Re: floating prime meridian

From: Rob Seaman <>
Date: Thu, 31 Aug 2000 15:29:35 -0700 (MST)

Michael Deckers notices the lack of subtlety of my argument:

> The concept is known in astronomy under the name of ephemeris meridian
> (using terrestrial time TT instead of the proposed "floating UTC").
> See eg the web site "".
> The ephemeris meridian is nowadays east of the prime meridian, which,
> incidentally, is no longer defined by the Airy Transit Circle.

The naive nature of my argument was the point, of course. I don't
really expect any significant international confusion about the prime
meridian (although my previous argument about the international date
line may not be so theoretical). But we aren't really discussing a
scientific or purely technical issue. We are discussing modifying
the civil definition of the day, not just the proposed retirement of
a technical entity called the leap second.

There may not be international confusion - but this would sure as heck
result in the lay population being even less in touch with the central
realities of science and planet Earth.

For instance the introduction of the "Report of the URSI Commission J
Working Group on the Leap Second", July 2, 2000, contained this wording:

    "UTC (Coordinated Universal Time), which the public commonly
     confuses with Greenwich Mean Time, ..."

In an earlier post (the start of the "leap second policy making" thread)
I commented:

    No. The word "confuses" is very slanted here. There is no reason
    the public *needs* to understand the difference.

Well, since then I actually went out and bought the "CCIR Recommendation
460-4 (1986)" standard from It has been asserted on
the list that this document, titled "Standard-frequency and Time-signal
Emissions", is the fundamental controlling international agreement.

I don't know if this slim document really bears the heavy weight of
defining the "day" for all the world, but it isn't actually too bad as
such things go. It does explicitly state that:

    (GMT may be regarded as the general equivalent of UT.)

Now if we can cast UTC loose from the shackles of UT1, we can certainly
do the same for GMT - all you have to do is add a "1":

    (GMT may be regarded as the general equivalent of UT1.)

But it isn't just the meaning of the wording "general equivalent" that
needs to be reconsidered, it is the civil population's definition of time.
A naive definition, sure, but are the advocates of the abolish leap second
movement (whoever they are) going to include the contingent costs in their
planning of reeducating 6 billion people?

Perhaps all that is really necessary is to revise Rec. 460-4 to
include better leap second scheduling and modern time distribution
methods? About half of the document is associated with radio time
signals. In any event the document will most surely have to be revised
since the section "Code for the Transmission of DUT1" (Annex II)
describes a method that can only be easily modified to support DUT1
up to about +/- 3 seconds.

My issue with this discussion is that the time and frequency community
would not have provoked the astronomical community (at least a few of
us) - and presumably others - if the leap second question had been
approached in terms of merely changing (hopefully improving) the
scheduling algorithm. Instead some opaque discussion has been taking
place in which various large constituencies were immediately polled
about only the most extreme option presented in the GPS world article.

My own opinion is that placing leap seconds on haitus is a poor idea
and that much larger legal, political, economic, social and scientific
forces will come into play than the survey suggested.

Rob Seaman
National Optical Astronomy Observatory
Received on Thu Aug 31 2000 - 15:29:33 PDT

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