What if Earth had another moon.

 

What would be the upper limit of it's size, that it's orbit yet would be stable. In which distance another big moon best would maintain it's orbit.

 

Another interesting question: let's forget another moon, but let's move our present moon nearer the Earth. How near it would come that it's orbit would be stable and it would not yet disturb Earth's revolving around the Sun. If the size of our Moon would be smaller, it's minimum distance could of course be smaller.

 

How near would for example 100 km diameter Moon revolve Earth.

It's also interesting to think what kind of scientifical or another meaning this small near moon would have to our society.

The upper limit of its size is a three body problem. Bugger.

 

I don't see why the orbit would become unstable closer to the sun; even with a constant force, you'll still get SHM, so why not orbits?

the earth does have two moons though, not including all the junk that we keep leaving up there.

Called Cruithne:

 

http://www.astro.queensu.ca/~wiegert/3753/faq.html

thanks dude.

Is this for real?

 

I mean... it's not really concidered to be another "moon" is it?

 

is it even big anough to fit the term?

 

btw, I wonder why no one's teaching that in school. Sounds important enough to me.. hah..

 

 

~moo

It's really not large enough to be classified as a moon, at least in the not-being-troll sense.

It says in the article that it does not have an orbit like a moon.

That's right it doesn't; it has a sort of horse-shoe-shaped, loopy orbit. It's a captured asteroid really, but many fondly regard it as a moon since it's more or less permanently trapped now.

Not sure you're addressing the point. Said it shares an orbit around the sun, not the Earth.

Fellow said in post # :

Another interesting question: let's forget another moon, but let's move our present moon nearer the Earth. How near it would come that it's orbit would be stable and it would not yet disturb Earth's revolving around the Sun. If the size of our Moon would be smaller, it's minimum distance could of course be smaller.

 

Can't you just use Newton for this? (to a good approximation)

 

Just work out at what point the moons gravity is larger than the suns gravity and you have the point of perturbation.

 

I know it's rough, but its a start and it doesn't have you dealing with the three body problem

MishMish said in post # :

Not sure you're addressing the point. Said it shares an orbit around the sun, not the Earth.

Who are you talking to? Which point? Who said what shares an orbit?

Sayonara, are you determined to get me to spruce up my grammar despite myself? Or just pointing out was being a bit of a nuisance

 

Was speaking to you, referring to the point made by Cap'n Refsmmat. The article said Cruithene shares orbit with the earth. "[Q] Does asteroid 3753 Cruithne orbit the Earth like a moon or satellite?

[A:] No. The asteroid's behaviour is more complicated than that: it doesn't orbit the Earth, but rather it shares the Earth's orbit. The relationship of a moon to its planet is called a two-body because there are only two important players (ie. the moon and the planet). However, in the case of Cruithne, the Earth and the asteroid both share the same orbit about the Sun, but are choreographed in such a away as to remain stable and avoid colliding with each other. This is called a three-body relationship as there are three main players: the Earth, the asteroid and the Sun."

 

My understanding was that, quite possibly simplistic, was that the shape of the orbit is not the determining factor in classigfying some body as a moon or planet but what it orbits. You answered by referring to the shape of the orbit instead of what it orbits, which I didn't think addressed the question of what it should be classified as

It wasn't really your grammar, just the fact that half the sentence was missing

 

He said "the article says it doesn't have an orbit like a moon". I agreed. There's little point in me duplicating the answers here for him if I have already posted a link to an article that contains them, and he has just posted evidence that he read it.

 

My comment on the shape of the orbit is merely a point of interest, and not an explanation of anything.

Okay, though was not clear to me that's what you were doing

Sayonara³ said in post # :

That's right it doesn't; it has a sort of horse-shoe-shaped, loopy orbit. It's a captured asteroid really, but many fondly regard it as a moon since it's more or less permanently trapped now.

but it's trapped by the sun, not earth. it's in symbiosis with Earth, but it's orbiting the sun, making it an asteroid, not a moon.

 

 

Why symbiosis? It isn't helping or hurting any!

iglak said in post # :

but it's trapped by the sun, not earth. it's in symbiosis with Earth, but it's orbiting the sun, making it an asteroid, not a moon.

Last time I checked the moon was orbiting the sun too.

 

Regardless, the insertion of the term "fondly" in there suggests the label is less than accurate.

But the moon and Cruithne are different. Obviously the moons is mostly effected by the Earth's gravity, whereas Cruithne is mostly effected by the Sun's gravity.

Even if you were right, which you aren't, my response would be "so?", because you are asking me to justify something I did not state as being fact. You seem to be under the impression that I called Cruithne a moon.

Well educate me on how I wasn't right then.

 

 

My take on this is if the Moon felt the Suns gravity more than the Earths then it would soon be ripped away (This is the upper Roche limit if I'm correct)

 

So since it's in a stable orbit around the Earth, then it feels the Earths gravity the most.

 

 

Cruithne on the other hand isn't in a stable orbit around the Earth. The Sun has the greater gravitational influence on it.

 

 

I was merely pointing out these differences.

Or, to make this even more interesting, lets put it this way: we've got the moon orbiting the earth, right? Well what if the earth obited the moon? Wud it make a difference?

<p>

But back to the original question, its an interesting concept and some things might be affected by it. Only problem is, the <i>gravatational force</i> of the Earth itself might not be able to handle the weight of both moons, unless the <b>totalled mass of the two moons equalled the same as the actual moon does</b>; i.e. Each moon was half the mass of the actual moon... does that make sense?

OrderInChaos said in post # :

So since it's in a stable orbit around the Earth, then it feels the Earths gravity the most.

Cruithne on the other hand isn't in a stable orbit around the Earth. The Sun has the greater gravitational influence on it.

Cruithne is in a stable orbit. Maybe not what you'd call regular, but it is in an equilibrium of some sort.

 

If the sun had more of a relative influence on Cruithne than it did on the moon (accounting for the difference in mass, obviously), and the Earth less so, don't you think Cruithne would have been torn away by now? Or, more likely, never reached a 3-body orbital state in the first place?