[LEAPSECS] the big artillery

[Michael Deckers]

   On 2014-11-04 22:26, Steve Allen wrote:

> Guinot explained this using the term "graduation second"
> in section 2.2 of 1995 Metrologia 31 431
> http://iopscience.iop.org/0026-1394/31/6/002
>
> He points out that the way the IAU has written the definitions of the
> time scales uses a subtly ambiguous notation.  He writes
>
>    The numerical value of UT1(IERS)-TAI does not
>    of course, express a duration. In this context, the "s"
>    only conveys the information that the readings of the
>    two time scales are expressed in graduation seconds.

   Thank you for that information!

   Yes, not every quantity with dimension time is a
   duration, let alone a duration of proper time. The
   difference between clock readings need not relate
   to proper time, and not even to the same time scale.
   A few operations with durations of differing time
   scales are considered to result in durations
   (eg, a weighted average of durations measured in
   different time scales), but most can not. And a
   sedimentation rate (a quotient velocity/acceleration)
   can not be considered as a duration, nor as
   the result of any other operation with time scales.

   Nevertheless, all these quantities have the dimension
   of time and can therefore be expressed with the SI
   unit for time, even though the SI second is (currently)
   defined as a duration of proper time. This is essential
   for the meaningful operations that one wants to perform
   with these quantities (differences of clock readings,
   averages of durations), but it also makes many
   meaningless operations possible (such as subtracting
   a sedimentation rate from a clock reading).

   Michael Deckers.