Moon sizes

[stonemonkeylives]

Firstly let me apologise if this is in the wrong place.

A work colleague and I were discussing the the relative sizes of astronomical bodies last night while at work (it was a slow night). And he asked if there was a minimum size for a moon. I couldn't find anything on here but said that I would anything over 200 metres would probably be class as a moon. But in honesty I have no idea, so I thought I would ask on here and bow to your superior knowleges'

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[JamesF]

There's no absolute definition of what constitutes a moon in terms of size, I believe. I think the generally accepted definition is that it must be a single distinct solid object in orbit about another larger body.

Some of the moons of Saturn are certainly very small.

James

[Stargazer_00]

and further to James post id add that or moon is comparatively large compared to the parent planet compared to other examples in our system.

[stonemonkeylives]

Cool thanks guys.

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[Pig]

I believe our moon is the biggest satellite in our solar system

[John]

I believe our moon is the biggest satellite in our solar system

It's the largest relative to the size of it's primary (the Earth). There are larger moons in the solar system, eg: Ganymede which is 50% larger in diameter than our Moon.

[Naemeth]

It's the largest relative to the size of it's primary (the Earth). There are larger moons in the solar system, eg: Ganymede which is 50% larger in diameter than our Moon.

Indeed, the Moon is about 1/3rd of the diameter of the Earth isn't it?

Both Phobos and Deimos are considered moons (of Mars), when they were probably once asteroids captured by Mars ..

[Stargazer_00]

Phobos and Deimos orbit Mars at approx 10,000km and 20,000km whereas The Moon orbits the Earth at 380,000km. I thought 380,000km was quite far from the primary for a moon to orbit until I just looked at Saturn's moon orbits. Iapetus' orbit is 3,500,000km!

[ArcticCircle]

he asked if there was a minimum size for a moon

Whatever you do, don't let him ask you bout the minimum definition for a planet!

I think the astronomical societies have only just recovered from arguing about Pluto.

[JamesF]

Phobos and Deimos orbit Mars at approx 10,000km and 20,000km whereas The Moon orbits the Earth at 380,000km. I thought 380,000km was quite far from the primary for a moon to orbit until I just looked at Saturn's moon orbits. Iapetus' orbit is 3,500,000km!

Orbing Saturn with an orbital radius of even 20,000km would be quite tricky, mind

James

[JamesF]

If someone asked me how many planets there are my reflex answer is still 9

Poor old Charon. Always the bridesmaid...

James

[Stargazer_00]

If someone asked me how many planets there are my reflex answer is still 9

[Knight of Clear Skies]

Sometimes the word 'moonlet' is used to describe a very small natural satellite, but there is no precise definition of the term. Nature often presents a continuum of objects rather than ones that fit into our sometimes slightly arbitrary categories.

[Stargazer_00]

What do you call an object which orbits a moon?

[Peter Drew]

A satellite?

[badgerchap]

I don't think, and I'm really not certain of this, that things orbit moons for very long and remain in a stable orbit. The more axes involved, the more likely the system is to be unstable. Please let me reiterate how uncertain I am of this.

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[Stargazer_00]

I appreciate the mechanics involved but it must be possible somewhere, I mean scroll it back one step.. the sun, the planets and the moons. not massively different from planets, moon and ting orbiting moon. It's definitely possible in my opinion, even if it's not evident in our locale.

[Knight of Clear Skies]

I don't think, and I'm really not certain of this, that things orbit moons for very long and remain in a stable orbit.

Yes, nature has taken pity on the poor taxonomers. Over time the primary would tend to pull the 'moon satellite' into a more elliptical orbit, until it hit something. It is possible for a small body to stay in orbit in a Langrangian point, but even these are not completely stable due to disturbance by the other planets.

[JamesF]

Or perhaps the two would end up being split so they both orbited the main body?

James

[Knight of Clear Skies]

I appreciate the mechanics involved but it must be possible somewhere, I mean scroll it back one step.. the sun, the planets and the moons. not massively different from planets, moon and ting orbiting moon. It's definitely possible in my opinion, even if it's not evident in our locale.

There probably are some small bodies in such orbits, but they will stay there for a comparatively short time. Distance is what makes the difference: the Sun is much further away from a planet's moon than the satellite of a moon would be from a planet. As a result, such an orbit isn't stable over solar system timescales.

Over very long timescales, the planets' orbits aren't completely stable either. Simulations suggest there is a very small chance one of the inner planets could collide with the Sun, another planet or be ejected from the solar system within the next billion years.

[badgerchap]

I think we often assume that planetary orbits are stable, but current data on other solar systems indicates that over large timescales there is a huge amount of planetary migration. We also know little of the permanence of moon orbits, seeing as the longest records of even our own moon are only thousands of years old. We didn't even know other moons existed until Galileo, so we still can't be sure just how long moons in general stay around. Saturn's rings may indeed be testament to the fleeting nature of smaller satellites.

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[Stargazer_00]

I'd suggest that Iapetus is far enough away from Saturn to be able to capture it's own small sattelites, possibly. It's a long long way from it's parent and it has a reasonable amount of mass. Something initially attracted to Saturn itself from a long way off may come close to Iapetus and be captured by it instead. i don't know, but even if it's not possible in our system I'm sure somewhere out there it is possible and is currently happening and those objects derserve a name!!

[Knight of Clear Skies]

I'd suggest that Iapetus is far enough away from Saturn to be able to capture it's own small sattelites, possibly.

Looking it up, it seem you're right: Iapetus is far enough from Saturn that a captured body could remain in a stable orbit for a very long time. What I said above is true for most moons, but not all of them.