When is a moon not a moon?

Nucular said:
In a debate on another site, a claim has been made that It's not central to the debate - in fact it's not important at all - but I was wondering if this is, in fact, true?
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I seem to remember Asimov's argument in this case had to do with what he called the "tug-of-war ratio" - which was the ratio of the graviational force due to the primary versus the graviational pull due to the sun. Luna is the only example of a satellite in the solar system which has a less-than-unity TOW ratio. Hence, it never "falls" away from the sun due to the earth's gravity. Instead the orbits of the Earth and the Moon braid together in such a way that the moon is always "falling" towards the sun.

--Terry
 
Nucular said:
By some other guy:
"Of course, the term "moon" for our own moon is a complete misnomer. Isaac Asimov has shown it is too large to be a moon. It is, in fact, a co-planet with the earth. They revolve around each other the way double stars revolve around each other. Their center of gravity is what revolves around the sun."
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Pigswill! It's the moon cos we call it the moon! Think definitions here.

Anyway, the common center of gravity is not coincident with the center of gravity of either body, regardless of mass (assumed finite).
 
If we were talking, say pluto and charon, then I could see this arguement being valid. The point around which they both rotate is not inside pluto, they do orbit each other by any definition of the term.
 
neutrino_cannon said:
If we were talking, say pluto and charon, then I could see this arguement being valid. The point around which they both rotate is not inside pluto, they do orbit each other by any definition of the term.
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What are you talking about? Are you saying that a moon is only a moon if the barycenter is found within the volume of one of the bodies?
 
So in other words, the moon is not a moon? Who'd a thunk it?

Incidentaly, your definition presumably excludes stars. If not, than surely the Earth is a moon of the Sun.
 
LucyR said:
So in other words, the moon is not a moon? Who'd a thunk it?

Incidentaly, your definition presumably excludes stars. If not, than surely the Earth is a moon of the Sun.
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No no, the barycenter between Terra and luna is firmly in terra. and yes, earth is a moon of the sun. My definition sucks.
 
Actually, I screwed up (but your username's still silly).

I think the point is that what significance the term has is really historical.
 
Our Moon is a moon. It was the 1st, and hence isthe definition of what we Earthlings view as a moon. I can't see how people can say that the one thing we called 'Moon' for centuries isn't one.

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.

Now, Pluto/Charion are different inthat one doesn't orbit the other, but rather (I believe) they orbit each other, in a kind of 'space dance'. Also, it gets even fuzzier because we are still not sure whether Pluto is even a planet at all. It may just be a KBO/asteriod. In that case, what would that make Charion, even if it DID otherwise fit the definition of a moon?
 
Larspeart said:
Now, Pluto/Charion are different inthat one doesn't orbit the other, but rather (I believe) they orbit each other, in a kind of 'space dance'.
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What, so the centre of gravity of the orbits of Pluto and Charon is not within either body, but between them?
 
Correct. It isn't a body orbiting another body. It is two bodies circling each other.

I hope that makes sense.

You have to remember that in size, they are quite similar (no other moon comes even close to Charion in terms of relative size to the planet it orbits) and they are very close together.

I'll try to find a link on some neat Pluto/Charion info for you guys.
 
Larspeart said:
Correct. It isn't a body orbiting another body. It is two bodies circling each other.

I hope that makes sense.

You have to remember that in size, they are quite similar (no other moon comes even close to Charion in terms of relative size to the planet it orbits) and they are very close together.

I'll try to find a link on some neat Pluto/Charion info for you guys.
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So why wasn't this very large perturbation noticed earlier? Seems like it would stick out like a sore thumb.
 
Actually Prester, I of of that opinion myself. The society that classifys them decided (for the sake of public opinion) to keep calling Pluto a planet, but they are starting to SERIOUSLY re-look at it as a KBO.

Still, for the sake of this thread, we'll call it a planet to show how hard it is to define moons, ;)
 
Sundog said:


So why wasn't this very large perturbation noticed earlier? Seems like it would stick out like a sore thumb.
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Until recently, even the best telescopes could only resolve the pair of them as a faint, starlike 'smudge' Even the photos that how show the pair are not all that distinct
 
I think <A href=http://www.math.nus.edu.sg/aslaksen/teaching/convex.shtml>this</A> is what Asimov was talking about.

--Terry
 
neutrino_cannon said:


No no, the barycenter between Terra and luna is firmly in terra. and yes, earth is a moon of the sun. My definition sucks.
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Coming back to the original question, does Terra revolve around the sun in an orbit fixed on its own centre of gravity, or fixed on the barycentre?

If it is the barycentre, then the Terran orbit wobbles, no?

( I need to know this information before I launch my attack on Terra. Would hate to miss and strike my friends on Luna)
 
It's only a Moon when BOTH cheeks are in contact with the automobile glass and flattened correctly. If there is only one cheek it the becomes a "Half Moon". I hope this helps
 
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