Light clock
If a spaceship were flying on a straight line passing over the North Pole at a large fraction of the speed of light, and it had a similar clock, people watching its passage would see that the mirror on top of the pole had moved from directly over the emitting light, so the light would travel along the line marked h in the diagram, and then it would follow the other hypotenuse back down to the base of the pole -- which would by now have moved some distance since the spaceship is moving so fast. We can figure out how much time one tick would take according to the people on Earth. We know that the spaceship's pole has a length of a since it is the same kind of clock as people are using at the North Pole. We want to figure out t' , the time it takes to make one tick of the clock on the spaceship.