Hello!
Your spirograph page http://wordsmith.org/anu/java/spirograph.html is very
interesting.

I am trying to understand "moving circle radius" and "moving circle
offset." I accidentally set the moving circle radius to a negative number
and got an interesting picture, but I don't know what a negative radius
represents. I am guessing that "offset" is a way to place the pen away from
the edge of the rolling circle, but I don't know what it is measuring.

Thank you,
Paul Boisen

cool! (how did i miss this! (thanks Paul!)

i don't know the math-- but i used to make spirograph designs (using a compass) when i was a kid.. and then make color wheels.
(spin them fast enough and the paper looks white)

i also had a spirograph toy a large toothed, circle, with smaller 'gears' that you placed a pen in, and ran round the inner diameter of the large circle..

i am going to have fun making designs (and maybe learning some of the math!) with this aplet.

I'm guessing the the sign of the radius for the moving circle indicates whether you put it inside or outside the fixed circle.
The moving circle offset, I think, refers to where the pen is placed relative to the moving circle's own center.

fun! I just wasted a lot of time there...