float cx, cy, x, y;
float R = 0.0, dR = 0.2, theta = 0.0, dTheta = 0.5;

void setup() {
  size(500, 500);
  background(255);
  cx= width/2; 
  cy = height/2;
}

void draw() {
  float x = cx + R*cos(theta);
  float y = cy + R*sin(theta); 
  strokeWeight(5);
  point(x, y);
  theta+=dTheta;
  R += dR;
}