Time.

Time. The earth's axial rotation is the phenomenon by which time is measured everywhere on the earth's surface (see DAY). Experiment and observation show that, if we assume the earth to rotate uniformly, there are many other phenomena which are as accurately isochronous in their periodicity. That is to say they pass again and again through all their phases in exactly the same interval of time as measured in terms of the earth's time of rotation. In the pendulum of a clock and the balance-wheel of a watch we have such isochronism very approximately realised (see HOROLOGY). A little consideration will convince us that the measurement of time is really a comparison of periodic sequences. We cannot conceive any other mode of marking off time intervals than by some kind of motion of a periodic character. Our practical unit of time is essentially terrestrial. We may, however, measure time in terms of a unit which could be as easily defined anywhere in known space as on the surface of our earth. The periodic time of a particular ray of light emitted by a universally distributed substance like hydrogen would be such a cosmic unit.

It has been long the custom among civilised races to divide the day into twenty-four hours, or rather into two sets of twelve hours, mean solar time (see DAY). Twelve o'clock noon corresponds to the meridian passage of the mean sun; and each day begins at twelve midnight. Now as we pass westwards the instant at which the sun crosses the meridian occurs later and later, as measured on a clock keeping, say, Greenwich time. That is, the noons at two places not situated on the same longitude line occur at different instants. Thus London and Dublin have different local times, the Dublin noon falling 25 min. 22 sec. later than the London (or Greenwich) noon. Similar differences exist for other parts of the United Kingdom, but within recent years, because of the development of rapid intercommunication by rail and steam, Greenwich time has gradually been adopted throughout Great Britain, and local times have practically ceased to exist. Similarly France has adopted Paris time as the civil time for the whole country.

Following the example of the United States, all countries are gradually adopting Standard Time. By this is meant time which differs from Greenwich mean time by whole hours. The globe is divided into zones of 15° or one hour breadth, the Greenwich meridian being in the centre of the zero zone. Thus Belgium and Holland keep Greenwich time; Denmark, Sweden, Switzerland, Austria, and Germany (Prussia temporarily excepted) keep the time of longitude 15° E.—i.e. one hour earlier than Greenwich. In North America again five zones are distinguished. Halifax falls within the time zone of 60° W. long. or four hours later than Greenwich; Montreal and New York fall within the zone of 75° or five hours west of Greenwich; and so on across the entire continent. New York, Chicago, Denver, and San Francisco may be mentioned as places whose times change by one hour as we pass from one to the next succeeding. The corresponding times are distinguished as Eastern ($67\backslash frac\{1\}\{2\}$—$82\backslash frac\{1\}\{2\}$°), Central ($82\backslash frac\{1\}\{2\}$—$97\backslash frac\{1\}\{2\}$°), Mountain ($97\backslash frac\{1\}\{2\}$—$112\backslash frac\{1\}\{2\}$°), and Pacific ($112\backslash frac\{1\}\{2\}$—$127\backslash frac\{1\}\{2\}$°) times. Standard time in Japan is nine hours earlier than Greenwich time.

In the western parts of Canada the twenty-four-hour system has been adopted, under which four P.M. becomes sixteen o'clock, and so on. Steps are being taken to introduce it generally in India, Belgium, and the United States. It is of special convenience in the construction of railway time-tables; and it has the further claim that it has long been used by the Italians and by astronomers. The system of universal time, in which all places would keep Greenwich time, has a growing number of advocates. The adoption of such a system would be a little awkward at first to all who have got to think of twelve o'clock as being necessarily either noon or midnight; but a little experience would correct this.

Time is earlier or later than Greenwich according as the locality is east or west of Greenwich. Thus places lying close together but on different sides of the longitude line of $180^\backslash circ$ differ nominally by a whole day in time. Sunday at one place occurs simultaneously with Monday at the other. A vessel sailing eastwards across the Pacific has two consecutive days of the same name as it crosses the critical meridian, or in other words has one day of nearly 48 hours' length. On the other hand, a vessel sailing westwards in the same longitude drops a day. Until a few years ago the Philippine Islanders held their Sunday on the day which was Monday to the inhabitants of Borneo. This arose from the historic fact that the Philippines were discovered by the Spanish voyagers coming from the east round Cape Horn; whereas Borneo was discovered by the Portuguese coming from the west. The matter is put most simply thus: To the eastward-bound traveller sunrises come at shorter intervals than to the stationary observer, while to the westward-bound traveller they come at longer intervals. Thus after one complete circuit the former traveller has experienced one sunrise more, and the latter one sunrise less, than has the stationary observer in the same lapse of time.

For sidereal time, see DAY; see also the articles on Astronomy, Calendar, Chronology, Clepsydra, Dial, Hour, Hour-glass, Latitude and Longitude, Month, Period, Seasons, Year. For the philosophical theories of time, see PSYCHOLOGY, Vol. VIII. p. 475. When it is twelve o'clock noon, Greenwich mean time, the hour (local time) at various important places on the globe is as follows: