This animation is based on a section of the book, SPECIAL RELATIVITY ILLUSTRATED, by John de Pillis.

A perfect clock ticks consistently, that is, the time interval between any two ticks is the exactly the same for any two successive ticks. The clock starts ticking by a passing photon.

Using the fact that the speed of light (a photon) in a vacuum is c = 186,000 mi./sec., we calibrate the clock so as to exectly define a one-second time interval.