In one of the public gardens of Paris a little cannon is set up with a burning-glass attached to it in such a manner that the sun itself fires the cannon as it reaches the meridian. This, of course, is the time of Paris noon—apparent noon—but it would be exceedingly imprudent of any traveller through Paris who wished, say, to catch the one o'clock express, to set his watch by the gun. For if it happened to be in February, he would find when he reached the railway station that the station clock was faster than the sun by nearly a full quarter of an hour, and that his train had gone; whilst towards the end of October or the beginning of November, he would find himself as much too soon.
Until machines for accurately measuring time were invented, apparent time—time, that is to say, given by the sun itself, as by a sun-dial—was the only time about which men knew or cared. But when reasonably good clocks and watches were made, it was very soon seen that at different times in the year there was a marked difference between sun-dial time and that shown by the clock, the reason being simply that the apparent rate of motion of the sun across the sky was not always quite the same, whilst the movement of the clock was, of course, as regular as it could be made.
This difference between time as shown by the actual sun and by a perfect clock is known as the 'equation of time.' It is least about April 15, June 15, August 31, and December 25. It is greatest, the sun being after the clock, about February 11; and the sun being before the clock, about November 2. Flamsteed, before he became Astronomer Royal, investigated the question, and so clearly demonstrated the existence, cause, and amount of the equation of time as entirely to put an end to controversy on the subject.
We had thus, early in the century, the two kinds of time in common use, apparent time and mean time, or clock time. But as the sun can only be on one particular meridian at any given instant, the time as shown by the clocks in one particular town will differ from that of another town several miles to the east or west of it. It is thus noon at Moscow 1 hr. 36 min. before it is noon at Berlin, and noon at Berlin 54 min. before it is noon in London.
This was all well enough known, but occasioned no inconvenience until the introduction of railway travelling; then a curious difficulty arose. Suppose an express train was running at the rate of sixty miles an hour from London to Bristol. The guard of the train sets his watch to London time before he leaves Paddington, but if the various towns through which the train passes, Reading, Swindon, etc., each keep their own local time, he will find his watch apparently fast at each place he reaches; but on his return journey, if he sets to Bristol time before starting, he will in a similar way find it apparently slow by the Swindon, Reading, and Paddington clocks as he reaches them in succession.
It became at once necessary to settle upon one uniform system of time for use in the railway guides. Apart from this, a passenger taking train, say, at Swindon, might have been very troubled to know whether the advertised time of his train was that of Exeter, the place whence it started, or Swindon, the station where he was getting in, or London, its destination. 'Railway time,' therefore, was very early fixed for the whole of Great Britain to be the same as London time, which is, of course, time as determined at Greenwich Observatory. At first it was the custom to keep at the various stations two clocks, one showing local time, the other 'railway,' or Greenwich time, or else the clocks would be provided with a double minute hand, one branch of which pointed to the time of the place, the other to the time of Greenwich.
It was soon found, however, that there was no sufficient reason for keeping up local time. Even in the extreme West of England the difference between the two only amounted to twenty-three minutes, and it was found that no practical inconvenience resulted from saying that the sun rose at twenty-three minutes past six on March 22, rather than at six o'clock. The hours of work and business were practically put twenty-three minutes earlier in the day, a change of which very few people took any notice.
Other countries besides England felt the same difficulty, and solved it in the same way, each country as a rule taking as its standard time the time of its own chief city.
There were two countries for which this expedient was not sufficient—the United States and Canada. The question was of no importance until the iron road had linked the Atlantic to the Pacific in both countries. Then it became pressing. No fewer than seventy different standards prevailed in the United States only some twenty years ago. The case was a very different one here from that of England, where east and west differed in local time by only a little over twenty minutes. In North America, in the extreme case, the difference amounted to four hours, and it seemed asking too much of men to call eight o'clock in their morning, or it might be four o'clock in their afternoon, their noonday.
The device was therefore adopted of keeping the minutes and seconds the same for all places right across the continent, but of changing the hour at every 15° of longitude. The question then arose what longitude should be adopted as the standard. The Americans might very naturally have taken their standard time from their great national observatory at Washington, or from that of their chief city, New York, or of their principal central city, Chicago. But, guided partly no doubt by a desire to have their standard times correspond directly to the longitudes of their maps, and partly from a desire to fall in, if possible, with some universal time scheme, if such could be brought forward, they fixed upon the meridian of Greenwich as their ultimate reference line, and defined their various hour standards as being exactly so many hours slow of Greenwich mean time.