View Full Version : What is a moon?

2003-Oct-11, 06:04 AM
What is a Moon? Definition Lags Behind Soaring Satellite Tally (http://www.space.com/scienceastronomy/moon_definition_040103.html)

We over a hundred moons in our solar system, the smallest ones are under a mile in diameter.

"More than 30 asteroids are known to have moons, too." We have moons orbiting asteroids too? Does that make these asteroids planets? We need a solid definition for the word 'moon'. Natural satellitie deosn't work any more, if all natural satellites were moons then every grain of dust in Saturn's ring are 'moons'. Which is ridiculas!

So I ask yo one seemingly simple question, what excactly is a 'moon'?

2003-Oct-11, 08:32 AM
The article seem to suggest that astronomers would not count small rocks as moons, even though they are satellites. So what about the rings of Saturn? Millions of satellites there... And what is a planet? Apparently we don't see the asteroids as "planets", and not the Kuiper objects either. Pluto might be a Kuiper object but it is considered a planet because they didn't know there would be a huge field of similar objects beyond Neptune's orbit. In my opinion, Pluto could very well be classified as a Kuiper object instead of a traditional planet.

And there are more problems when talking about planets and moons, for example what if the size and mass between a planet and its moon are so similar that the centre of gravity around which they orbit are outside and in between them both? Is it a binary planet? Is the smaller object the moon? Are they each other's moons?

I have a few ideas. I think that atoms and molecules can not be considered moons, and not small rocks either. I'm thinking one could draw the line at one meter, perhaps? I suspect that our need to classify these things has more to do about who we are and how we function, maybe we need to write down things, to sort them in neat columns... :) So any attempt at drawing a line has to do with us and what our preferences are etc. Because really, there is no clear line, it's not like we can say "this is a neutron, this is a proton". It doesn't seem to be an exact science, it seems to be more of a classification problem, what label to put on what objects. (Not that it's not important to do that, of course!)

Or, maybe we need a new way of going about this. What if we classified satellite as anything that completes at least one orbit around a planet. Then, the definition of a moon would be something like "a satellite larger than radius X but no larger than the size of planet Y". A planet, then, would be any object orbiting a star, but not larger enough to be a star itself. A Kuiper object would be "an icy small planet, smaller than radius X, beyond the distance Z from the sun", and an asteroid would be "a rockey small planet, smaller than radius X, pretty much anywhere in the solar system", etc.

2003-Oct-11, 11:39 AM
A celestial body acting under another celestial body's gravity.
Or something like that :)

2003-Oct-11, 03:34 PM
Ahh, moons. I always like talking about moons. :D
It has occurred to me, since we have many levels of knowledge amongst the members, some may not know which moon belongs to which planet.
So I decided to post a 'short list' of the most significant moons orbiting the planets with moons in decending order of size. I won't cover asteroids, somone else may do that if they like.

Moons of MARS only 2
Deimos (That's all)

Moons of JUPITER 61 total

Moons of SATURN 30 total

Moons of URANUS 24 (As of Sept 25th)

Moons of NEPTUNE 11 total

Moon of PLUTO only 1

Of these 32 listed;
2 are bigger then the planet Mercury and nearly as large as Mars,
4 are known to be bigger then our Moon, and one more may be bigger,
7 are bigger then the planet Pluto,
19 are bigger then Ceres, the biggest asteroid in the asteroid belt,
3 are thought to possibly have water,
1 is known to have an atmosphere, and 3 more may have an atmosphere,

2003-Oct-16, 09:54 AM
So what is a moon? What makes a moon a moon?

Does a moon need to be spheretical? Does it need to be a certain percentage in size of its mother planet?

Dan Luna
2003-Oct-16, 03:52 PM
I agree with Parker that these distinctions are mainly a matter of human convenience. For example, if we saw the Moon split into two bits, we would call them "the two bits of the Moon", but if it had split 4 billion years ago we would be saying "the two moons". Also, I think it was still called "the Moon" in "Space 1999" even after the nuclear fuel dump exploded and it went whizzing away so fast they got to a new solar system every week (although not too fast for them to land on the planets :lol: ).

2003-Oct-16, 04:18 PM
Originally posted by matthew@Oct 16 2003, 09:54 AM
So what is a moon? What makes a moon a moon?

Does a moon need to be spheretical? Does it need to be a certain percentage in size of its mother planet?
There are moons that aren't really spherical, for example Deimos seems like an asteroid, and Phobos too. There are probably other examples, mostly the small moons which are too small to be spherical.

2003-Oct-17, 09:05 AM
Isn't Charon, Pluto's moon, larger than Pluto?

2003-Oct-17, 09:50 AM
Pluto's diameter is 2274 km and Charon's is 1172 km, which still means that Charon is a large moon relatively to its planet. If the moon was the larger one, it would be the planet, or possibly they would be a binary planet system, depending on their relative sizes, I guess.

2003-Oct-17, 10:02 AM
Charon is so large in relation to Pluto that they are frequently considered a double planet system. Charon is also big enough to lock up Pluto's rotation, so only one side of Pluto ever faces Charon, the other side of Pluto won't see Charon.

2003-Oct-17, 10:04 AM
Ohhhhh ok! Thanks:)

2003-Oct-19, 12:55 AM
Glad to help. :)

2003-Oct-20, 02:45 AM
So what is a moon? What makes a moon a moon?
A moon is a natural orbiting celestrial body of a subtantually larger celestrial body which in turn orbits a star.

If one rock in space orbits one other larger rock in space, or even a couple larger rocks and all of coarse orbit the Sun, then you have a moon. :rolleyes:
It is possible for a moon to have a moon, but I know of no instances where that occurs. :blink:

Does a moon need to be spheretical? Does it need to be a certain percentage in size of its mother planet?

No, it can be any shape as long as it orbits the planet. :P

If the center of gravity is subtantually in favor of the larger object, then you have a planet-moon relationship. If the center of gravity is near the center of the two bodies, then you have a double planet or double planetoid relationship. :rolleyes:
It is possible for a moon or moons to orbit both planets, but I never heard of it.
Or if a moon or moons orbit only one of two double planets, then the moons are moons of only the planet it orbits. :ph34r:

Please don't ask about what if it first orbits one, then the other, because I don't know, and don't think that is possible. :unsure:

2003-Oct-23, 07:40 AM
Ok, a moon has to orbit a planet. What is a planet? (http://www.universetoday.com/forum/index.php?showtopic=1010)

But if a 'moon' can be any object that orbits a planet then Saturn has millions of moons. Anyone willing to count?

2003-Oct-23, 07:46 AM
For convinience I think we need to draw the line somewhere. Maybe have different classes of moons, such as "small moons", which would be from any detectable size up to small rocks, and then "middle sized moons" and then "large moons". For examplethe stuff in the rings around Saturn wouldn't be moons but rings, and there would have to be a definition of rings as well (which could be difficult, since they can be rings of rocks and small stones, or rings of dust and gas).

2003-Oct-23, 11:09 AM
Hi , first
I'm not sure if i i got the question u want the scientific definition for the moon?