I'm working on code to render the path of the sun and moon where they travel slowly at the top of the screen and move faster as they approach the horizon at 1/2 screen height.  What I've got is working, but, about every three iterations through the calculation of y for the position of x, it generates a NaN error. 

What is a NaN error?  What does the acronym NaN mean?  What effect do they have on memory use?  Is it necessary to trap them?

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;i++)
15.   {
16.     x = width - x-i;
17.     y = -((float)(height/2)/(width/2)) * sqrt((float)sq(width/2) - sq(x-((float)width/2)))+height/2;
18.     if (x <= 0) {x *= -1;}// y *= -1;}
19.     //if (y <= 100) {y *= -1;}
20.     ellipse(x,y,14,14);
21.     ellipse(x,height-y,14,14);
22.     println(str(y));
23.   }
24. }

Ultimately, Im going to synchronize the movement with a clock and sunrise and sun set times scraped from a weather info site.  If I am unable to figure out how to use javascript code for calculating reasonably precisely the moon and sun setting and rising times.  Apparently, the phase of the moon can be calculated (instead of scraped) so I figure the sun rise and setting times might be calculatable as well.  Whatever, I'm still interested in knowing more about NaN.