Light clock
Before we study the light clock, consider another kind of relativity. Imagine that someone is dribbling a basketball in the hold of a large cargo jet. The basketball player is moving in the same direction as the jet airplane. The other people in the plane see him moving a meter or two as he makes one dribble. Between the time the ball bounces the first time and the ball bounces the second time, about one second has passed. But when the first bounce occurred the basketball was over Gibraltar, and when the second bounce occurred the basketball was over the water nearer to Spain. So the basketball has moved 280 meters with respect to Earth.