Defining a moon

[paranoid marvin]

Why not just in your introduction tell the listener/reader about which/what type of moon you are going to talk? Or discuss the various types of moon and the confirm which are the kind you are about to discuss?

This maybe better than a one-definition-fits-all approach, as no single definition will stop people saying, "But what about..."

 

[Swank]

There's a wikipedia page on this, if anyone is curious about the topic.

"Moon" isn't a science concept. It is the borrowed name of Luna informally applied to natural satellites.

The problem with writing informal science is that you quickly butt up against actual science, or you write baloney.


If I wanted to write a kids book of planetary astronomy, I would just say that planets go around suns, and moons go around planets. That is accurate enough, just incomplete.

 

[Brian G Turner]

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.

These are called Trojans.

Trojan (celestial body) - Wikipedia

en.wikipedia.org

 

[Brian G Turner]

Why not just in your introduction tell the listener/reader about which/what type of moon you are going to talk? Or discuss the various types of moon and the confirm which are the kind you are about to discuss?

So far I've decided to simply state that I will call anything around 250 miles in size as a moon - as it's large enough for gravity to make it a rounded shape - and anything smaller as a moonlet. Will see how far that approach gets me when I arrive at Jupiter and Saturn.

 

[Swank]

These are called Trojans.

Trojan (celestial body) - Wikipedia

en.wikipedia.org

Trojan is the point or an object at that point, but "Trojan moon" is a real term.

And if we had a moon size object hanging out at one of the Trojans, we'd likely call it a moon.

 

[Ursa major]

which has a moon orbiting it called Dactyl.

If an astronaut lands on it one day, can they claim to have reached terra Dactyl...?

And given the moon's size, will they then fly off?

 

[AllanR]

moon size object hanging out at one of the Trojans, we'd likely call it a moon.

like 624 Hektor - Wikipedia (note: Hektor has a moon, but is in no way round)

 

[Lady of Winterfell]

If an astronaut lands on it one day, can they claim to have reached terra Dactyl...?

And given the moon's size, will they then fly off?

I'm going with yes on both accounts.

 

[Swank]

So far I've decided to simply state that I will call anything around 250 miles in size as a moon - as it's large enough for gravity to make it a rounded shape - and anything smaller as a moonlet. Will see how far that approach gets me when I arrive at Jupiter and Saturn.

You're inventing your own astronomy definitions?

 

[BAYLOR]

Well , on the plus die , the moon didn't break from Earth on September 13th 1999 due to a freak nuclear accident .And theres still no Moonrise Alpha .

 

[Brian G Turner]

You're inventing your own astronomy definitions?

There is no official definition of a moon. So I'm writing in a way that makes the subjects covered easier to understand.

 

[paranoid marvin]

There is no official definition of a moon. So I'm writing in a way that makes the subjects covered easier to understand.

There are definitions of what a moon is, but they aren't conclusive and universally agreed upon. What is the difference (other than size) between a large asteroid and a moon? What makes a naturally created satellite orbiting a planet a moon and not something else? And why can't a large enough artificial body orbiting a planet be classed as a moon?

Hopefully your book will help answer that.

 

[Swank]

There is no official definition of a moon. So I'm writing in a way that makes the subjects covered easier to understand.

The problem I see is that someone is going to read that and later quote that number as a fact that they read in a book - and it won't be.

 

[Parson]

Just to confuse things further, what about a "moon" orbiting a planet, which in turn has a moon orbiting it? It would seem to be at least a possible scenario with moons the size of some of Jupiter's moons.
(If this was dealt with upthread I didn't notice it.)

 

[Venusian Broon]

Just to confuse things further, what about a "moon" orbiting a planet, which in turn has a moon orbiting it? It would seem to be at least a possible scenario with moons the size of some of Jupiter's moons.
(If this was dealt with upthread I didn't notice it.)

A moonmoon you say? (Yes, that's what people have called such objects - see Subsatellite on Wikipedia)

Theoretically possible, although I don't think one has been observed.

I would hazard a guess that they do exist out there in the cosmos, but are few and are probably relatively temporary because the moonmoon has to contend with the tidal forces and gravity of two bigger objects, thus making it very likely for it's orbit to easily destabilise.

 

[Swank]

Just to confuse things further, what about a "moon" orbiting a planet, which in turn has a moon orbiting it? It would seem to be at least a possible scenario with moons the size of some of Jupiter's moons.
(If this was dealt with upthread I didn't notice it.)

Those are also moons.

 

[Robert Zwilling]

Once you get past moons orbit planets, it gets kind of hazy. Moons in our system have a lot of variety in size and shape. A couple of moons are bigger than Mercury. You'll reach the twilight zone when you get to Jupiter with 95 moons although many are tiny, way under the 250 limit. There are thousands of things orbiting Jupiter, the moons probably got their own space environment interacting with everything around them. The biggest ones are nicely rounded, classic shaped. Saturn has 146 moons, some with smorgasbord shapes such as a big potato and others that are ravioli shaped. Io has volcanic activity, a couple of moons got water on them. Some of the larger moons could be classified as planets if they orbited the sun per wiki.

 

[Swank]

A moonmoon you say? (Yes, that's what people have called such objects - see Subsatellite on Wikipedia)

Theoretically possible, although I don't think one has been observed.

I would hazard a guess that they do exist out there in the cosmos, but are few and are probably relatively temporary because the moonmoon has to contend with the tidal forces and gravity of two bigger objects, thus making it very likely for it's orbit to easily destabilise.

There are trojan moons in our solar system that are moons of moons.

 

[BAYLOR]

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?

In this case , what you'd have is a Double Planet/BinarySystem .

 

[Swank]

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?

I wonder if this is really possible. It would require two equal bodies to fall equally into the other's orbit and that orbit to be stable despite the pull of the star causing them to wobble. Between the unlikelihood of the initial pairing and the high likelihood of the orbit decaying into an impact - it is a real long shot.

I would not be surprised if there is a mass ratio between primary and satellite that is necessary for stability.