Re: Orbital simulator alogorithms

From: Jim McCauley (jematfriidotnet_at_frii.com)
Date: 09/24/04 

To: sci-space-tech@moderators.isc.org
Date: Fri, 24 Sep 2004 08:27:31 -0600

"Hop David" <hopspageHATESSPAaMmM@tabletoptelephone.com> wrote in message
news:41531C4A.5010801@tabletoptelephone.com...
> I'm trying to model orbits with Lunar perturbations in Microsoft Excel

This does not really answer the question, but I am reminded of an algorithm
for simple orbits developed by some middle-school students of mine about
twenty years ago. They had challenged themselves to create a program in the
Logo computer language to cause the graphical "turtle" to behave like a
satellite in orbit. Here is what they came up with:

  1. Remember three things:
     a. the direction you're pointed in now
     b. the "sideways speed" you're traveling at now
     c. the position you occupy now.
  2. Point toward the earth.
  3. Fall for a distance determined by the influence of the inverse
     square law at your current altitude.
  4. Turn in the direction you remembered from Step 1.
  5. Go forward the amount of the "sideways speed" from Step 1.
  6. Remember where you are right now.
  7. Jump back to your position at Step 1.
  8. Point toward the position you remembered at Step 6.
  9. Jump back to the position you remembered at Step 6.
 10. Go back to Step 1.

Essentially, it's vector addition carried out in a sequence of steps.

Provided that the values for planetary mass and satellite speed are chosen
with care, the small line segments produced by this algorithm approximate an
orbital ellipse quite nicely. The algorithm and the Logo program in which
it is realized are comprehensible by students as young as third grade.

Jim McCauley

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