Seeker s;
void setup() {
  size(250, 250);
  s = new Seeker();
 mouseX = width/2;
 mouseY = height/2;
}
void draw() {
  background(200);
  s.move(mouseX, mouseY);
  s.draw();
}

class Seeker {
  float x, y, angle, sz;
  Seeker() {
    x = width / 2;
    y = height / 2;
    sz = width / 20;
  }

  float TURNSPEED = .1;
  float SPEED = 1;

  void move(float tx, float ty) {
    float dx = (tx -x);
    float dy = (ty - y);
    float wantangle = atan2(dy, dx);
    float anglediff = (angle - wantangle);
    //normalize things so the difference is between -PI and PI...
    if (anglediff > PI) {
      anglediff -= TWO_PI;
    }
    if (anglediff < -PI) {
      anglediff += TWO_PI;
    }
    if (anglediff > TURNSPEED) {
      angle -= TURNSPEED;
    } 
    if (anglediff < -TURNSPEED) {
      angle += TURNSPEED;
    }
    x += cos(angle) * SPEED; 
    y += sin(angle) * SPEED;
  }

  void draw() {
    ellipse(x, y, sz, sz);
    line(x, y, x + sz*cos(angle), y + sz * sin(angle));
  }
}