Thread View: mod.std.unix
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Started by Moderator, John Quarterman
Tue, 23 Dec 1986 17:26
Re: time for time details
Author: Moderator, John Quarterman
Date: Tue, 23 Dec 1986 17:26
Date: Tue, 23 Dec 1986 17:26
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From: seismo!nbires!vianet!devine (Bob Devine) Date: Wed, 17 Dec 86 19:39:53 EST This is in response to Ron Tolley's article that appeared in mod.std.unix last week. My reply corrects the errors. Bob Devine ------------------------------------------------------------------ > GMT and UTC are not the same. Yes they are (within a very small delta). GMT (Greenwich Mean Time) is maintained by the UK while UTC (Universal Coordinated Time) is maintained by the International Time Bureau in Paris (BIH). The WWV* broadcasts in the US are not exactly UTC but neither is GMT. However, they are within nanoseconds of UTC. WWV (and WWVB and WWVH) are the US's official distributors of the time according to the US's clocks. Some confusion results from the use of "GMT". In common usage, it means what time it is in the timezone centered on the Greenwich Observatory which defines zero degrees longitude. It also means the official UK time. GMT is no longer the global standard for time; UTC is (since 1972). UTC is an average of all the contributing countrys' clocks (US, UK, France, Italy, Japan etc all contribute to UTC). The change of UTC to stay close to UT1 (the "spinning earth" time) is through the adding or subtracting of leap seconds. BIH makes recommendations for such leap seconds and it is up to the individual countries to follow them. I don't know of any case where a leap second recommedation was not followed by a country for its clocks; it doesn't make sense to disregard them. > The following is a list of leap seconds which have been added to > Universal Coordinated Time (UTC) in order to keep it relatively close to > solar time. Note that with Greewich Mean Time, such corrections were > made by stretching or contracting the length of seconds. UTC is > generally available through time standards, GMT not readily available. > This is data derived from an AP story from May 1985. No data since then > is known. There is also no indication whether the insertions were made > in local time or in UTC. Local time is assumed. (Wouldn't Australia, > Newfoundland, and other half-hour-off places have fun with inserting an > extra second in the middle of a pseudo-random hour.) A second is not stretched/contracted for leap second adjustments. The selected minute will have 59 or 61 seconds. There are agreements as to which minute is selected and the BIH issues its recommendation far in advance selecting the minute. Currently, and unless the earth goes wacko, a second is usually added once a year. Bob Devine Volume-Number: Volume 8, Number 73
Re: time for time details
Author: Moderator, John Quarterman
Date: Wed, 07 Jan 1987 23:46
Date: Wed, 07 Jan 1987 23:46
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From: ames!pyramid!nsc!nscpdc!nscpdc.nsc.com!djg Date: Sun, 28 Dec 86 11:47:06 pst > From: seismo!nbires!vianet!devine (Bob Devine) > Date: Wed, 17 Dec 86 19:39:53 EST > > This is in response to Ron Tolley's article that appeared in mod.std.unix > last week. My reply corrects the errors. ..... > > solar time. Note that with Greewich Mean Time, such corrections were > > made by stretching or contracting the length of seconds. UTC is > > generally available through time standards, GMT not readily available. .... > A second is not stretched/contracted for leap second adjustments. The > selected minute will have 59 or 61 seconds. Note as above it was G.M.T that was stretched. I used to work at the R.G.O. Whenever possible a zenith tube reading of Polaris was used (up to the installation of caesium clocks 20? years ago) to correct an oscillator defining a 10MHz signal sent via land line the Rugby time centre for broadcast. On every hour the signal was inverted 5 seconds before the hour to synchronise clocks (This is still done but from atomic clocks). Since using U.T.C the leap seconds are manually added or subracted at the appropriate time (Yes someone at midnight dec 31 has to go down to the time computer and press a button). Note that since UTC is the calculated best fit of many atomic clocks it can never be given in "real time" but only after the fact. (Most laborities(observatories) use quartz clocks set against a reference and then post-calibrate it). Volume-Number: Volume 9, Number 9
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