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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]> MIME-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Disposition: inline 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 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 rotation. Consider, for instance, the transit of Venus of 14 June 2984. First exterior contact (for the Earth's center) will take place at 10:10:23 Dynamical Time. 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 UTC. (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)