, present somewhere, at rest relatively to
. But we cannot fix the time, by means of this clock, of an event whose distance from the clock is not negligible; for there are no "instantaneous signals" that we can use in order to compare the time of the event with that of the clock. In order to complete the definition of time we may employ the principle of the constancy of the velocity of light in a vacuum. Let us suppose that we place similar clocks at points of the system
, at rest relatively to it, and regulated according to the following scheme. A ray of light is sent out from one of the clocks,
, at the instant when it indicates the time
, and travels through a vacuum a distance