When is a moon not a moon?

[c0rbin]

Cool moon thin (LO)

I like crashing them into earth, or skimming them out in to the void

 

[Nucular]

Oops, didn't mean to bump this thread. Though now I have, everyone look at the lunar orbiter bit on the first page, it rocks (that was what I was trying to work out what it was, but hit "reply" by accident, d'oh!)

 

[drkitten]

No, if my memory serves me right, Asimov did not hold any official titles, but he was very good (if a tad long-winded) at explaining scientific concepts.

Associate Professor of Biochemistry, Boston University.

Which of course, has little to do with "moons" unless their composition is radically different than what I expect.

 

[drkitten]

Asimov was an honest-to-goodness scientist. He was professor of Biochemistry at Boston University until 1958, when he went into writing full time.

Actually, he retained the title until his death -- he was just on an extended (and unpaid) leave of absence. There were a few political problems -- some university presidents didn't like the idea of having non-teaching/research faculty on staff. Other, more sensible ones, recognized the drawing power of being able to claim Isaac Asimov as a faculty member....

 

[drkitten]

I think <A href=http://www.math.nus.edu.sg/aslaksen/teaching/convex.shtml>this</A> is what Asimov was talking about.

That's pretty good, yes.

One of the statements I've seen offered as a definition of a "moon" -- although I have no idea how authoritative this definition is -- is that a true moon must at some point move backwards in its orbit relative to the sun.

 

[SquishyDave]

Now, taking that, we have to apply the characteristics of our Moon when trying to classify others moons (what the capitals). In that regard, I am not sure whether we can say size is a factor. A moon is an orbiting body around a PLANET, so Earth is not a moon, but a planet, because Earth orbits a star. The Moon does not.

But the Moon does orbit a star. And so does the earth, they just sort of get in each others way as they both orbit the sun. See Terry's link <A href=http://www.math.nus.edu.sg/aslaksen/teaching/convex.shtml>here.</A>

Where as Io orbits a planet, very clearly, it moves around and backwards in relation to the sun. At least as I understand it.

So where does that leave us?

 

[davidhorman]

One of the statements I've seen offered as a definition of a "moon" -- although I have no idea how authoritative this definition is -- is that a true moon must at some point move backwards in its orbit relative to the sun.

Doesn't that raise the prospect of a planet with two satellites, one close in which, at some, point goes backwards in it's orbit around the star, and another further out which doesn't?

Or two identical planets with identical satellites, one orbiting close and one further way - one would be said to have a moon and the other wouldn't.

The fact that the above definition requires a sun should rule it out too, IMHO.

I'll stick with my definition (I say "my" - I'm sure I'm not the first but I don't remember reading it anywhere else) that a moon is a moon if the centre of gravity of the planet-moon system lies under the surface of the planet. Otherwise they are co-planets.

Of course, that raises the prospect of an object in an eccentric orbit that's a part-time moon...

David

 

[drkitten]

Doesn't that raise the prospect of a planet with two satellites, one close in which, at some, point goes backwards in it's orbit around the star, and another further out which doesn't?

It does. And?

The fact that the above definition requires a sun should rule it out too, IMHO.

Why?

Free bodies wandering in interstellar space that aren't orbiting anything are usually just called "object," not planets or moons.

 

[c4ts]

Don't all moons technically orbit around a cocenter of gravity?

 

[davidhorman]

Don't all moons technically orbit around a cocenter of gravity?

Their orbit would be centred on the centre of gravity of the system, yup.

Free bodies wandering in interstellar space that aren't orbiting anything are usually just called "object," not planets or moons.

I only meant that the definition was a bad one (at least, it doesn't sit well with me) because it has to make reference to a sun - making it more complicated, and leading to the above situations in which otherwise identical pairs of objects are evaluated differently because of their relationship with an entity outside the system.

David

 

[Jon the Geek]

A side note on the cool orbiter sim thingy: If I set something at or near L4 or L5, regardless of whether I give it any starting velocity (well, as long as I don't give it so much that it escapes from L4 or L5), shouldn't it stay at L4 or L5? I tried putting my mouse over the moon and clicking repeatedly (so one of the satellites I placed would end up at or near L5 once the moon was 60 degrees away), but they all fell toward the Earth (some deflected slightly toward the moon, but not nearly as much as something at L5 would be). Do they have to have some initial velocity in order to stay there? I thought objects near L4/L5 would be "pulled" into those points, thus making them stable equilibrium points. Or maybe they're pulled into an orbit AROUND L4/L5?

ETA: If the International Astronomical Union (or whatever it's called) calls it a moon, it's a moon. But I think the "if the center of mass in the system is in one of the two objects, that object is the planet and the other is a moon" idea is a good start. Unfortunately, doesn't that mean man-made satellites are moons? Or is it that any natural satellite that meets that definition is a moon? In that case, isn't any bit of dust in a ring around a planet a moon?