standard time, civil time used within a given time zone. The earth is divided into 24 time zones, each of which is about 15° of longitude wide and corresponds to one hour of time. Within a zone all civil clocks are set to the same local solar time. Adjacent zones typically differ by a whole hour, although there are instances, such as in Newfoundland and South Australia, of half-hour zones. Standard time is based on universal time. Standard time was largely the creation of the Canadian railway engineer Sir Sandford Fleming (1827-1915). Its establishment in the United States was mainly due to the efforts of the educator Charles Dowd and William Allen, secretary of the American Railroad Association. Standard time officially came into existence after a 19-nation White House meeting in 1884, with the prime meridian established at Greenwich, England. In the United States, time zones are regulated by the Dept. of Transportation.

See also daylight saving time.

Official local time of a region or country. Local mean solar time depends on longitude; it advances by four minutes per degree eastward. The Earth can thus be divided into 24 standard time zones, each approximately 15° in longitude. The actual boundaries of each time zone are determined by local authorities and in many places deviate considerably from 15°. The times in different zones usually differ by an integral number of hours; minutes and seconds are the same. Seealso Greenwich Mean Time.

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The Standard Time and Frequency Signal (STFS) is a service available in the United States (centered at 10.000MHz? .jg) which provides standard time and frequency signals, broadcast on very precise carrier frequencies by the U.S. Naval Observatory and the National Institute of Standards and Technology (NIST), formerly the National Bureau of Standards (NBS).

A similar service is operated in the United Kingdom by the National Physical Laboratory, broadcasting from Anthorn VLF transmitter in north-west England, and by the BBC using the 198kHz carrier of the Radio 4 national radio station with a frequency accuracy of 1 part in 10^11.

External links

• NIST Time and Frequency Division