"Martin Lewicki" wrote in message
...
Hi

I would like to draw a realistic representation of an elliptical orbit with
a simple computer drawing program. Given the semi-major axis and
eccentricity, is there a formula that converts these into the lengths of
the major and minor axes so that I can enter these values into the the
"circle" function to make the ellipse?

If a = semimajor axis
b = semiminor axis
c = half the distance between foci
e = eccentricity

then

a^2 = b^2 + c^2 (True for any conic section orbit)
and e = c/a (True for any conic section orbit but the
parabola, where c -- infinite)

so that

b^2 = a^2 - (e*a)^2

b = a*sqrt(1 - e^2)