public void progress(int delta) {

    if (this.keyDown[Input.KEY_UP]) {
      this.centerY--;
    }
    if (this.keyDown[Input.KEY_DOWN]) {
      this.centerY++;
    }
    if (this.keyDown[Input.KEY_LEFT]) {
      this.centerX--;
    }
    if (this.keyDown[Input.KEY_RIGHT]) {
      this.centerX++;
    }
    if (this.keyDown[Input.KEY_RBRACKET]) {
      this.scale = this.scale - 8;
    }
    if (this.keyDown[Input.KEY_LBRACKET]) {
      this.scale = this.scale + 8;
    }
    if (this.keyDown[Input.KEY_E]) {
      float x = earth.getX();
      float y = earth.getY();
      this.centerX = -(x / this.scale);
      this.centerY = -(y / this.scale);
    }

    for (BaseUnit b : this.units) {
      b.update(delta);
    }
  }