Re: [LEAPSECS] The real problem with leap seconds

From: Francois Meyer <fmeyer_at_obs-besancon.fr>
Date: Wed, 18 Jan 2006 08:17:54 +0000 (UTC)

On Mon, 16 Jan 2006, Mark Calabretta wrote:

> On Fri 2006/01/13 11:17:52 -0000, Michael Deckers wrote
> in a message to: LEAPSECS_at_ROM.USNO.NAVY.MIL
>
> > I must get TAI, up to an integration constant. This is correct.
> > The integral of d( UTC ) is TAI (up to an integration constant),
> > but this integral is UTC only where UTC is a continuous function
> > of TAI.
>
> You're still not getting the point that UTC is just a representation
> of TAI.

Maybe it should be, but this is far from being
obvious from its current definition.

The actual situation corresponds to :

1. UTC and TAI share the same rate, the same
   origin, the same second. And therefore :

        UTC - TAI = 0

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.

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"

-- Francois Meyer
Tel : (+33) 3 81 66 69 27 Fax : 3 81 66 69 44
Observatoire de Besancon - BP1615 - 25010 Besancon cedex - FRANCE
**** Université de Franche-Comté ****** CNRS UMR 6091 *****
Received on Wed Jan 18 2006 - 00:43:37 PST