See also:time is determined by the relation of the meridian of a place to the
See also:sun .
See also: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
See also: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
See also:hours . It follows that noon, and therefore any other
See also:hour of the
See also:day, is later by four minutes for every degree of longitude towards the west, so that a
See also:watch carried east or west will be found to deviate from local time by an amount proportional to the
See also:change of longitude . Before the time of
See also:railways this deviation was not productive of inconvenience . But when railway travelling became
See also:train schedules had to be more exact than those of a
See also:mail eoach, 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
See also:part of railways of using the time of some central or important city on its
See also:line for all places not too distant, which time would naturally be adopted by the inhabitants of the region through which itpassed . 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
See also:Greenwich time has long been in use throughout England, and all the railways of France are run by
See also:Paris time .
But inconvenience was still unavoidable in passing from one
See also:country to another, or in travelling through long stretches in the same country . The inconvenience was especially
See also:felt in the
See also:United States, where every railway, and even every long stretch of several
See also:great railways, had its own time
See also: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 . A
See also:device now being generally adopted to do away with this confusion was planned in 1878–1879 by Mr (afterwards
See also:Fleming, and published in the Journal of the
See also:Canadian Institute of
See also:Toronto for 1879 . On the initiative of this organization, Mr Fleming's proposals were officially communicated to the leading governments of the
See also:world with a view of securing an
See also:international unification of the method of designating the. hour of the day for common use . Naturally connected with the proposal was that of a
See also:prime meridian, from which all longitudes should be reckoned . United States invited an international
See also:conference, which was held in
See also:Washington in 1884, for the purpose of proposing a standard meridian to which longitudes and times should be referred . Before this conference was called the railway managers of the United States, after long: discussion, adopted the system . Its fundamental idea was that twenty-four standard meridians should be established 15° apart in longitude, starting from the meridian of Greenwich and extending
See also:round the globe . Then on each meridian the local time would differ from Greenwich time by some entire number of hours . At every point of the globe the time to be adopted for common use was that of the nearest standard meridian . These meridians would therefore mark the central lines of twenty-four zones, within each of which the time to be adopted would be
See also:uniform, but which would change by an hour on passing from one zone into another .
The inhabitants of each zone naturally use the time of the zone instead of their local time, the maximum difference between the two being
See also:half an hour . When the system was first established in the United States a delicate legal question arose as to whether the business of
See also:banks and courts should be legally adjusted to the new time . This was soon settled by state
See also:laws making the standard time legal within the limits of each zone . A similar system is being adopted in
See also:Europe, the standard meridians being those of 15°, 30°, &c., east of Greenwich . France, however, still adheres to Paris timel but Belgium and
See also:Holland use Greenwich time, and
See also:Switzerland, Italy and central Germany use the time of 15° E., and therefore one hour in advance of that of Greenwich . This is termed
See also:European time . The system we have described is that adopted for, the purposes of the railways and of• daily
See also:life . For scientific and for some international purposes yet other modifications are desirable . An important distinction must be made between the cases in which convenience requires that the time have some relation to the hour of the day, and those where no such relation is required . The former is the case in designating acts or occurrences which depend upon our daily routine of
See also:rest and wakefulness . But if nothing is necessary except the designation. of some moment. of absolute time, irrespective of our daily routine of life, then only a single measure for the whole world is necessary . At , the Washington Meridian Conference of 1884 it was proposed that Greenwich time should be adopted as a standard for the whole world in all matters of this class, especially in astronomical practice and in
See also:cable despatches .
But this system does not seem to have been extensively adopted outside ofastronomy, the I A
See also:bill adopting Greenwich time in ' France, which had already seed the Chamber of Deputies, was favourably reported on in the Hate in
See also:December, 1910 . 988 cultivators of which are most accustomed to the conversion of local into standard or Greenwich time . An unavoidable inconvenience associated with the system is the uncertainty in many cases whether local or Greenwich mean time is under-stood . This must be especially the case with magnetic and seismic phenomena, the designation of which should be uniform for the whole earth; at
See also:present, however, we cannot invariably expect local observers to convert their observations from local into Greenwich mean time . Associated with this question is that of the moment when the day should begin, or from which the hours should be counted . The
See also:civil division of the day into a.m . (ante meridiem, before mid-day) and p.m . (
See also:post meridiem, after mid-day), now practically universal in
See also:household and ordinary civil life, is impracticable for scientific purposes, where a count of the hours from o up to 24 is necessary . In railway schedules the
See also:necessity of distinguishing a.m. from p.m. when our civil time is used is found so troublesome that in some countries, especially Italy and
See also:Canada, the 24-hour system is used . Hours after noon are there designated as 13, 14, &c., up to midnight, at which moment a new day begins . On the other
See also:hand, with some few exceptions, astronomers have almost from time immemorial begun their day at noon, and navigators have very generally adopted the same practice, but for a quite different reason . In astronomy the day begins at noon for two reasons of convenience .
One is that as the day is fixed by the transit of the sun over the meridian, it is more natural to start the count of the hours from this moment than from that when the sun is on the invisible antimeridian at midnight . This practice also coincides with that of counting the hours of sidereal time from the transit of the vernalequinox, and leads to the
See also:rule that the local mean time is equal to the hour
See also:angle of the mean sun . The other reason is that, as the astronomer makes most of his observations at
See also:night, and often after midnight, it is inconvenient to begin a new day at the latter hour . This
See also:consideration is however reversed in day observations, especially those on the sun, but these are few in number . Navigators began the day at noon because their latitude is determined by observations of the sun, while the longitude is also generally determined during the daytime . Thus, in doing the " day's
See also:work " in the
See also:log, the position of the
See also:ship was always computed for noon . Such being the case, it was found more convenient to begin the count of a new day at this hour, to be continued through the night until the following noon . But the navigator's count of days was one day in advance of that of the astronomers; for example,
See also:March the loth, astronomical time, begins on the loth day of March at noon, and this count continues until noon of the day following, so that the forenoon of March the rrth, civil time, is still March the roth, astronomical time . But the navigator begins March the firth at noon on March the roth . This difference is worthy of mention because a widespread misapprehension exists that the navigator was forced to count his days from noon owing to the adoption of the same system in the Nautical
See also:Almanac . The fact is that the practice of the navigator, like that of the astronomer, was adopted purely for his own convenience, and for the reasons just set forth . It is, however, being changed so as to conform to civil time, but as yet no general
See also:law prescribes the change .
At the Meridian Conference of 1884, it was proposed that the practice of beginning the day at midnight should be adopted universally in astronomy andnavigation, and that the hours should be counted from that moment in all the nautical and astronomical ephemerides . The question of adopting this system became a subject of international
See also:correspondence . The views of the
See also:directors" of the astronomical ephemerides, so far as elicited, were strongly against the change . The considerations which determined them were the confusion which the change would introduce into the tables and the count of time in the ephemerides, including the relation of sidereal and solar time; the unavoidable doubt as to whether the one or the other system was used in astronomical publications; and the danger of placing in the hands of the navigator an
See also:ephemeris in whichthe hours should have a different meaning from that to which he was accustomed . On the other hand, the reasons of convenience which led to the practice of beginning the day at noon still continued, so that nothing could be shown to counterbalance these drawbacks Still, in
See also:works to be used by the public, especially almanacs an: other astronomical annuals, it is necessary to convert astronomical into civil time . This must continue to be done, but offers no difficulty to the authors of such works, who are acquainted with the difference, nor to the public, which has no
See also:interest in the ephemerides and
See also:measures of time used by the professional astronomer . (S .
MEASUREMENT OF TIME
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