new beginning

From: Rob Seaman <seaman_at_noao.edu>
Date: Wed, 3 Aug 2005 16:11:06 -0700

There has been a suggestion that this discussion has become too
dogmatic - on both sides. I might reiterate that the discussion has
never really had a chance to get started, but it is certainly true
that we have so far failed to explore all our options.


Scott Moore wrote:

> All that matters is that the time reference generators like WWV,
> and Internet time servers have good to excellent accuracy, I.e.,
> they can calculate solar time from atomic time with enough places
> to satisfy everyone concerned.

Ed Davies replied:

> Using UT1 or somesuch (Earth angle time) as you suggest results in
> variable length seconds. This was what was done prior to 1972,
> when UTC was introduced to fix this.

I agree that UTC has proven to be a good compromise demonstrating how
solar time can be preserved while relying on a constant length
standard unit of time. It may not be the only possible compromise.


Now read again what Steve Allen has said:

> The whole point of the 24:60:60 notation is to match up with the
> day. In order to do that all civil time scales have always had
> leaps or seconds of non-uniform length or both. Dynamical time and
> atomic time have no such concept and should always have been
> communicated as a decimal count of elapsed seconds.
>
> [1] Confusing the one entity with the other in the original
> definition of Ephemeris Time was perhaps the biggest timekeeping
> mistake ever made by astronomers.
>
> [2] Not following the suggestion to define the cesium-based,
> Ephemeris-Time-inspired unit of duration as a new unit called the
> Essen was perhaps the biggest timekeeping mistake ever made by
> physicists.
>
> [3] Allowing civilian entities to believe that the broadcast
> time signals originally intended for navigation could also be used
> for setting civil time was perhaps the biggest timekeeping mistake
> ever made by the CCIR (now ITU-R) and all the national time services.


I believe Moore's suggestion is the same as Allen's scholarly reply.
Davies points out one of the two main constraints on civil time - it
serves as an interval reference standard. I will point out the
other: civil time IS mean solar time.

To clarify - even those arguing for the most extreme position of
dismantling UTC and ceasing leap seconds only do so because TAI (+
offset) forms a close approximation to solar time in the near
future. Hence the three contributory mistakes cataloged by Allen.

For any time scale to ever be considered as a candidate for replacing
UTC, its rate must approximate UTC's to within milliseconds per day
at any given epoch. Only then could leaps or rate changes ever be
considered a possibility for synchronizing the two clocks. Whether
the leaps occur frequently with one second amplitude or infrequently
with one hour amplitude, a near perfect initial match in rates is a
prerequisite. For example, sidereal time would never be considered
an option for civil time, simply because four minutes per day is too
large of a mismatch.

(I assert that apparent solar time on the one hand and standard/
daylight time on the other hand are each simple equivalents of mean
solar time, averaged over the course of a year. An analemma is not
rocket science. It is the solar cadence that matters, not periodic
latencies.)


So, what steps might we take to correct these mistakes made by the
astronomers, physicists and timekeepers?

1) Recognize that interval time (TAI) and solar time (UTn) are both
necessary. (Please don't make me explain it again.)

2) Reserve sexigesimal notation for civil (solar) time. TAI and
similar unsegmented time scales should always be communicated as
elapsed seconds. Choice of epoch and numeric representation should
be interesting debates. Surely I'm not alone in wanting to have
interesting debates again?

3) Clarify the relationship between the civil second and the SI
second. It may be too late to define a new unit of duration -
whether Essen or Fressen - or perhaps it isn't. In any event, there
are 86400 seconds per solar day, and that usage of the word "second"
clearly differs from the SI unit which happens to have the same
name. What are we going to do about it? (Certainly the ITU proposal
does not address such issues.)

4) UTC performs the neat trick of conveying BOTH TAI and an estimate
of UT1 in one convenient package. If we are no longer going to do
this, what is the alternative? Shall both TAI and UT1 be separately
conveyed? Shall one be conveyed and the other calculated from it
using standard algorithms incorporated into, for instance, consumer
electronics? This simply must be clarified before a functioning
system for conveying both is dismantled.


I accept the criticism that I am too often dogmatic, and am
attempting to be otherwise. Suggest that dogmatic replies to this
message be disregarded - I will.

Rob Seaman
National Optical Astronomy Observatory
Received on Wed Aug 03 2005 - 16:11:36 PDT