It still seems to be common among groups outside the computing and scientific worlds to talk about “GMT” rather than “UTC”. The trouble with “GMT” is that it doesn’t seem to be clearly defined; it might or might not differ from UTC by up to 0.9 seconds. The (unadjusted) time reference defined by an official collection of atomic clocks worldwide is called “TAI”. This pays no heed to the slowing rotation of the Earth. Astronomers prefer a time reference called “UT1”, which is based on the actual rotation of the Earth (subject to some time-averaging, I think). The definition of “UTC” comes from TAI, adjusted by applying leap seconds. These accumulate as needed so as to keep UTC within 0.9 seconds of UT1, and we are currently up to 36 of them. That is, at 0:00:00 UTC today, TAI was actually 0:00:36. The trouble with GMT is that it is not officially clear whether it is the same as UT1, or the same as UTC. Given that it was originally defined at the Greenwich Observatory, which was (is?) run by astronomers, one would think they would make it the same as UT1. But the practical fact is, our usual everyday timekeeping is based on standard time zones, which are all defined as whole-second (indeed, whole multiples of a half-hour) offsets from UTC.
Hi Lawrence, Recently had this discussion at work in relation to a what we should call the approximate (nearest 10 secs) UTC time reference that we display in our application. We decided on GMT because we figured that more people would know what that is. In this case the accuracy doesn't matter. On 01/07/15 17:53, Lawrence D'Oliveiro wrote:
It still seems to be common among groups outside the computing and scientific worlds to talk about “GMT” rather than “UTC”. The trouble with “GMT” is that it doesn’t seem to be clearly defined; it might or might not differ from UTC by up to 0.9 seconds.
Outside the scientific and computing worlds why does it matter? Isn't 0.9 seconds near enough for most non science or computer related uses?
The (unadjusted) time reference defined by an official collection of atomic clocks worldwide is called “TAI”. This pays no heed to the slowing rotation of the Earth. Astronomers prefer a time reference called “UT1”, which is based on the actual rotation of the Earth (subject to some time-averaging, I think). The definition of “UTC” comes from TAI, adjusted by applying leap seconds. These accumulate as needed so as to keep UTC within 0.9 seconds of UT1, and we are currently up to 36 of them. That is, at 0:00:00 UTC today, TAI was actually 0:00:36.
The trouble with GMT is that it is not officially clear whether it is the same as UT1, or the same as UTC. Given that it was originally defined at the Greenwich Observatory, which was (is?) run by astronomers, one would think they would make it the same as UT1. But the practical fact is, our usual everyday timekeeping is based on standard time zones, which are all defined as whole-second (indeed, whole multiples of a half-hour) offsets from UTC.
In which context would this matter? Anyone concerned about the accuracy of their time reference would surely be educated about it and therefore would use the appropriate reference. According to wikipedia: "Today GMT is considered equivalent to UTC for UK civil purposes (but this is not formalized) and for navigation is considered equivalent to UT1 (the modern form of mean solar time at 0° longitude); these two meanings can differ by up to 0.9 s. Consequently, the term GMT should not be used for precise purposes" Glenn
participants (2)
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Glenn Ramsey -
Lawrence D'Oliveiro