Achiim, The Time Standards page of my Clocks and Time Web site links to the U.S. USNO (Washington, DC) and NIST (Boulder, CO) sites, which describe their atomic clocks: http://www.ubr.com/clocks/freqstd.html . These sites have much information on time.
In addition to the PTB, other major national standards laboratories are in the UK (NPL), Japan (NRL), and Italy (ETL). The International Bureau of Weights and Measures (BIPM, Paris), which determines Coordinated Universal Time (UTC) publishes a list of the atomic standards which are used to determine UTC. UTC is International Atomic Time (TAI) adjusted by leap seconds to conform to the earth's rotation (UT: Universal Time). The national standards laboratories usually have an ensemble of cesium clocks from which they determine their national time. See the USNO site for details on the ensemble of cesium clocks and hydrogen masers which they use to maintain time. These days cesium atomic clocks are rather common, and rubidium clocks even more so. With good GPS receivers being accurate to about 50 nanoseconds the need for local atomic primary standards is less critical, except that data networks require increasingly accurate time for synchronization. Atomic clocks determine time interval, defined in terms of the frequency of cesium atoms. One of the best references on time is the "Explanatory Supplement to the Astronomical Almanac," edited by Seidelmann, 1992. It discusses many time scales. In a technical sense a system of time or time scale comprises a time interval (such as the second) and an epoch or date from which time is measured. Epochs have included the beginning of the year 1900, and the origins of TAI, GPS and various other atomic time scales. What time is, the philosophy of time, is a separate topic; see the books by Fraser, for example, and G. J. Whitrow's "Time in History." Except in the technical sense Seidelmann does not discuss it. Gordon At 06:03 PM 10/14/98 , Achim Loske wrote: >Dear friends: > >I would like to know the addresses of some institutions having atomic >clocks. The only one I know is at the PTB in Braunschweig, Germany. >Probably you can help me, even if I am not asking about sundials. > >Another question: >Does somebody know what TIME is? >The 1994 ENCARTA encyclopedia defines time as: >Period during which an action or event occurs. >I am not happy with that, because I could ask what the word PERIOD means >and the response might be: >Time between two events! > >Thanks in advance - Achim. > >Achim M. Loske >INSTITUTO DE FISICA, UNAM >A.P. 20-364 >01000 MEXICO D.F. >MEXICO >Phone: (525) 6 22 50 14 >Fax: (525) 6 16 15 35 > Gordon Uber [EMAIL PROTECTED] 3790 El Camino Real, Suite 142, Palo Alto, CA 94306 Reynen & Uber Web Design http://www.ubr.com/rey&ubr/ Webmaster: Clocks and Time http://www.ubr.com/clocks/
