An ideal superball clock features a ball bouncing back and forth between two walls in gravity-free space. The walls are a distance D apart, and the ball moves with constant speed vo, so the round-trip time is T = 2D/vo. An identical clock in a spaceship moves past us to the right at speed V. The walls are separated by a distance D in the ship frame, and the ball moves with speed to relative to the ship. From our point of view the ball moves diagonally, as shown below.

An ideal superball clock features a ball bouncing back and forth between two walls in gravity-free space. The walls are a distance D apart, and the ball moves with constant speed vo, so the round-trip time is T = 2D/vo. An identical clock in a spaceship moves past us to the right at speed V. The walls are separated by a distance D in the ship frame, and the ball moves with speed to relative to the ship. From our point of view the ball moves diagonally, as shown below.