In article <404178f6$1@news.povray.org>,
 "Greg M. Johnson" <gregj;-)565### [at] aolcom> wrote:

> Take a "sun".
> Take any mass and throw it in any direction not exactly at its center at any
> velocity.
> 
> If the mass doesn't sink into the sun,  will it:
> 
> i)  undertake an "orbit"  "right there" in the sense that it will undertake
> an elliptical orbit which intersects that point of release,

In a 2-mass system, just the sun and the orbiting mass, it will either 
do exactly that or escape the system entirely.


> OR
> ii) will a particle "destined" for a stable orbit sometimes "drift" out
> towards the radius of its stable orbit.

2 body systems are stable. Maybe you're mistakenly thinking of 
elliptical orbits as unstable?


> If I had  been much, much more patient, might some of those orbits which
> looked like they were decaying be just wandering off to something "close
> enough" to their own stable orbit?

If a 2-body system drifts significantly, it is your simulation which is 
unstable. In the real world, none of the planets in our solar system 
follow perfect elliptical orbits, and they do drift around and transfer 
energy to each other, but the level of instability is too small to 
matter. When the sun goes into its red giant stage and swallows up the 
inner planets, they will still be in very similar orbits to what they 
have today.

The p2 = p1 + v1*dt + 1/2*a1*(dt^2) equation will give a more accurate 
simulation than p2 = p1 + v1*dt, but for my simulation of the inner 5 
planets, I found it insufficient. The two-step method I described was 
sufficient, the simulation was stable for at least several decades 
without trying very hard.

-- 
Christopher James Huff <cja### [at] earthlinknet>
http://home.earthlink.net/~cjameshuff/
POV-Ray TAG: <chr### [at] tagpovrayorg>
http://tag.povray.org/

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