Time

Time can be measured by any phenomenon which is periodic and regular. The rotation of the earth, or the apparent rotation of the celestial sphere, was naturally used from the earliest days as a measure of time. [STANDARD OF TIME.] Whatever instrument may be used as a measure of time, some means must be adopted to control it. Thus, though the swing of a pendulum of a clock or the balance-wheel of a watch may be perfectly regular, we must compare the readings on the instrument with some standard, in order to avoid conflusion between different timekeepers. In modern times the positions of the sun and certain standard stars have been accurately determined for every day with regard to the meridian of Greenwich. A comparison of the reading of a watch or clock with an observation made on one of these fixed objects enables the error of the watch to be found and so corrected. Several observations made in this way will give the rate of gain or loss of the watch. Sundials have been used from very early days to determine the time, and water or sand clocks were the rough gauges in remote antiquity. The regulation of these primitive timekeepers can have been at best only an approximation, but in the time of Hipparchus the right ascensions of certain stars - one of which culminated every hour were used as references.

Nowadays elaborate systems of astronomical measurements have given a means of accurately obtaining mean solar time, and it is quite easy to correct the time as shown by the sun on a dial to this standard time by the application of the equation of time, the value of which is given for every day in the year in the Nautical Almanack. The time at different places on the earth s surface is naturally different, only those places on the same meridian being alike in this respect. Thus, when it is 12 o'clock at Greenwich, it is an hour later 15 degrees to the east and an hour earlier the same distance to the west, a difference of four minutes of of time corresponding to every degree in longitude. Birmingham time will therefore be earlier than Greenwich time by 7 minutes 24 seconds. On account of the rapidity with which different places are now put into communication with each other, both by means of railway and telegraph, the local time of a place has gradually fallen into disuse, and it is customary to use Greenwich time over the whole of Great Britain; this naturally greatly facilitates the construction of railway time-tables, and in many ways is extremely convenient. The legend "Synchronised hourly from Greenwich" now seen on clocks all over the conutry shows how important this convention is considered. Paris time is adopted all over France, while Holland and Belgium use Greenwich time, and Germany and Switzerland adopt "Central European" time, which is one hour earlier than Greenwich. It has been suggested that Greenwich time should be used all over the world, which would mean that people would have to get accustomed to the new time of day suggested to their minds by the mention of any hour; in New York, for instance, noon would not then be 12 o'clock, but would be 3 minutes 56 seconds past 7 P.M. This suggestion has not, however, been favourably received. The United States extend between longitude 65° W. and 125° W. approximately, thus covering about 60°; hence there will be a difference of four hours between the times in the eastern and western States. A conventional division of the States into four time-regions is therefore adopted. Between longitude 67-1/2° and 82-1/2° that time is accepted which corresponds to longitude 75°, i.e. 5 hours earlier than Greenwich - this is known as Eastern time; from 82-1/2° to 97-1/2° it is Central time - an hour earlier than Eastern; between 97-1/2° and 112-1/2° Mountain time prevails, and lastly between 112-1/2° and 127-1/2° they have Pacific time. It is thus 5 o'clock in the Eastern region when 2 o'clock is registered in the Pacific. On board ship it is customary to make the time correspond to the longitude at noon every day. In crossing from Liverpool to New York the boat will increase its longitude about ten degrees a day; so, when the clocks and watches show that 24 hours have elapsed since the previous noon, it yet wants 40 minutes to the true noon of that meridian in the ocean; when the true noon arrives, all the ship's watches and clocks are promptly put back 40 minutes. If this were not done, the time shown by the clock would bear no relation to the actual part of the day, and on arriving at New York an error of five hours would have accumulated. The ship's chronometer, however, goes steadily on, and the difference in time shown between it and the time obtained from corrected observations of the sun gives the longitude of the ship. The importance to navigation of an accurate chronometer is therefore enormously great. In travelling round the world with the sun the hands of a watch would have to be put back 24 hours in all by the time it got back to its starting-point; this is popularly expressed by saying that a day is lost in the journey. This leads to a curious confusion in the name of the day. Imagine that it is 12 o'clock on a Wednesday at Greenwich; at New York about 7 o'clock in the morning of the same day, while Pacific time gives it as only 4 A.M. Further on it would be, say, 1 A.M. of the same Wednesday, while in Japan it might be considered as 9 P.M. on Tuesday. But if we took countries to the east of Greenwich, we should find that when it is 12 o'clock on Wednesday there, St. Petetsburg records two hours later, on the meridian of Calcutta it is 6 o'clock on Wednesday evening, and in Japan later on the same day. According to this, it may be 9 o'clock in Japan on either Tuesday or Wednesday, while it is noon here. To overcome this inconvenience, the meridian of 180° is chosen as the critical point where the day suddenly changes its name. A ship, travelling westward, say long. 179° W., on a Tuesday, will, after crossing the meridian of 180°, suddenly assume it to be Wednesday, although only a few hours have elapsed. If it arrive there at midnight on Monday, Tuesday becomes avoided altogether. But a vessel travelling to the east across this line has one day twice over; thus, if it reaches the line at 11 P.M on Tuesday, the time is suddenly said to be 11 P.M. on Monday, and so Tuesday comes over again.