// Learning Processing
  // Daniel Shiffman
  // http://www.learningprocessing.com
  
  // Example 14-18: Object-oriented solar system
  
  class Planet {
    // Each planet object keeps track of its own angle of rotation.
    float theta;      // Rotation around sun
    float diameter;   // Size of planet
    float distance;   // Distance from sun
    float orbitspeed; // Orbit speed
    
    Planet(float distance_, float diameter_) {
      distance = distance_;
      diameter = diameter_;
      theta = 0;
      orbitspeed = random(0.01,0.03);
    }
    
    void update() {
      // Increment the angle to rotate
      theta += orbitspeed;
    }
    
    void display() {
      // Before rotation and translation, the state of the matrix is saved with pushMatrix().
      pushMatrix(); 
      // Rotate orbit
      rotate(theta); 
      // translate out distance
      translate(distance,0); 
      stroke(0);
      fill(175);
      ellipse(0,0,diameter,diameter);
      // Once the planet is drawn, the matrix is restored with popMatrix() so that the next planet is not affected.
      popMatrix(); 
    }
  }