Defining a moon

[Brian G Turner]

So it occurs to me it may be useful to have a definition for a moon - not a law or rule, but a general guideline principle.

My first attempt was this:
1. A moon is a body that has enough mass to be a rounded shape, that orbits another body that also has enough mass to be a rounded shape.

However, I wondered if I needed to be more specific:
2. A moon is a body that has a rounded shape due to the action of gravity on its mass, that orbits another body that also has a rounded shape due to the action of gravity on its mass.

However, that seems like it may be something of a mouthful.

Maybe it just needs tweaking a little differently to something like this:
3. A moon is a body that has enough mass to be pulled into a rounded shape, that orbits another body that also has enough mass to be pulled into a rounded shape.

Would be interesting to get other people's thoughts on this.

 

[SilentRoamer]

A moon is a natural satellite for any Planet or other object with the exception of a star, whose natural satellites are Planets.

 

[paranoid marvin]

Not a Death Star.

 

[Harpo]

If a coconut is a rounded object and it orbits around another rounded object, such as a meandering search for the holy grail, is it a moon?
Coconuts are hairy and produce milk, so….

If coconuts have hair and produce milk why they aren't classified as mammals? - Ask Us! Answer Service

askus.library.wwu.edu

 

[Venusian Broon]

A moon is a natural satellite for any Planet or other object with the exception of a star, whose natural satellites are Planets.

Okay, it took me a while to parse this sentence, but I think, if I've read this right, is the easiest and best definition

There's still a bit of confusion perhaps. Your implication that moons orbit planets and everything smaller in mass. But what is a natural celestial object that orbits a Brown dwarf - which sits in-between and is neither a star or a planet? (I'd think of satellites of Brown dwarfs more as planets, I suppose.)

But I think the important thing to pull out from this is that a moon does not need to be massive enough to be pulled into a nearly rounded shape. Mar's moons and a great number of Jupiter and Saturn's moons would like to have word with you if you thought otherwise!

The mass and shape definition are important to make a sensible cut-off for planets, both dwarf and normal, but not moons.

However...

...there is a vagueness in the very low limit of mass, as people have used the term 'moonlet' to describe a very, very small natural satellite of an asteroid. And the rings of Saturn, which technically could be made up of a huge number moons under the definition. Therefore it feels right to call ring components as 'moonlets' too.

 

[Dave]

What if both bodies have equal mass? Which is the Moon? Are they both planets? How much smaller does the second body need to be to become a Moon?

 

[Orcadian]

Not all moons are rounded - look at Phobos and Deimos...

 

[SilentRoamer]

Okay, it took me a while to parse this sentence, but I think, if I've read this right, is the easiest and best definition

There's still a bit of confusion perhaps. Your implication that moons orbit planets and everything smaller in mass. But what is a natural celestial object that orbits a Brown dwarf - which sits in-between and is neither a star or a planet? (I'd think of satellites of Brown dwarfs more as planets, I suppose.)

But I think the important thing to pull out from this is that a moon does not need to be massive enough to be pulled into a nearly rounded shape. Mar's moons and a great number of Jupiter and Saturn's moons would like to have word with you if you thought otherwise!

The mass and shape definition are important to make a sensible cut-off for planets, both dwarf and normal, but not moons.

However...

...there is a vagueness in the very low limit of mass, as people have used the term 'moonlet' to describe a very, very small natural satellite of an asteroid. And the rings of Saturn, which technically could be made up of a huge number moons under the definition. Therefore it feels right to call ring components as 'moonlets' too.

Yeah to reword it for easier parsing: "A moon is a natural satellite for Planets, as Planets are natural satellites for stars."

Brown Dwarfs are still Stars so still fit the definition. Agree with the mass and shape though, I mean the above definition (without lower mass and shape requirements) would mean a clump of dust would constitute a moon.

Imagine future real estate, you buy a cool moon, only for some annoying galactic bureaucrat to downgrade your moon and reduce your real estate value... Vogons....

 

[Venusian Broon]

Brown Dwarfs are still Stars

I disagree. The IAU doesn't' think they are either. But that's another definition discussion and not about moons.

And I'd put planets around a Brown dwarf anyway, as I said.

 

[paranoid marvin]

My (slightly more sensible) definition is a nafurally created body that orbits, and either A: has a definable impact on the planet or B: is at least of a certain size/mass (say 5%). This then excludes space flotsam like asteroids which definately aren't moons.

 

[paranoid marvin]

Btw if small planets are 'dwarf' planets, then moons should be dwarf moons.

But I digress. In my opinion a planet is either a planet or it's not . And a moon is either a moon or it's not; no dwarf moons or moonlets.

 

[Ursa major]

Not all moons are rounded - look at Phobos and Deimos...

Indeed:

 

[Orcadian]

What if both bodies have equal mass? Which is the Moon? Are they both planets? How much smaller does the second body need to be to become a Moon?

Indeed. At what point do two objects in orbit around their common CoG stop being a planet & moon and become a binary planet?

 

[paranoid marvin]

Two large masses close to each other would just lead to one, larger mass. Which is pretty much how our own planet was formed

 

[Ursa major]

At what point do two objects in orbit around their common CoG stop being a planet & moon and become a binary planet?

Given that the barycenter of the Plutonian system lies outside Pluto, Pluto-Charon would be a binary planet... if Pluto and Charon were planets.

Instead, the rules on what a planet must be seem to disallow the existence of binary planets (specifically, the "orbital clearing" rule). And given that there are likely** to be what should be called binary planets, including ones composed of bodies that would be much larger than, say, the Earth, it would be silly not to call them what they really are.

I wonder if us calling Pluto and Charon a binary planet for a long time*** would have prevented Pluto from being crossed off the list of planets. It sounds far more interesting than a dwarf planet that happens to have a (relatively) oversized moon... and we'd have ended up with ten planets, instead of eight.


** - After all, we keep discovering all sorts of (what we thought were) unlikely bodies, so why wouldn't there be large binary planets?

*** - How recently did we discover that the barycenter of the Plutonian system was not inside Pluto?

 

[SilentRoamer]

I disagree. The IAU doesn't' think they are either. But that's another definition discussion and not about moons.

And I'd put planets around a Brown dwarf anyway, as I said.

having spent the last 30 minutes reading about brown stars, which aren't what I thought when I first wrote my comment, I would have to agree with you now. They're wannabe stars though, they just didn't want it enough. Their attempts weren't quite stellar.... I'll see myself out.

 

[BAYLOR]

Ramsey Campbell wrote about the moon in one of his novels and, it was very hungry.

 

[Brian G Turner]

Whoops - I should have named this thread "Redefining a moon" as I was looking at a way to talk about the moons of the Solar System in a way that would allow me to focus on just the larger moons.

A moon is a natural satellite for any Planet or other object with the exception of a star, whose natural satellites are Planets.

That shows a pretty fundamental flaw in my original attempt as stars are usually considered rounded, too.

And the rings of Saturn, which technically could be made up of a huge number moons under the definition. Therefore it feels right to call ring components as 'moonlets' too.

This is why I want to focus on the larger moons, because at present there's no formal definition so any pebble orbiting a planet can be called a moon, which doesn't sit well with me.

 

[Swank]

The problem with all the definitions offered is that a moon doesn't have to "orbit". You could have a moon static at a lagrange point, if it is the lagrange point of a planet. Unless you want to define a Lagrange as a type of orbit.


It would be more correct to say that a moon is a large body that is in the primary gravitational influence of a single planet.

I don't know why you would need to define a planet as round. A planet spinning fast enough would be a domed saucer shape.

 

[Lady of Winterfell]

I might go with the definition that a moon is an object that orbits a larger object that isn't a star.

As others have pointed out, the moons of Mars are not round yet we still consider them moons, so roundness may not be necessary. And there are examples of small objects orbiting larger objects that aren't planets, such as the asteroid Ida which has a moon orbiting it called Dactyl.

I hadn't realized there is no 'official' definition for a moon, I guess I just assumed there must be one. Perhaps with all the ensuing kerfuffle after officially defining a planet, they aren't in any hurry to do that again.