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.