Many explanations of time dilation use light sent from mirror to mirror on moving spaceships to explain that since the distance the light travels increases then time must increase in order to keep the speed of light the same . . . however, in order for the light to leave one spaceship and hit the other one, the light must have a horizontal velocity equal to the speed of the spaceship . . . if it has c as its speed toward the other ship and it has a horizontal velocity . . . then, in fact, its speed is greater than c . . which is not allowed. How does this work?