The Maxwell Theory predicted that the speed of light (in vacuum) is always, under all conditions, the same, 186,000 miles per second, fast enough to get to the moon in just over a second.
Because it is always the same, this speed can be used as a measure of time.
A device that uses the speed of light to measure time is called a light pulse clock.
When one imagines using such a clock (one doesn't have to actually build one), it turns out that movement of the measuring instrument relative to the clock affects how much time elapses. This is not the sort of thing that it is easy to accept, but it is so. It is said that moving clocks run slow, but this isn't really the case.
What really happens is that if you are holding a clock, then an interval of time that you measure will be longer if it is measured by someone who is moving with respect to you.
In our book, we tell the story of Timus Ybatu, a mile runner, who carries his own watch because he doesn't trust the judges' clocks. Because of the relativistic effect, Timus' watch gives him a shorter time than the judges do (by a preposterously small amount -- but it's a good story, and tells you a lot about relativistic time).
Onward to Chap 8: Length
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