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Recently some new moons of Jupiter were discovered. However the moons are pretty damn small indeed - only a few kilometers wide. That makes me wonder if they should even be called moons in the first place. The Wikipedia article on natural satellites says there's no clear lower limit (or upper limit for that matter). But if we accept that, then Jupiter already has countless numbers of moons in its ring system, and even the Earth has multiple moons since the International Space Station is a "moon".

What exactly is a moon? Is it really a vague notion of "something that goes around a planet"? If so, why hasn't the IAU clarified what exactly a moon is? They've already done so for planets; it seems natural to do so for moons as well.

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Unlike "planet" the IAU hasn't attempted to precisely define "moon". General usage requires that a "moon" is a natural satellite of a planet (or dwarf planet, asteroid, or perhaps even of another moon?) and it is big enough for us to have seen it as an independent body.

This contrasts with the "moonlets" that have been detected in Saturn's rings by their graviational effects on the even smaller ring particles. We know that a body exists there, but it is too small to be directly imaged.

As such the question "How many moons does Jupiter have" is probably unanswerable, since it depends on the sensitivity of your telescope and how close you are to the planet.

The smallest moons are about 10km across, but moonlets in the rings exist that are only a few hundred metres across. So in current usage, 10k might be roughly where people stop saying "moon" and start saying "moonlet".

Language exists to serve us, the definition of planet was only made to solve a particular naming problem. No such problem has arisen for moons. So there is no authoritative answer.

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    $\begingroup$ "Language exists to serve us" is a very good point which can hardly be repeated too much. (Though whether the IAU's naming proclivities have helped or not is debatable.) $\endgroup$ – Mark Olson Jul 20 '18 at 12:42
  • $\begingroup$ What is the "particular naming problem" the definition of planet was made to solve? I thought it was because of the discovery of a planet larger than Pluto that led to the IAU redefining planets. $\endgroup$ – Allure Jul 23 '18 at 3:33
  • $\begingroup$ Previously there had been no definition of "planet". The IAU didn't want Eris and other kuiper belt objects to be called a planets, so chose to define "planet" in such a way as to exclude them. $\endgroup$ – James K Jul 23 '18 at 6:26
  • $\begingroup$ "The IAU didn't want Eris and other kuiper belt objects to be called a planets". Sure. But why? What scientific end is served? $\endgroup$ – Mark Olson Jul 24 '18 at 15:14
  • $\begingroup$ Just as they didn't want Ceres, Vesta, Juno and so on to be planets. But naming isn't really about science. It is about language (or less politely, its about stamp collecting) $\endgroup$ – James K Jul 24 '18 at 16:26
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According to NASA - What is a Moon?:

Planets and asteroids orbit the Sun. Moons—also known as natural satellites—orbit planets and asteroids. Moons come in many shapes, sizes and types. Most are airless, but a few have atmospheres and even hidden oceans. There are hundreds of moons in our solar system—even a few asteroids have small companion moons.

The same question on Universe Today - What is a Moon? says:

A moon is defined to be a celestial body that makes an orbit around a planet, including the eight major planets, dwarf planets, and minor planets. A moon may also be referred to as a natural satellite, although to differentiate it from other astronomical bodies orbiting another body, e.g. a planet orbiting a star, the term moon is used exclusively to make a reference to a planet’s natural satellite.

Many other sites have similar explanations. By any of the definitions, though, the International Space Station is not a moon, as a moon is defined as a natural satellite orbiting a planet, dwarf planet or minor planet.

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    $\begingroup$ The particles in Saturn's rings meet all the criteria you listed above. They're generally not considered moons but part of a ring system. I think your answer is generally correct, but it misses the point of the question, on whether there is a specific and clear cut definition where a moon ends and debris in orbit begins. $\endgroup$ – userLTK Jul 20 '18 at 8:20
  • $\begingroup$ A fleck of paint from the Apollo 10 mission... We get down to absurdity. $\endgroup$ – Wayfaring Stranger Jul 27 '18 at 16:43
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I wont attempt to propose a complete definition, but only suggest a few criteria, that could be meaningful when defining a moon. As I see it, a formal definition, should either serve a very specific purpose (none given in question) or clarify a term already in general use. as a clarification of an everyday term, the formal definition should strive to be consistent with how, the word is already in common use, and not break drastically with existing understandings of the word. I would propose, that moons are first and foremostnatural satelites orbiting around planets, dwarf-planets and exo-planets (a satelite orbiting smaller objects, such as an asteroid, would probably not meet most people's idea of a real moon. As for size, and clearing it's orbit, I would suggest, that the satelite, should be large enough and distinct enough from other objects in its orbital neighbourhood, to be clearly visible, as a distinct object, with the naked eye of a hypothetical human being, standing on the surface of the orbited (parent) object(at least, during part of its orbit).

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