void setup() {
  size(500, 500);
  rectMode(CENTER);
  smooth();
}

void draw() {
  float a = (frameCount/100.0)%TWO_PI;

  float x = 150*cos(a);
  float y = 75*sin(a);

  translate(width/2, height/2);

  background(200);
  rect(x, y, 50, 20);
}

thanks! worked nicely!