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Perhaps a moon could be required to meet any 2 of the following criteria:

- Tidal locked to planet
- Circular, prograde orbit
- Spherical
- More than a certain threshhold size or mass

But honestly, I don't see why it's so important to make any such distinction; it is not common knowledge how many moons there are in the solar system and there are no mnemonics AFAIK that attempt to list all of them.

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Won't work. Every planet (and the Sun) meets the last two criteria. Why the focus on a prograde orbit, by the way? Is there a reason retrograde orbits wouldn't count?

I guess anything orbiting a planet or an asteroid or a KBO is nowadays called a moon (if it's not a ring), and the only way to stop the addition of more ones is perhaps to get a minimum size (mass), either absolute or relative to the parent.

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How bout any object that orbits a body that the body orbits a star.

Also spilt the moon term into moon and moonlets. This is siiliar to planets and planetiods.

Moons will be go from some type of size limit and spherical. While moonlets will be anything under that size and not spherical.

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On another forum we got into a discussion about the defining difference between a planet-moon system and a double planet system. We wound up deciding that center of gravity might be as good an indicator as any. If it's below the surface of one object, then you've got a planet-moon system. If it's between the two then you've got a double planet.

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Still not a problem. Personally, I only bother to remember Moon; Phobos, Deimos; the 4 Galileans; MET DR THIP; MAUTO; Triton, Nereid, and Charon, and look up the others when I need to.

Well, it'd have to orbit something other than the Sun to count in the first place. But even still, those criteria would exclude Phoebe.

Well, it was suggested in the OP, but since all retrograde really amounts to is inclination > 90°, it's perfectly reasonable to allow retrograde orbits I suppose.

I guess not only is Pluto's historical status as a planet being taken away, but so is Charon's as a moon.

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Any definition needs to exclude the objects in Saturn's rings, else there's an awful lot of naming to do.

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What about shepard moons?

Alternatively, you could define that objects that are closer to each other than a given distance (1 meter, 10 meters) should not to be considered as separate astronomical entities. Rings around planets would be defined as... rings!

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When it comes to naming some common sense should be used. If object are ~10m or less or even under an km, they really should not be named. Especially rings.

If naming everything we came upon maybe we should also name all the trash that is circling around our planet?

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Yes, I would agree with you. Notwithstanding, Saturn rings are "named" (G, F, A, B, and C).

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Yep they are named, but they're for whole sections of the rings which are different by composition, density and other things. Naming sections of rings is not that bad. But naming every little thing that is I really dont' know what to say.

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You are right. Meaningless things shouldn´t be named.

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I've always felt anything too small to have a spherical shape should not be a called a moon. Moonlets works for me.

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It is possible, given a sufficiently eccentric orbit, for the center of gravity to be above the surface at some times, and below the surface at other times. Whether this actually occurs, I don't know...

It works out to a radius of about 170 km and there are 20 such Moons including the Earth's. That works for me. Everything smaller than that is debris.

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