Here I have a cheep simulation of the moon orbiting the earth by graphing the location of the moon using the equation of an ellipse. My problem is that toward the edges the moon "speeds up" and then slows down.

How do I make it so that an image moves in a circular path smoothly?



/**Ellipse
*
*Created By: David A. Perez
*
*Notes:
*h = x of center point
*k = y of center point
*a = x +- a
*b = y +- b
*
*blank.x = h - a;
*/

PImage sun;
PImage moon;
PImage earth;
orbit a;

void setup(){
   sun = loadImage("sun.jpg");
   moon = loadImage("moon.gif");
   earth = loadImage("earthWest.gif");
   
   size(700, 600);
   frameRate(100);
   noStroke();
 
   a = new orbit();
      a.a = 200;
      a.b = 75;
      a.h = 500;
      a.k = 400;
      a.x = 500 - 100;
   }
   
void draw(){
   background(sun);
   a.orbit();  
}

class orbit{
   float a, b, h, k, x;
   int direction;
   
   void orbit(){
      fill(255,0,0);
     
      if(x == h-a){direction = 1;}
      else if(x == h+a){direction = 0;}
     
      if(direction == 0){float y = k - sqrt(sq(b) - sq(b)*sq(x - h)/sq(a));
                              image(moon, x, y, 50, 50);
                              x--;}
      image(earth, 500-100, 400-125, 250, 250);
      if(direction == 1){float y = k + sqrt(sq(b) - sq(b)*sq(x - h)/sq(a));
                         image(moon, x, y, 50, 50);
                         x++;}
   }
}