This program uses the trigonometric functions sine and cosine to draw a circle.
Why, you might ask, use trig, when there is a built-in Javascript function for drawing circles?
Well, it is because by drawing a circle point-by-point, be can change the radius while the circle
is being drawn!
From 11th grade math, you know the equation of a circle, in as a function of the radius and
angle is: x=r*cos(θ) y=r*sin(θ)