On Wed 2006/01/18 08:17:54 -0000, Francois Meyer wrote in a message to: LEAPSECS@ROM.USNO.NAVY.MIL
>Maybe it should be, but this is far from being >obvious from its current definition. I agree that the current definitions leave a lot to be desired in terms of clarity and rigour - an uncharitable person might even describe the extract of ITU-R TF.460-6 cited the other day by Michael Deckers as inconsistent. However, you have to consider how UTC is actually used in practice and this is what my comments are based on. >1. UTC and TAI share the same rate, the same > origin, the same second. And therefore : > > UTC - TAI = 0 They both count SI seconds but the question of the origin is a bit muddy. You could argue that there is a fixed 10s offset between UTC and TAI because UTC post-1972 (everything I've said about UTC only applies post-1972) started with 10 leap seconds, and before 1972 UTC wasn't simply a representation of TAI. There's no simple way of fudging radixes that I can think of to make them match up, but if this worries you then simply think in terms of proleptic UTC (post-1972), see http://en.wikipedia.org/wiki/Proleptic. However, this is not important and really only confuses things further. (It's why I couched the original example as a graph of a person's age vs UTC rather that TAI vs UTC.) >2. UTC only differs from TAI by its definitions of > the minute, hour and day. > >3. the TAI day, hour, minute are the SI > day, hour, minute of 86400,3600,60 > SI seconds. > >4. the UTC minute is defined to ensure that dhms > expressions of UTC match UT1 at .9 s; it can be > either 59, 60 or 61 SI seconds long. This > definition of the minute is realized > by (positive or negative) leap seconds and > ensures that the mean UTC day is the mean solar > day in the long term. The UTC hour has 60 UTC > minutes, the UTC day has 24 UTC hours. OK, but using the word "definition" doesn't seem quite right, I'd couch it in terms based on counting seconds. If you consulted the wikipedia entry for "Factoradic" that I referenced the other day you will see that each digit in the factoradic representation of a number has a subscript indicating its (fixed) radix. The fields (not the digits!) of a UTC representation should have something similar (likewise calendar dates written as 2006/01/19), except that for the seconds field (and the day field) the radix would be variable and there's no simple way to write it, so people assume it's 60 which is what you do anyway if you want your approximation of UT1. >>From that point of view, the sentence from the ITU460-6 : > > "[UTC] ...differs from it [TAI] from an integer of seconds" > >should read : > >"representations of UTC involving minutes, hours, >days differ from equivalent representations of TAI >by an integral number of seconds" I agree that the ITU-R TF.460.6 text needs fixing, but I would argue for a stronger statement emphasising that a UTC representation (of TAI) has two interpretions, one giving TAI (exact) and the other UT1 (approx), the latter being offset from TAI by an integral number of seconds. Mark Calabretta ATNF
