[LEAPSECS] the big artillery

[Steve Allen]

On Tue 2014-11-04T21:52:05 +0000, Michael Deckers via LEAPSECS hath writ:
>    Then which unit would that be? When the IERS compute a difference
>    TAI - UT1, how do they do it? Do they convert the UT1 reading in
>    any way before they subtract? Or, if they don't, what is the unit
>    of the difference, SI seconds or "second of UT1"?  The IERS
>    Conventions certainly do not mention any of this. How could they
>    if the units would really differ?

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.

This is basically saying that UT1(IERS)-TAI is only a difference of
two numbers.  I think it's pretty much the same as supposing that a
place with a comfortable temperature is cooling off and when the
weather is freezing then the difference
	Delta T = T(deg Fahrenheit) - T(deg Celsius) = 32 degrees
and when the temperature reaches -40 then
	Delta T = 0 degrees

That difference is not a temperature, just like Delta T for eclipse
timing predictions is not a duration.

--
Steve Allen                 <sla at ucolick.org>                WGS-84 (GPS)
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