I don't see 0, I see negative values, then NaN.
Make the values positive, and you will see the ellipses.

Ah, good catch! I used a square area, so it worked for me...

Instead of drawing circles in an elipse shape touching the centers of each edge of the window all around the window, they only draw for 1/2 the elipse shape and then there are NaN errors.  The negative is important because it has to start ("rise") on the right ("east") side of the window and travel towards the left side of the window where it "sets" ... its supposed to represent a sun or moon rising and setting ... 1/2 the window height is the horizon line so when it's on it's return after reaching the left side of the screen it's travelling back left to right under the imaginary horizon line.

1. float x;
2. float y;
3. void setup() {
4.   size(600,200);
5.   noLoop();
6.   //frameRate(1);
7.   x = width;
8.   y = height/2;
9. }
10. void draw(){
11.   ellipse(width/2,height/2,width,height);
12.   ellipse(x,y,10,10);
13. //
14.   for (int i = 0; i <= width*2;i++)
15.   {
16.     x = x-i;
17.     y = -((float)(height/2)/(width/2)) * sqrt((float)sq(width/2) - sq(x-((float)width/2)))+height/2;
18.     ellipse(x,y,14,14);
19.     println(str(y));
20.   }
21. }