do you like the solar system

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I can never keep track of how many moons Earth has. It seems that even Cruithne is no longer regarded as a proper moon.

Well, the Earth only has one moon, the Moon. Cruithne is an asteroid and orbits the Sun. It's true that it has a rather peculiar orbit with almost the same period as the Earth; but to be a moon of the Earth, Cruithne should be orbiting the Earth and that's not the case, so it's not a moon.

I'm afraid you do not grasp the fact that an "orbit" is only a relative definition.

As soon as you have more than two bodies, every one of them, in a sense, orbits every other one.

Cruithne has a most peculiar orbit about the Earth, but it is still an orbit (rather drastically perturbed by the presence of that inconveniently large nearby object - sol - which itself orbits Uranus, and the centre of the Milky Way, and the centre of the local cluster, and so on).

As to whatever the current flavour of definition for a "moon" is, I guess something in that definition currently debars Cruithne from being considered as a moon of Earth. That's all. It's just a choice of body size and orbital components that happens to be picked on. Nothing inherent.

saturn. I like Gas Giants and the possible art; and flying through endless clouds..

Deaconblues, excellent point, life is really unfair.

Out of the available choices I chose Mars because it can be explored. Saturn is also cool, but Pluto has been rightfully demoted - with its abnormal rotation and out of whack orbit it always was a piss poor example of a planet. Pluto relates to planets like like the people of Maine speak english. I piss in Pluto's general direction. The Solar System as a whole however, I tend to like, I refuse to hold the rest of the Solar System responsible for the actions of Pluto.

Cruithne has NOT a most peculiar orbit about the Earth, but instead it has a most peculiar orbit about the orbit of the Earth. This means that Cruithne does NOT go around the Earth, but the Earth and Cruithne share an orbit around the Sun.

Cruithne is an asteroid with an elliptical orbit around the Sun. Cruithne and the Sun are gravitationally bound. The gravity of the Earth perturbs the orbit of Cruithne, but this does not make it a moon of the Earth. Earth's orbit is perturbed by the gravity of Jupiter, but this does not make the Earth a moon of Jupiter.

Here's is a simulation of Cruithne's orbit around the Sun:
http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=Cruithne;orb=1
The wikipedia article for Cruithne also has interesting information:
http://en.wikipedia.org/wiki/3753_Cruithne
If we look at the orbit of Cruithne from a from a reference frame corotating with the Earth, will see how the orbit has a horseshoe shape. Careful with that, in this frame the Earth stands still. It's very helpful to see the unusual orbit of Cruithne. This is explained at the site of Paul Wiegert:
http://www.astro.uwo.ca/~wiegert/3753/3753.html

All this makes Cruithne a very special and very interesting asteroid but still, not a moon. We can agree or disagree with this classification, but this is not gonna change it. Maybe one day the definition of "moon" will be changed to include bodies with orbits like Cruithne's.

I'm very interested in astronomy and I'm glad you are interested too.

Yes it does.
This is a meaningless sentence.
It does.
No. They are three bodies (perturbed by each other and everything else). Earth and Cruithne have approximately matched orbital periods. They vary. Other than that, there is no "sharing".
No. Replace that with "approximately elliptical".
As are all masses.
As the gravity of Cruithne perturbs the orbit of the Earth.
According to a definition which I have yet to see.
The current definition of "moon" would help.

Interesting. This simulation seems to show Cruithne with a much closer perigee to Earth than the Wikipedia article diagram.
Yes. This is the link I gave at the start. Unfortunately, their animations don't seem to make any sense. They also show far to close a perigee and show the "kidney bean" orbit as if it does not enclose Earth.
wording... not "corotating". That would be much messier... a reference frame spinning once a sidereal day.
Horseshoe? That tiny concavity? No. The "horseshoe" shape refers to the overall "filled in" pattern one gets from the 385 year precession of the basic "kidney bean" orbit.
Why should I be careful?

The movie "From the non-rotating to the rotating frame" shows clearly the kidney shaped orbit of Cruithne about the Earth, when a non-rotating coordinate system with Earth as the origin is employed, as is done to show the orbit of our Moon about Earth.
Definition? (Actually, the Wiegert article you linked refers to it as a companion, which sounds a good description.)
Who is agreeing/disagreeing? You seem to want to belabour the point that Cruithne happens not to currently fit some definition of "moon". That is what I said in my first post. As to what that definition is, I don't know.

Why? It would seem pretty pointless to do so. A moon as a very nearby, smaller mass, with a reasonably good approximation to elliptical arbit is a bit of a waffle, but is what we want to have the word to mean. When you get to Pluto and Charon, that's where "who is the moon of whom?" becomes awkward. They are more a "binary planetisimal", I guess.


How about this as a definition:

In a system of multiple bodies, in turn, take each possible pair of bodies and examine the orbit of the less massive body of that pair in a non-rotating coordinate system with origin at the more massive body. Whichever of these pairings yields an orbit that is most closely modelled by an ellipse, nominate the smaller mass as a satellite (moon) of larger mass, and replace that pair with their combined mass at their centre of gravity. Repeat until all bodies have been organised into the hierarchy.

For the solar system, the "moons" of the sun are the planets.

I don't think we have any moons with sub-moons, in the solar system. I guess there will be such things elsewhere. I don't know of any reason why such a thing should be inherently unstable.

There's no point in trying to contradict what I wrote in the last post because I wasn't expressing an opinion. I was describing what the astronomers have discovered so far about Cruithne. All my sentences are right and make perfect sense.

The status of Cruithne has been decided by the International Astronomical Union (through the Minor Planet Center). Cruithne is classified as an Aten asteroid and it received a minor planet number (#3753). When a new moon is discovered it does not receive a minor planet number.

In relation to the definition of moon. As you may know, there's no "official definition" but there is an informal one and that's what the astronomers use to decide if a newly discovered body should be classified as a moon or not. In simple terms, it says something like this (the wording my change because it's not official, as I said): "A moon (or natural satellite) is a celestial body with an elliptical orbit around a planet or a smaller body (like an asteroid)." As you say, when the two bodies have masses that are so similar that it is difficult to tell who orbits who, it can be called a double body.

In your definition, when you say "In a system of multiple bodies, in turn, take each possible pair of bodies..." it seems that we should take any pair of bodies and try to see which one is a moon of which one. Like, is the Earth a moon of Mars? or is Mars a moon of the Earth? Did I get this right? We can consider the entire universe a system of multiple bodies (gravity extends to infinity) so, should we take every pair of bodies in the universe and try to see which one is a moon of which one? This would make the definition amazingly (and unnecessarily) complicated.

I think we should try to find points where we can agree, instead of trying to convince each other that the other is wrong.
So you said:

Exactly. This is what I'm saying. It's just the commonly used definition of moon (I wrote it above). So if this is what you said in your first post, then there's something we can agree on.
You also said:

I assume you mean because of the perturbations in its orbit. So if we replace "elliptical" with "approximately elliptical", will you agree with that? If you agree with that, the argument is settled with this sentence:
"Cruithne is an asteroid with an approximately elliptical orbit around the Sun. Its orbit doesn't fit the current (informal) definition of "moon" commonly used by the astronomers."

I also think that the term "companion" (used by Wiegert) is a good one to describe such an special body as Cruithne.

Why isn't Earth an option?

Yes. I did say that one only would consider the less massive as a moon of the more massive body.
Yes.
(Although I'm unclear about the "infinity" - that doesn't really agree with current theory.)

No. My definition is extremely simple.

Computationally intensive, yes. Complicated, no.

There are simple, justifiable heuristics that simplify the task considerably. Conversely, all systems being unstable, eventually (when do we cross paths with Andromeda?), one can only use the definition to identify moons as of a particular epoch, with various concession to relativistic effects.

Your definition is interesting but it radically changes the concept of "moon". For example, under the current concept, nobody would say that Mars is a moon of the Earth.

(Oh, I think the collision with Andromeda is expected in 2 or 3 billion years. I'm looking forward to that! )

Ah. I see you have not understood my definition.

It produces EXACTLY the current allocation of moons to planets and planets to sol, when applied to that set of objects... i.e. those objects of a reasonable minimal mass. Why you would suggest "Mars is a moon of the Earth", I don't know.

With Pluto relegated to planetisimal, throw Pluto, Charon and the other Kuiper belt larger objects into the set, with "planetisimal" as a cutoff dependent on mass (or is the cutoff best done with eccentricity?), and it would get those classified "properly" too.

I think I made my point clear enough. Anyone who reads this argument should be able to see the difference between what you say and what I say:

You created a new definition for the term "moon" and then classified celestial bodies according to this definition. It makes no sense to try to convince you that a certain body is or is not a moon because I will use the usual commonly accepted definition and you will use yours.

There's not much left to say.

You really have trouble understanding this.

I stated that Cruithne was not a moon of the Earth, giving a link to the Wikipedia article about it. Read my post.

You then proceeded to try to argue with me. About what, I don't know.

You waffle, above, that you "will use the usual commonly accepted definition", but will give no indication what that is. You seem to be just saying that the magic "they" will say what's what. Is that "The Committee"? The president of the USA? X is a moon of Y because someone told you so?

My suggested definition can work perfectly well for other planetary systems. No committee has yet sat on making choices for such things, so far as I know.

It would appear, to me, that you have very little interest in astronomy, if you cannot be bothered to inform me of the "official" definition. I'm only interested in so far as I would like to know how close it comes to my suggested method of definition. However, my interest is mainly from the theoretical, mathematical side. The detail, specifics, rulings, etc for our planetary system... they are essentially boring.

No, lau. I didn't proceed to argue with you. I just replied to your first post to clear your doubts about how many moons the Earth has and to confirm that Cruithne is really not a moon. It was just a little clarification.

Then you replied saying that:

which is wrong. I corrected you and I tried to explain why this is wrong.

I gave you the definition (it's here). Moon or natural satellite is a very common term, its definition is no secret. You can find it anywhere. But I gave it to you anyway, so don't say I didn't. I'll rewrite it here:
"A moon (or natural satellite) is a celestial body with an orbit around a planet or a smaller body (like an asteroid)."
[EDIT: the word "elliptical" was superfluous. There's no need for it.]

If Cruithne had an orbit about the Earth (as you said), it would be considered a moon of the Earth (because of the definition).

I have no reason to believe that your definition wouldn't work. But I hope you realise that, as of today, is not widely used, and so it's of little use in arguments like this one.

My initial intention was just to explain something. There is nothing wrong with that.
I tried to stop the argument and reach an agreement with you. You ignored it.

Some of your sentences in your last post seem to be directed to hurt me or insult me. Please don't do that. I hope we will be able to finish this in a friendly manner.

Last edited by Thor on 13 Apr 2008, 3:08 pm, edited 1 time in total.

So do I. :3
*wishes Pluto was still a planet T)T*