thumb|right|worldwide time zones at present standard time is the synchronization of clocks in different geographical locations within a time zone to a common time standard, usually based on the meridian at the centre of the time zone, rather than using a local meridian as a local mean time or solar time standard. historically, this helped in the process of weather forecasting and train travel. the concept became established during the 19th century. the time so set has come to be defined in terms of offsets from universal time. where daylight saving time is used, the term standard time typically refers to the time without the offset for daylight saving time. the adoption of standard time, because of the inseparable correspondence between time and longitude, solidified the concepts of halving the globe into an eastern and western hemisphere, with one prime meridian (as well its opposite international dateline) replacing the various prime meridians that had previously been used. ==history of standard time== ===great britain=== a standardized time system was first used by british railways on december 11, 1847, when they switched from local mean time, which varied from place to place, to greenwich mean time (gmt). it was also given the name railway time reflecting the important role the railway companies played in bringing it about. the vast majority of great britain's public clocks were standardised to gmt by 1855. ===north america===  until 1883 each united states railroad chose its own time standards. the pennsylvania railroad used the "allegheny time" system, an astronomical timekeeping service which had been developed by samuel pierpont langley at the western university of pennsylvania's allegheny observatory in pittsburgh. instituted in 1869, the allegheny observatory's service is believed to have been the first regular and systematic system of time distribution to railroads and cities as well as the origin of the modern standard time system. by 1870 the allegheny time service extended over 2,500 miles with 300 telegraph offices receiving time signals.{{cite book|url=http://www.nps.gov/history/history/online_books/butowsky5/astro4m.htm|title=astronomy and astrophysics|chapter=allegheny observatory|first=harry|last=butowsky|publisher=national par
Time_scales