From: jematfriidotnet@frii.com   
      
   "Hop David"  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   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)