time

TIME, STANDARD. Local time is determined by the relation of the meridian of a place to the sun. Noon at any place is defined as the moment when either the true or mean sun passes the meridian of that place, according as apparent or mean time is used. Practically, the use of mean time is now universal, so that we may regard the mean sun as that by which noon is determined. As the earth revolves, all its meridians are brought under the sun in succession or, relative to the earth, noon continually travels around the earth, making the circuit in twenty-four hours. It follows that noon, and therefore any other hour of the day, is later by four minutes for every degree of longitude towards the west, so that a watch carried east or west will be found to deviate from local time by an amount proportional to the change of longitude. Before the time of railways this deviation was not productive of inconvenience. But when railway travelling became common, train schedules had to be more exact than those of a mail coach, and the traveller was rapidly carried to places where the local time continually deviated from that shown by his watch. The use of such time thus had to be modified in places where intercommunication with others of a different longitude was frequent. Thus arose a practice on the part of railways of using the time of some central or important city on its line for all places not too distant, which time would naturally be adopted by the inhabitants of the region through which it passed. For a similar reason, in countries which did not extend through a large fraction of an hour of longitude, it was natural to use the time of the capital throughout all or a large part of its extent. Thus Greenwich time has long been in use throughout England, and all the railways of France are run by Paris time. But inconvenience was still unavoidable in passing from one country to another, or in travelling through long stretches in the same country. The inconvenience was especially felt in the United States, where every railway, and even every long stretch of several great railways, had its own time system. Thus it happened not infrequently that in a single station clocks would be found set to the time of three different meridians, one for the road toward the east, another for the road toward the west, and a third for the meridian of the place, or local use.