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

**GIVEN:**

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.