float x,y,cx,cy,radius,angle;

void setup() {
  size(300,300);
  cx=width/2;
  cy=height/2;
  radius=width/2;
  angle=0;
  
  framerate(20);
  rectMode(CENTER_DIAMETER);
}

void loop() {
  background(200);
  x=cx+cos(radians(angle))*radius;
  y=cy+sin(radians(angle))*radius;
  angle+=2; // == angle=angle+5;
  
  rect(x,y, 10,10);
}