Elliptical orbits for planets

[mwforum@xxxxxxxxxxxxxxxxxx (Mike Sandy)]

Jeff wrote:
>Note towards always sets heading to an integer value, 
>which is a terribly rough approximation.  If you are 
>keeping a running total of deflections based on 
>values supplied by towards, very large errors 
>can accumulate.

In fact ,with the ellipse, I found this is not a problem.
The error occurs also with MW Pro so I've redefined
TOWARDS


to h.towards :t
 let[p pos
     p1 ask(list :t)(list "pos)
     x (first :p1) - first :p
     y (last :p1)- last :p]
 seth 90 - atan :x :y
end

to atan :x :y
 if and :x = 0 :y = 0[op 0]
 if :x = 0[if :y > 0[op 90]
           op 270]
 if :y = 0[if :x > 0[op 0]
           op 180]
 if :x > 0[op arctan :y / :x]
 op 180 + arctan :y/:x
end

Mike