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When is a moon not a moon?

Nucular

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In a debate on another site, a claim has been made that
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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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?

I do like to smooth out incidental inaccuracies, cos it makes me feel big and clever.

By the way, I am aware that Asimov is a good SF author, but I am unaware whether he has any scientific credentials.

Anybody care to comment?
 
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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Doesn't the same hold true for every other planet/moon combo we know of?

Maybe the rule is that if the centre of gravity of the system is below the surface of the major body, then it's a planet and its moon, otherwise they're dual planets. That's the only thing I can think of that comes close to being non-arbitrary.

David
 
davidhorman said:


Doesn't the same hold true for every other planet/moon combo we know of?
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Yeah, you're right!
Maybe the rule is that if the centre of gravity of the system is below the surface of the major body, then it's a planet and its moon, otherwise they're dual planets. That's the only thing I can think of that comes close to being non-arbitrary.
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Sounds feasible. I was thinking it might come down to relative masses or something, but that would be extremely arbitrary.

So if they both seem to orbit each other - around a central point between them, instead of one of the bodies staying on a steady orbit of the sun? Is that what it means?
 
The moon is about eighty times less massive than the Earth. The centre of mass about which they rotate (the 'barycentre') is inside the Earth - about 1000 km below the Earth's surface, IIRC.
 
Ahh, how was it? According to some Clarke-Asimov agreement, Arthur C. Clarke was the World's best science-fiction writer and the World's second-best popular science writer, and Isaac Asimov was the World's best polular science writer the World's second-best science-fiction writer.

Since Asimov has now left the building, I suppose Clarke currently holds both titles.

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.

On moons or no moons: This is a very simple matter of definition. A matter of words. Classification of bodies as moons, coplanets, captured astroids or whatever can mainly be based on two things:

1) Size

2) Origin

The problem about #2 is that we do not now and may never get to know the origin of all bodies in the Solar system, so while more arbitrary, the size criteria is much simpler to handle. According to that, the Moon is a moon till such time as we choose to redefine it.

Hans

Edited to compensate for my non-deterministic keyboard which randomly leaves out or adds letters, or swops them, especially if I type fast, heheh.
 
As far as I can tell, browsing around on Google, there are a few people out there who consider Luna a "co-planet", but the majority seem to be calling it a moon.

And, at the lower end of the size spectrum...

What is a Moon? Definition Lags Behind Soaring Satellite Tally

In the old days of astronomy, before Galileo, there was just the Moon. Then scientists had to accept the clear and visible evidence of four objects orbiting Jupiter, satellites the master saw through a crude telescope in 1610.

Things have only grown more complex, especially of late.

With a spate of discoveries in the past three years, the number of known moons in the solar system has jumped to 118 as of this writing. That figure won't stand long. Astronomers expect the tally to double, at least, in the next few years. Most won't be surprised if it eventually triples. And that's not counting the small stuff -- boulders the size of football stadiums and countless smaller rocks that are surely trapped in orbit around the big outer planets.

Meanwhile, the complexity of moon types and behavior grows with the tally, and astronomers are struggling to sort out what it all means.

Several interviews with moon hunters and top theorists reveal that the definition of a moon is not clear and that there has been almost no discussion, professional or casual, about whether there should be any lower size limits set to separate real moons from miniature imposters. And nobody is in a rush to do anything about it... [more]
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As far as I can tell, Asimov was just a writer.
 
Thanks all - great responses, I knew I could rely on the collective wisdom of the JREF board! :)

For whomsoever may be interested, adding "asimov" to my google search terms turned up this - it doesn't add any new info, but it seems to be a very short review of where the reference probably came from.

Again, thanks!
 
so what would we call an object revolving around our moon? would it be a "moon" to the moon and the earth or just to the moon and something else to us?
 
HarryKeyogh
so what would we call an object revolving around our moon? would it be a "moon" to the moon and the earth or just to the moon and something else to us?
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Like this? I'd call it a LO (lunar orbiter)
<p align="center">
<applet codebase="http://www.mround.pwp.blueyonder.co.uk/programming/orbits/" code="orbits.class" width="512" height="512">
<PARAM name="pixelSize" value="1.7E6"> <PARAM name="fps" value="50">
<PARAM name="simSpeed" value="5e4">
<PARAM name="G" value="6.67E-11">

<PARAM name="image1" value="bigEarth.gif">
<PARAM name="r1" value="6.3781E6">
<PARAM name="m1" value="5.97E24">
<PARAM name="x1" value="0">
<PARAM name="y1" value="0">
<PARAM name="vx1" value="0">
<PARAM name="vy1" value="0">

<PARAM name="image2" value="moon.gif">
<PARAM name="r2" value="1.7374E6">
<PARAM name="m2" value="7.35E22">
<PARAM name="x2" value="-3.74E8">
<PARAM name="y2" value="0">
<PARAM name="vx2" value="0">
<PARAM name="vy2" value="1023">

<PARAM name="image3" value="small.gif">
<PARAM name="r3" value="1">
<PARAM name="m3" value="1">
<PARAM name="x3" value="-3.92E8">
<PARAM name="y3" value="0">
<PARAM name="vx3" value="0">
<PARAM name="vy3" value="1523">
</applet>
</p>
 
Of all of the known moons in the solar system, the only one I really have a hard time with in terms of classifaction is Charion (Pluto's moon). The Fact that Pluto itself is 'barely' a planet at best, and that Charion is nearly the same size as it (as well as VERY close) makes me wonder if they are either;

1. a dual planet

2. two asteriods that simply circle each other.
 
MRC_Hans said:

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.
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Well, let's give the good Doctor his due: he had a Ph.D. in biochemistry.
 
MRC_Hans said:

snip...

2) Origin

The problem about #2 is that we do not now and may never get to know the origin of all bodies in the Solar system, so while more arbitrary, the size criteria is much simpler to handle. According to that, the Moon is a moon till such time as we choose to redefine it.

...
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Btw, great article about this in this month's Natural History magazine.
 
That LO is a heck of a lot of fun too!

I have managed to stick about a dozen 'moons' in HUGELY elliptical orbits. It takes a couple a full FIVE MINUTES to come back formoff the screen, lol.

I have not been able to get a moon to orbit the big moon, but I have been able to put one in a steady 'circular' orbit around the Earth. Not a perfect circle, but darn close.
 
Nucular said:
What? he didn't even fly out to NYC to explain it to you! Call that an answer? :p
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Have you tried adding extra satellites, by clicking and dragging in 'space'? It's a great time waster.
 
I am sure that someone like 'Stimpson J. Cat' or 'The Bad Astronomer' could speak to this issue better than I, but I expect that there are a great many of technical terms that would be added and/or re-defined if the people who have a really good grasp on these terms were somehow allowed to re-write the dictionary.

'Planet' for example, originally meant 'wanderer' as in how the planets wandered about the pattern of stars as people watched them over the years. To ancient people in the northern hemisphere, planet probably seemed like a very good term since all of the stars swept about the North Star in a very regular fashion except for the few 'Planets' that zigged and zagged against the background of the stars and that varied in brightness and color.

However, we now know that that motion of the planets is not random at all, it can just appear that way when the only data one has about them is based on the tiny bit of light that we can see with our eyes.

By the same token, moon probably seemed a quite appropriate term centuries ago, but to define a moon as a smaller body orbiting a larger body is rather broad. Using this rationale, one could say that the Earth is a moon of the Sun. And that our solar system is a moon of our galaxy. While these statements may be in line with the definition, they convey a poor level of description.
 
Larspeart said:
That LO is a heck of a lot of fun too!

I have managed to stick about a dozen 'moons' in HUGELY elliptical orbits. It takes a couple a full FIVE MINUTES to come back formoff the screen, lol.

I have not been able to get a moon to orbit the big moon, but I have been able to put one in a steady 'circular' orbit around the Earth. Not a perfect circle, but darn close.
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Oh wow, it's interactive! I didn't realise! Well, there goes my afternoon...
 
Goshawk said:
As far as I can tell, browsing around on Google, there are a few people out there who consider Luna a "co-planet", but the majority seem to be calling it a moon.

And, at the lower end of the size spectrum...

What is a Moon? Definition Lags Behind Soaring Satellite Tally


As far as I can tell, Asimov was just a writer.
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Asimov was an honest-to-goodness scientist. He was professor of Biochemistry at Boston University until 1958, when he went into writing full time.
 
Well, here's something to consider. The word "moon" was invented to refer to that big white ball we see in the sky. When we discovered that it was really a gigantic chunk of rock orbiting our planet, and that other planets had big chunks of rock orbiting them too, we started calling them moons as well.

So the real question should not be "is our moon too big to qualify as a moon?", but instead "are the moons of those other planets too small to qualify as moons?"

Either way, our moon is a moon, by definition.


Dr. Stupid
 
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