, a whole lot.

Enceladus librates so much that it cannot be solid all the way through. Instead, there must be a vast ocean of liquid water sloshing around inside, with only a thin, icy crust floating on top.

That’s a big deal because with all that liquid water, there’s a chance that maybe—just maybe—Enceladus could support life.

Next time on Sciency Words: A to Z, we’ll talk about metal. Everyone knows what metal is. Everyone except astronomers.

19 thoughts on “Sciency Words: Libration (An A to Z Challenge Post)

  1. So that’s why Enceladus has a thin layer of ice crust. Yesterday I read an article about Enceladus… and then here you go adding more information as to why it might support alien life. Very nice read!

    Liked by 1 person

    1. Cool! I feel like with every story I read about Enceladus, the estimate for the thickness of the ice layer gets thinner and thinner. We’ve gone from maybe just a thin lens of liquid near the south pole to a global subsurface ocean maybe a hundred kilometers down to an ocean that is… what?… like, two or three kilometers down?

      Liked by 1 person

      1. Or maybe the ice layer is thin after all… only the instruments we use to observe it change and so our estimates too. But well, the next question is if that ocean can harbor simple-cell organisms because that would freak me out if it does!

        Liked by 2 people

      2. Someone should start making a probe now before that ice thickens (might be possible because of Saturn’s location now? I dunno if it is in its perihelion…)

        Liked by 1 person

      3. I know NASA’s launch schedule is pretty full for the next decade or so, so I doubt they have the time or resources for Enceladus at the moment. But there are plenty of other space agencies out there. I hear China’s been making a lot of progress with their space program.

        Liked by 1 person

      4. Oh the Tiangong Program? From what I heard they’re on the second phase of the program (they’d launched a space lab last year). The third and last phase will be a space station (…to replace the ISS), set for launch before 2020. I dunno what the ESA is up to.

        Liked by 1 person

      5. From what I hear, ESA’s looking to build a moon base, possibly in partnership with Russia and/or the US. They’ve also got a Jupiter mission in the works, I think.


    1. Glad to hear it! This was a real eye-opener of a term for me. I had no idea there was a special term for this because I had no idea moons even did this sort of thing in the first place. It’s a case of science turning out to be even more interesting than I could have imagined.

      Liked by 1 person

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