With permission from the author. (Jean is not subscribed to [leapsecs] ).
Chris
Date: Fri, 8 Jul 2005 02:47:00 -0400
From: Jean Meeus <[EMAIL PROTECTED]>
Subject: About UTC
Sender: Jean Meeus <[EMAIL PROTECTED]>
To: Daniel Gambis <[EMAIL PROTECTED]>
Cc: Daniel Crussaire <[EMAIL PROTECTED]>,
Claude Doom <[EMAIL PROTECTED]>, Fred Espenak <[EMAIL PROTECTED]>,
Bruce McCurdy <[EMAIL PROTECTED]>,
Hermann Mucke <[EMAIL PROTECTED]>,
Roger Sinnott S&T <[EMAIL PROTECTED]>,
Thorsteinn Saemundsson <[EMAIL PROTECTED]>,
Denis Savoie <[EMAIL PROTECTED]>,
Christian Steyaert <[EMAIL PROTECTED]>,
Felix Verbelen <[EMAIL PROTECTED]>,
Aldo Vitagliano <[EMAIL PROTECTED]>, Jan Vondrak <[EMAIL PROTECTED]>
Message-ID: <[EMAIL PROTECTED]>
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Dear Colleague,
On July 5, I received the following text from Dr. Daniel Gambis, of the
Paris Observatory and the IERS.
Letter to IERS Bulletins C and D users
Dear colleague,
As you are probably aware, international discussions are in progress on a
redefinition of UTC, including a possible suppression of the leap seconds
procedure.
To reply to the request of several international organizations on a
possible
evolution of UTC the International Telecommunication Union, ITU decided in
October 2000 the creation of a Working Group called "Special Rapporteur
Group",
SRG chaired by Mr Ron Beard of the Naval Research Laboratory (NRL-USA), in
charge of studying by a large consultation the different possible
solutions.
The SRG reports its conclusions to the ITU working party WP-7A.
The Special Rapporteur Group (SRG) organized a colloquium in Torino, Italy
in
May 2003. No general agreement emerged from this colloquium.
In parallel, the US delegation to the WP-7A submitted, at the last meeting
in
Geneva in October 2004, a proposal which contained the following items:
1 - Maintenance of a time scale called UTC.
2 - Suppression of the leap seconds adjustments which maintains UTC close
to UT1,
a time scale based on the Earth's rotation (currently UT1-UTC < .9 s)
3 - The difference of UT1 from UTC should not exceed 1 hour.
4 - The change should take effect at 21 December 2007, 00:00 UTC
If your activity is affected by the content of the US proposal which will
be
discussed in November 2005 at the WP-7A, you are urged to react. This could
be
the last opportunity before a recommendation is issued by the WP-7A.
If you wish you can express your opinion to your representative(s) at the
WP-7A
of ITU (for the list see the ITU website,
http://www.itu.int/home/index.html)
with a copy to Daniel Gambis ([EMAIL PROTECTED]), IERS EOP Center.
Daniel Gambis
Head
Earth Orientation Center of the IERS
Observatoire de Paris
-
This is what I think about that proposal.
Acceptance of the US proposal would be a disaster for classical astronomy,
and below I mention several inconveniences. The proposal to adopt the
change
on 21 December 2007 is a dirty trick: for me, it is evident that this date
has been chosen in order that no leap second could be introduced at the end
of December 2007.
(1) A first inconvenience of the change would be that the mean Sun would no
longer transit the meridian of Greenwich at 12:00, "official time", and
this
would be the end of a long tradition. Very regrettable!
(2) Acceptance of the change would result in three separate uniform time
scales running "parallel" to each other, indeed a ridiculous situation.
Presently, we already have the Dynamical Time and the Atomic Time (TAI)
which differ by a constant value, 32.184 seconds. If the US proposal is
accepted, then we would have a third parallel time scale, the (new) UTC.
There is no need to have a proliferation of those parallel time scales.
(3) As long as the difference UT-UTC remains smaller than 0.9 second, this
difference can be neglected for many applications where no extremely high
accuracy is needed. This the case for the instants given for occultations,
the phenomena of the satellites of Jupiter, etc., as published in various
astronomical almanacs. These instants are given in UT, and we can simply
use
the time given by the radio signals which are in UTC, and consider them to
be equal to UT. That would no longer be the case if a new definition of UTC
is accepted.
(4) A newly defined UTC would be a problem when constructing long lists of
astronomical phenomena such as lunar eclipses or transits of Mercury and
Venus. Suppose we want to construct such a list for the years 1000 to 3000.
What time scale should we use?
If we choose UT, then times given for, say, the year 2500 would not be
consistent with the official time, which will be the (new) UTC.
If, instead, we choose UTC, then there would be another problem: of course
for years before 1900, UTC would be meaningless, as in those
