By this definition, is every small rock orbiting Saturn considered a natural satellites? What is the widely accepted terminology for moons? The Wikipedia article is ambiguous on this. So, what terms do actual astronomers use?

  • $\begingroup$ This seems like a duplicate of astronomy.stackexchange.com/questions/27016/… $\endgroup$ Commented May 8 at 13:48
  • $\begingroup$ @DarthPseudonym Yes, that is part of what I was asking. Thanks. One thing that question doesn't cover the last part of my question. $\endgroup$
    – EMS
    Commented May 8 at 13:56
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    $\begingroup$ Does this answer your question? What exactly is a "moon"? $\endgroup$
    – James K
    Commented May 8 at 19:39
  • $\begingroup$ EMS - there's simply no definition of the word "moon". So that's that. $\endgroup$
    – Fattie
    Commented May 10 at 11:51

2 Answers 2


Technically speaking, every grain of sand in orbit of another body is a "satellite". The IAU definition is essentially any object in orbit of a planet where the barycenter between the two objects is below the surface of the planet. (If the barycenter is above the surface, in the space between the two bodies, then it's a double planet.) We don't usually talk about orbital dust grains as 'satellites', but it's not inaccurate.

The term moon is not well-defined in astronomy. While it usually indicates a natural satellite large enough to be observed as a separate body, that definition makes the meaning of moon depend entirely on the resolving power of our instruments and the distance to the object, which doesn't make much sense as a definition.

But many terms in astronomy aren't strictly defined with clear limits. There's a continuum of objects from galaxy to dwarf galaxy to globular cluster and we still can't agree on where those dividing lines ought to be. (There's an ongoing debate about whether the Large Magellanic Cloud is a dwarf galaxy or true galaxy -- albeit a small one -- for example.) There are similar ongoing taxonomic debates about how to define planets and dwarf planets and many other terms. The IAU hasn't even tried to tackle the problem of how to define a moon.

That said, terminology sometimes doesn't benefit from extremely strict definitions. Terms are a tool for communication between human beings and useful only in as much as they benefit that communication. If we tangle a term in so much technicality that it becomes difficult to use in standard communication, we have not benefited anyone.

In the case of Saturn, it's often useful to talk about each individual ring as a distinct object for most purposes (or even to refer to the entire ring system as a single object). If we're getting very technical about the rings as structures, it's understood that they're made up of chunks that range from millimeters to kilometers in size. Some of the largest chunks we might call moonlets, but that's a practically meaningless term, given that we don't strictly define "moon" to begin with.

In actual usage, it doesn't matter that much. If you're writing a paper about the rings of Saturn, you're going to define what you mean by your terms within the paper. You wouldn't talk about "moonlets" unless you specified exactly what that means; more likely you'll simply talk about objects within a range of masses or diameters.

  • $\begingroup$ So, whenever using terms 'satellite' & 'moon' closely together, I should just add a sentence or two to clarify what I mean by them? $\endgroup$
    – EMS
    Commented May 9 at 12:30
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    $\begingroup$ I would think it would be clear from context what you're talking about just from context, but if you feel it might be misunderstood, it's always fine to use a clarifying statement. It depends on what you're writing about and for whom. I don't think there's one universally true answer to that. $\endgroup$ Commented May 9 at 14:28
  • $\begingroup$ I find it amusing that if you take the IAU definition too literally, Jupiter is not a satellite of the Sun. (And I suppose this illustrates your broader point about how it's hard to come up with a simple definition to cover all cases.) $\endgroup$ Commented May 9 at 21:06

There is no sharp definition for a lower boundary of 'moon'.

Practically, one usually draws the lower boundary of a moon somewhere where the internal gravity is strong enough to keep the mass together and withstand shear forces due to tides and differential Keplerian motion. The boundary is actually quite visible and discussed in literature in the Saturnian system. In the rings one finds quite a few ephemeral moonlets (e.g. see Tiscareno et al (Nature, 2006)), bodies of a few hundred meters consisting of rubble (thus smaller stones). However the Saturnian tides and differential Keplerian motion are strong enough that to overcome cohesion and self-gravitation of these bodies so that they eventually fall apart again.

This definition obviously depends on the size of the parent body and the distance to it. Thus practically the lower end of what is called a moon is somewhere around a (few) hundred meters (e.g. see the list of moons on Wikipedia), depending on parent body and situation. One does not want to name every indistinguishable car- or house-sized boulder in the Saturnian ring a moon.

  • $\begingroup$ Besides falling apart, not having cleared their orbit could help define moons like it did planets (sorry Pluto). $\endgroup$ Commented May 11 at 1:39

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