Robert I. Eachus dice...
> Bj�rn Persson wrote:
> 
> > I didn't mean to say that the Sun doesn't affect the Moon. "If what J-P 
> > wrote were true, the Moon would not orbit both the Sun and the Earth, as 
> > it does, but only orbit the solar system's center of gravity (if we 
> > ignore the rest of the universe)." Is that better?
> > 
> > You can't choose your words too carefully when there are Ada language 
> > lawyers in the audience. :-)
> 
> I think you missed my point.  Earth's moon is unique in the solar 
> system.  It is the only moon whose path around the Sun doesn't cross 
> itself.  In fact, the effect of the Earth's gravity is so slight that if 
> you are calculating the Moon's orbit, it is better to compute it the 
> same way as other planets.  (Treat it as orbiting the sun, with 
> perturbations by the gravity from other planets.  For all other moons, 
> you are better off calculating them as orbiting some planet, with the 
> Sun's gravitational effect as a perturbation.)

I had never stopped to think about it, but if I understand you 
correctly, that means that the Moon's orbit has a zig-zag path. Nice :)

I'll look further into your Runge-Kutta suggestion, thanks.