Earth has a mini orbit around the moon?

[Seb]

I've recently realised that the earth must have its own mini orbit with the moon.

Since the moon seems to have orbit diameter of 770000km, and the moon is 0.012 the mass of the Earth, doesn’t this mean that Earth must also have an orbit diameter with the Moon of around 9240km

Is this already a known fact? Does Celestia allow for this?

Here's my estimation of our orbit in green:

[eburacum]

The Earth and the Moon do both orbit an invisible barycentre, or centre of mass, which is beneath the surface of the Earth;

the orbit of the Earth around this barycentre is about the size you state-

presumably Celestia takes this into account, but I don't know for sure.

[selden]

Yes, the VSOP87 theory used by Celestia does take into account the gravitational effects of the Earth and Moon on one another. All mutual gravitational effects are taken into account for all of the bodies which are defined with a CustomOrbit. Their positions are very accurately calculated for about +/- 2000 years or so from now

Note that you can't just specify a CustomOrbit in an SSC file and have it work. Each such declaration has to be accompanied by a corresponding routine within Celestia that actually performs the appropriate calculations.

[Seb]

Is Jupiter calculated like this? - that would be some amount of calculations to include all the moons. It sounds like Celestia uses a custom routine for every orbit in our galaxy. Can't a general function be created that calculated the positions correctly for all objects just by looking at the current objects and masses? - mm.. I guess this might need a bit of processing power. Well, maybe where a CustomOrbit is not used, there could be a default VSOP87 orbit calculations done.

In case I'm talking nonsense, I don’t really know what I'm talking about..!!

[Evil Dr Ganymede]

Keep in mind that Jupiter is about a million times more massive than all its moons combined, so the barycentre of the orbits is pretty much bang in the middle of the planet.

Earth and the Moon on the other hand, are much closer in mass.

[granthutchison]

Is Jupiter calculated like this? - that would be some amount of calculations to include all the moons. It sounds like Celestia uses a custom routine for every orbit in our galaxy.

There are CustomOrbits for the solar system planets and major moons. The point is that the calculations have been done once, already, off-line, and fitted to a mathematical expression that Celestia knows. Celestia therefore doesn't need to think about all the masses and where they are, but just needs to evaluate a single (big) equation for each planet at a given time.
Properly calculating the effects of gravity in real time would be prohibitively complex; doing a simple approximation would soon diverge so much from reality as to be useless. Hence CustomOrbits.

Grant

[Tetzauh]

Isaac Asimov wrote about this in his book "of time, space and other things" (a really cool book, if I may add). The discussion is too lengty to reproduce here, but in a nutshell with fairly simple mathematics Asimov finds out that there should be no moon beacause it is in a position where the sun should "win the gravitiational tug-of-war" with earth. Since the moon is there, the logical explanation is that Moon and earth must orbit around each other.

Even if the result is the same as stated in replies above, I truly reccomend you read this section of Asimov's book. It explains about this "tug of waer" in jupiter, mars, and venus among other interesting details.

[Joe (Oh)]

for the same reason that the barycentre is so close to the center of jupiter for its moons, is the same one that earth sized planets have that same close barycentre near the center of the sun? Are we able to detect such faint barycentres or is that why we are launching those 2 sattilites someone else mentioned?

[Michael Kilderry]

It is interesting to note that Pluto and it's moon Charon also do orbits around each other. But in this system the barycenter is actually located past Pluto's surface, about 1/4 the distance to Charon! This is because Charon is very massive to it's planet and also quite close.

Michael Kilderry

[selden]

Joe (Oh),

A barycenter isn't a physical object. It's not something you can literally see.
.
It's the "center of mass" of a collection of bodies.
It's exactly the same as the "center of gravity" of an irregularly shaped object, except that several separate objects are involved instead of just one.

The "center of mass" of any two bodies is somewhere
between them. When the two bodies have almost the same mass (weight), then the barycenter is almost exactly halfway between them. Usually, though, the bodies involved have quite different masses, so that the barycenter, the center of mass of the two of them, is somewhere inside one of the two. The barycenter of the Earth-Moon system, for example, is somewhere inside the Earth, on the line between the center of the Earth and the center of the Moon.

When you're watching a system of several different bodies from a distance, they seem to be orbiting around their mutual barycenter -- around their center of mass.

Does this clarify things at all?