About UTC / a disaster for classical astronomy

From: Christian Steyaert <steyaert_at_vvs.be>
Date: Fri, 08 Jul 2005 18:23:01 +0200

With permission from the author. (Jean is not subscribed to [leapsecs] ).


Date: Fri, 8 Jul 2005 02:47:00 -0400
From: Jean Meeus <JMeeus_at_compuserve.com>
Subject: About UTC
Sender: Jean Meeus <JMeeus_at_compuserve.com>
To: Daniel Gambis <Daniel.Gambis_at_obspm.fr>
Cc: Daniel Crussaire <Daniel.Crussaire_at_obspm.fr>,
         Claude Doom <cdoom_at_skynet.be>, Fred Espenak <espenak_at_MrEclipse.com>,
         Bruce McCurdy <bmccurdy_at_telusplanet.net>,
         Hermann Mucke <astbuero_at_astronomisches-buero-wien.or.at>,
         Roger Sinnott S&T <rsinnott_at_skypub.com>,
         Thorsteinn Saemundsson <halo_at_raunvis.hi.is>,
         Denis Savoie <denis.savoie_at_palais-decouverte.fr>,
         Christian Steyaert <steyaert_at_vvs.be>,
         Felix Verbelen <felix.verbelen_at_online.be>,
         Aldo Vitagliano <alvitagl_at_unina.it>, Jan Vondrak <vondrak_at_ig.cas.cz>
Message-ID: <200507080247_MC3-1-A3AE-A657_at_compuserve.com>
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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

To reply to the request of several international organizations on a
evolution of UTC the International Telecommunication Union, ITU decided in
October 2000 the creation of a Working Group called "Special Rapporteur
SRG chaired by Mr Ron Beard of the Naval Research Laboratory (NRL-USA), in
charge of studying by a large consultation the different possible
The SRG reports its conclusions to the ITU working party WP-7A.

The Special Rapporteur Group (SRG) organized a colloquium in Torino, Italy
May 2003. No general agreement emerged from this colloquium.

In parallel, the US delegation to the WP-7A submitted, at the last meeting
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
discussed in November 2005 at the WP-7A, you are urged to react. This could
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
of ITU (for the list see the ITU website,
with a copy to Daniel Gambis (daniel.gambis_at_obspm.fr), IERS EOP Center.

Daniel Gambis
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
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
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
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 years there
were no leap seconds and even no time signals! And events taking place in,
say, 1975 and 2500 would not be comparable, because the UTC used in 1975
would not be the new UTC.

(5) It is proposed that "The difference of UT1 from UTC should not exceed
1 hour." This means that a leap *hour* should be introduced when the
difference between UT and UTC becomes too large, which would be the case
somewhere between the years 2800 and 3200. Of course the exact year is not
yet known presently, as it depends on the slowing down of the Earth's
Consider, for instance, the transit of Venus of 14 June 2984. First
contact (for the Earth's center) will take place at 10:10:23 Dynamical
This will be 10:09 UTC if the US proposal is accepted. However, if the
leap hour is introduced before A.D. 2984, then the instant would become
09:09 UTC. Consequently, presently we don't know whether the transit will
begin at 09:09 or at 10:09 in the proposed UTC scale, and hence it is not
possible to create a long list of events with the instants expressed in

(6) Finally, for sundials, too, the situation would be complicated.
Presently, to convert true solar time (as given by a sundial) to "official"

time, we have to take into consideration: the longitude difference with
Greenwich, the equation of time, and the fact that we use or not the
"summer" time. But if the US proposal is accepted, a further correction
would be needed: the difference between UT and UTC, a difference that is
now negligible, but that will gradually increase over the years if the US
proposal is accepted.

Finally, I don't understand why the ITU and the people of GPS insist to
suppress the leap seconds. Are they really unable, notwithstanding the
modern technique of the 21th century, to handle this "problem"? Should
astronomy suffer because those guys cannot handle the leap seconds easily?

Jean Meeus (Belgium)
Received on Fri Jul 08 2005 - 09:33:28 PDT

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