Celestia Forums

t00fri wrote:

BobHegwood wrote:Good Doctor,

Can the Brain-Dead ask WHY the Moon is different? What makes it
so different from any other orbiting body in the Solar System?

Just curious...

Thanks, Bob

Bob,

first you should recall what VSOP87 qualitatively does. It is some kind of "perturbation expansion", as we say, which successively takes into account the cumulative effects of all sources of a gravitational field (other planets, moons,...) around a given body in form of hundreds of "perturbative" terms. [It's precisely the analog of the so-called Hartree-Fock method in atomic physics.]. These correction terms due to the other bodies are relative to elliptical Keplerian orbits which are assumed as a starting approximation.

The specific problem with the Moon is quite intuitive and more general than VSOP87, actually:

The Moon moves noticeably differently from a simple Keplerian ellipse, because of the competing gravitation of the Earth and the Sun.. So many more correction terms are actually needed. That and many other more technical issues form the basis for why the Moon orbits are difficult to calculate at high accuracy. You will perhaps also recall that the gravitational force is a long-range force (decreasing relatively slowly with distance) and its strength is proportional to the masses of the bodies under consideration. So the Sun is still very competitive at the location of the Moon. And so is the Earth, of course.

Bye Fridger

I though, so all we need to do is solve the unsolved n-body problem in physics, and we should be able to get rid of all the custom orbits right? Sounds simple enough.
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Sorry could not resist. Embarassed