Sciency Words: Moon

Today’s post is part of a special series here on Planet Pailly called Sciency Words. Each week, we take a closer look at an interesting science or science-related term to help us expand our scientific vocabularies together. Today’s term is:


There are three things I want to cover with today’s post. Firstly, for anyone who may not already know, Earth’s moon is officially called the Moon (with a capital M). Unless you don’t speak English, in which case it’s called whatever it’s called in your language, provided that you treat the word as a proper noun. This according to the International Astronomy Union (I.A.U.), the one and only organization with the authority to name and classify astronomical objects.

Phases of the Moon.

Of course the Moon is not the only moon out there, so I also want to talk a little about the official I.A.U. sanctioned definition of the word moon. Unfortunately there isn’t one, which seems odd given how the I.A.U. are such stickers about their official definition of the word planet.

A common unofficial definition is that a moon is any naturally occurring object orbiting a planet, dwarf planet, or other kind of minor planet (such as an asteroid or comet). Except this definition creates some problems:

Saturn has like a bazillion moons!

Since there’s no lower limit on size or mass, you could consider each and every fleck of ice in Saturn’s rings to be a moon.

The Moon isn’t a moon!

In a very technical sense, the Moon does not orbit the Earth. The Earth and Moon both orbit their combined center of mass, a point called a barycenter. In the case of the Earth-Moon system, the barycenter happens to lie deep inside the Earth, so this distinction may not seem important, but…

Pluto is Charon’s moon, and Charon is Pluto’s!

The barycenter of the Pluto-Charon system is a point in empty space between the two objects. Pluto is the larger of the pair, so we generally consider Charon to be Pluto’s moon; however, you could argue that Pluto and Charon are moons of each other. You could even write a love song about their relationship.

Of course I’m not seriously arguing that Saturn has billions upon billions of moons, nor am I arguing that our own Moon is not really a moon. There does seem to be some ambiguity about Charon’s status (is Charon a moon, or are Pluto and Charon binary dwarf planets?), but I’m not sure if this ambiguity has caused any real confusion in scientific discourse.

Still, as we learn more about moons in our own Solar System and also moons in other star systems, I think the I.A.U. will eventually have to come up with an official definition. And that brings me to the third and final thing I wanted to cover today: exomoons.

An exomoon would be defined as a moon (whatever that is) orbiting a planet or other planetary body outside our Solar System. Finding exoplanets is hard enough, so as you can imagine, searching for exomoons really stretches the limits of current telescope technology. But astronomers are trying, and next month (October, 2017) the Hubble Space Telescope will be making special observations of a planet named Kepler-1625b in an attempt to confirm a possible exomoon detection.

20 thoughts on “Sciency Words: Moon

  1. A counterpoint to your “Moon isn’t a moon” argument (even though I know you’re not actually proposing it). I would say that the definition of orbiting something includes the condition that the barycenter is somewhere within the larger object, not specifically at the larger object’s center. Otherwise no two objects could ever orbit each other, ever.

    Liked by 2 people

    1. I think that’s fair. My point was really just that the common “one thing orbits another” idea is not technically accurate. It’s fine if you’re dealing with the Earth and the Moon, or Jupiter and all of its moons, but you start running into problems with Pluto and Charon. You could also conceivably have 3-body problems or n-body problems where the barycenter is sometimes inside the larger object and sometimes is not. The problem for the I.A.U. will be finding a definition of moon that includes all these possible cases.

      Liked by 1 person

  2. I had a similar post not too long ago, when it was demonstrated (I won’t say discovered) that the asteroid Florence had two moons of its own. The problem (?) of multiple objects orbiting each other leads to some real mental gymnastics on what to call which object, and why… the Pluto-Charon combo is a great example. I just like how it points out limitations in our attempts to neatly categorize and label things!

    Liked by 2 people

      1. Florence is actually straightforward – it’s the “big” asteroid that passed really close to Earth about a month ago, close enough that we could use radar to verify that it has two small moons orbiting it. But the idea of a “moon” implies “planet” in most cases, so it was still a little unusual!

        Liked by 2 people

    1. Hi J.S. Pailly and Troy (Flying Squirrel),

      Moon orbiting a moon orbiting a moon and so on . . . . sounding like a recursive or fractal phenomenon!

      Considering the relevance and quality of what you have discussed in your posts, I have also hyperlinked this post and the other one entitled “Sciency Words: Moon Village” to my post entitled ” If My Name Were Moon Tonight… ” published at

      Liked by 1 person

  3. Interesting post. I do not like it that Pluto has been categorized as not a planet. Love the song about Pluto and Charon. The NASA mission to Pluto was amazing. I wrote a short post, “It sure looks like a planet to me.” The lead investigator of this mission feels that Pluto is a planet.

    Liked by 2 people

    1. If you’re asking for my personal opinion, I’d like to call them both planets. I’d also like to call Titan, Europa, Ganymede etc planets, and have it established that an object can be a planet and a moon at the same time. That’s part of the geophysical definition of planet that’s been proposed to the I.A.U., though I very much doubt the I.A.U. is going to accept that definition any time soon.

      In the meantime, I guess Charon’s probably a moon, and Ceres is officially a dwarf planet.

      Liked by 1 person

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