eyeofmidas.com/htdocs/files/orbitdemo.pde

int x;
int y;
int r = 200;

void setup()
{
  size(640, 480);
  background(0,0,0);
  x = 0;
  y = 0;
}

void draw()
{

  float t = millis() / 1000.0f;
  x = (int) (350 + (r * cos(t) * 1.3));
  y = (int) (240 + r * sin(t));
  
  background(0,0,0);
  
  //orbit
  stroke(0, 0, 255, 128);
  fill(0,0,0,0);
  ellipse(240, 1240, 2000, 2000);
  
  //planet
  stroke(0,255,0, 255);
  fill(0,255,0, 0);
  ellipse(250, 240, 150, 150);
  stroke(255,0,0, 255);
  fill(255,0,0,255);
  rect(250 - 5, 240 - 4, 10, 8);
    
  //orbit
  stroke(0, 0, 255, 128);
  fill(0,0,0,0);
  ellipse(350, 240, 520, 400);
  
  //moon
  stroke(128,128,128,255);
  fill(128,128,128,0);
  ellipse(x, y, 48, 48);
  stroke(255,255,0,255);
  fill(255,255,0,255);
  rect(x - 5, y - 4, 10, 8);
  

}