At last count (last I heard anyway), Jupiter has 62 moons, while Saturn has 35. Mars has 2 moons.

However, the majority of the moons of Jupiter and Saturn, and all of Mar's moons are little more than captured asteroids. They're not even spherical.

Tradition aside, should these satellites be counted as moons?
Or should the term "moon" be restricted to bodies large enough to become spherical under their own gravity, aka big and round.

And if not, what exactly would the lower cut off limit be for determining if something is a moon, or just a satellite body? Surely a rock the size of a baseball cannot be called a moon...

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AFAIK, there is no precise definition, but IMO there is room for the smaller irregular-shaped moons to be called moons. Probably the biggest criteria to be called a moon is that it is an object in a stable orbit around a planet, which brings us to another definitional connundrum. What is a planet?

I would argue that Titan is more a planet than Pluto or Mercury.

Well, just like a moon is an object orbiting a planet, a planet must be orbiting the sun. So, Titan doesn't qualify.

So what do you call an object in stable orbit around a moon?

By who's definition? IIRC, there is no formal definition as yet.

Tomba: I don't think there would be a stable orbit like that.

???? Doesn't a dictionary qualify? http://dictionary.reference.com/search?q=moon

I don't believe you can have a stable orbit around a moon, the gravitational attraction of the parent planet would drag on it every time it orbitted nearest to said planet. That field would drag it out of any hope for a stable orbit, unless it somehow, miraculously, stabilized into a figure 8 orbit without tearing itself apart.

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But he that sows lies in the end shall not lack of a harvest, and soon he may rest from toil indeed while others reap and sow in his stead. -J.R.R. Tolkien, The Silmarillion

One weakness with the current definition of moon is that there is no guidance for a small chunk of rock or ice. When is an object a moon versus just another particle in a ring?

There have been numerous spacecraft that have orbited our Moon, though admittedly I'm not sure what sort of adjustments they had to make to maintain their orbits.

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I guess that apart from a stable orbit aruond a planet, it also has to 'fly' on its own, not in a larger group in approximately the same orbit. Thus you exclude rings. As for minimum size, that's harder to decide...

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Interesting, as I understand it a few of Saturn's and Jupiter's moons are co-orbital.

http://www.solarviews.com/eng/janus.htm

Take Janus and Epimethius for example.

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Russ: The Dictionary is certainly source to consider, when determining how a word is used in common parlance. However, as anyone with any science or technical background can verify, it is not an authoritative source for professional jargon.

So, if we are applying the common man's definitions, you are quite correct. However, since this is an Astronomy forum, I assumed we were talking in terms of the jargon of Astronomers. Astronomers have no formal definition of planet. It is a topic that is actually in hot debate in recent years with the discoveries on Titan and extra-solar planets, particularly the planets that roam free from any star.

I am not sure if there is a formal definition of "Moon", but my suspicion is that it is as nebulous as "planet". I will have to look it up.

That's what I feared... Allright, my definition doesn't work! There goes my chance to make it in the science history books: "The generally agreed on definition of what is a moon and what isn't, informally called 'Fram's definition'... ".

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LOL, this discussion came up related to Cassini over the possibility of finding new shepard moons in the rings. Its probably this mission that will start to put a hard line between what's a moon and what's orbital debris.

I suppose my own personal take on it would be, any object in orbit around a planet which is individually identifiable as separate and distinct from other bodies orbiting a planet.

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The last time I felt a warm fuzzy feeling, I was informed by my doctor that it was just gas.

Honestly, I doubt any universal definition will ever be satisfactory to everyone involved. People interested in celestial mechanics will always insit that a moon orbits a planet, and a planet orbits a star. However, people more interested in the composition of the planet will look at Titan, and be hard pressed not to call it a planet.

Edit: As to the size limit for a moon, I propose that we set the minimum at a size where a human could walk/jump on the surface without risk of flying off into space, and without noticably altering the orbit of the "moon". I am not sure whether all of the currently identified moons would meet this size limit.

If this were necessarily true, one could make the same argument for the Earth/moon/sun system. Every time the Moon's orbit brings it directly between us and the sun, it is closest to the sun and thus experiences more gravitational pull in that direction. However as we all know, the gravitational attraction of the sun has never yanked the moon out of its orbit around the Earth. Without doing the math involved it is simple enough to say that there is a certain critical distance, after which the gravitational pull of the sun would outpull the gravity of the Earth. Anything within this point is not pulled away from the Earth because the Earth pulls harder.

There must therefore be a region of space around the moon where its gravitational pull is greater than that of the Earth. If there was not, then the rockets we fired to the moon would never have been able to land there, and would have fallen back to the Earth instead. This region of space is likely rather small, and the chances of our Moon capturing a satellite that would orbit within this critical distance are equally small, nevertheless there do exist stable orbits around the Moon (and thus any moon of any planet), its just that a configuration happening naturally is unlikely.

I'd suggest the minimum size be set at the diameter of the Death Star. After all, we know that it's the size of a "small moon", and that's good enough for me.

Yes, size is the big source of ambiguity. The definition just says "larger than an asteroid or comet," but what does that really mean? My criteria would probably include a gravity field strong enough to make it roughly spherical (which would exclude many moons, including Mars's).

A better word (and often used by astronomers) may be satellite, for which size is irrelevant.
I suspect astronomers simply don't care, which is why you rarely see any controversy over the issue. Planet, asteroid, minor planet? Not that important. Moon or just a satellite? Not that important either.

But you won't ever see moon and planet being interchanged.

This is all well and good, but how big is this "moon around a moon" we're talking about? Yes, a satellite can be made to orbit in a stable and reasonably circular orbit, 50 miles from the moon - but are you suggesting we call a 30m rock at that same distance another moon? Any "moon around a moon" of reasonable size would be larger, orbit much further away, and thus be perturbed quite a lot by its neighboring planet.

Oh, I'm not saying that there do exist "moons around moons" as much as I was suggesting that orbital configurations for them can exist. I agree that the liklihood of any object large enough to be considered a "moon" would be found orbiting another "moon" to be very small. Of course, this only further illustrates why it'd be nice to have a firm, scientific definition for the term "moon"

what about clasifing them by % of the size of the parent and also accounting for gravitational pull of the parent object?

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You still get inconsistent results. Some of the largest planets known might have moons the size of Ganymeade that would be disallowed as moons, but then if you use those percentages on a gas giant like Uranus or Neptune, large ring debris may qualify.

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Well, I guess if you think of the technical values between an earth moon (our moon: with dead planet like characteristics) and a big peice of battered rock, then astroid moons are just astroids ... If a moon is called a moon only simply becuase it orbits a planet then I guess they're moons but if a moon has to be spherical and kinda planet like rock with craters or without or an atmosphere perhaps, then I think maybe that is a more appropriate understanding of moons rather than a rock that just so happend to get pulled into the planets gravitational pull and begin to orbit a planet. What can we begin to say about that? It's not really much of a moon. But hey I leave that up to the brilliants minds at NASA and any other Astronomic Facilities and space agencies that study space and its components. If the moon has an atmoshpere and some sort of life growth wether it be an airborne substance other than dust , oran actual living organism would it be more like a planet especially if it carries the consistancy of water or with Titan water ice or whatever it was referred as.

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After reading a few other post let me review some simple information that might help resolve A Planet Or A Moon... If you look at our Planet and moon The moon we have has an atmosphere of some sort right? It is a smaller dead planet like structure and an astroid is that simply of a big chunk of rock. Would it be safe to call a moon a Dead planet and that of a dead planet alone to classify a moon? A rock that is just a chunk of astroid whether it is in full orbit, in a stand still or just simply passing by should be called an astroid. A moons structure ressembles a planet in so many ways. We first descovered moons becuase of our moon right? Well does it not have the characteristics of a dead planet? If the moon has an atmoshpere and some sort of life grwoth wether it be an airborne substance other than dust , oran actual living organism would it be more like a planet especially if it carries the consistancy of water or with Titan water ice or whatever it was referred as.

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Not to stir the pot on the planet/moon bit, but personally, I'm in the "If it doesn't directly orbit a star, it isn't a planet" camp. A moon can be a world, but a moon cannot be a planet.

Actually, that might be a good way to classify something as a moon or not a moon.

If you took the body and set it in orbit around a star, would it be considered a planet? If not, then perhaps it shouldn't be called a moon either, so much as orbiting debris. Big orbiting debris, but still just debris.

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But he that sows lies in the end shall not lack of a harvest, and soon he may rest from toil indeed while others reap and sow in his stead. -J.R.R. Tolkien, The Silmarillion

Where is the cut-off between big orbiting debris and planet then if both orbit a star? What would Saturn be if it were to orbit Jupiter like a moon?

Well, for starters, Saturn couldn't orbit Jupiter like a moon. Its large enough that they would orbit each other in a dual-planet arrangement.

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But he that sows lies in the end shall not lack of a harvest, and soon he may rest from toil indeed while others reap and sow in his stead. -J.R.R. Tolkien, The Silmarillion

Like Earth and the moon? :-k

The moon is only what, a quarter the size of the Earth? The center of rotation is still within the body of the Earth. Jupiter and Saturn are much closer to each other in scale, so I highly doubt one could be seen to orbit around the other at all, so much as they would rotate around an empty point in space between the two bodies.

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But he that sows lies in the end shall not lack of a harvest, and soon he may rest from toil indeed while others reap and sow in his stead. -J.R.R. Tolkien, The Silmarillion

Uranus and Neptune are roughly 1/16 the mass of Jupiter, if either of them orbited Jupiter (center of rotation within Jupiter) would it be considered a moon or a planet?