From owner-leapsecs@ROM.USNO.NAVY.MIL Sat May 10 02:09:48 2003 Received: from [192.5.41.253] (juno.usno.navy.mil [192.5.41.253]) by santo.ucolick.org (8.11.7+Sun/8.11.7) with SMTP id h4A99ew07164 for ; Sat, 10 May 2003 02:09:40 -0700 (PDT) Received: from rom.usno.navy.mil by [192.5.41.253] via smtpd (for santo.ucolick.org [128.114.23.204]) with SMTP; 10 May 2003 09:20:48 UT Received: from ROM.USNO.NAVY.MIL (rom.usno.navy.mil [10.1.4.27]) by rom.usno.navy.mil (8.12.8/8.12.5) with ESMTP id h4A99ZTA019178; Sat, 10 May 2003 09:09:36 GMT Received: from ROM.USNO.NAVY.MIL by ROM.USNO.NAVY.MIL (LISTSERV-TCP/IP release 1.8e) with spool id 15169 for LEAPSECS@ROM.USNO.NAVY.MIL; Sat, 10 May 2003 09:09:35 +0000 Received: from TS-FW.usno.navy.mil (TS-FW.usno.navy.mil [10.1.1.3]) by rom.usno.navy.mil (8.12.8/8.12.5) with SMTP id h4A99YT8019175 for ; Sat, 10 May 2003 09:09:35 GMT Received: from geneva.ucolick.org ([128.114.23.183]) by TS-FW.usno.navy.mil via smtpd (for rom.usno.navy.mil [10.1.4.27]) with SMTP; 10 May 2003 09:20:42 UT Received: (from sla@localhost) by geneva.ucolick.org (8.11.6/8.11.2) id h4A99V918201 for LEAPSECS@ROM.USNO.NAVY.MIL; Sat, 10 May 2003 02:09:31 -0700 Date: Sat, 10 May 2003 02:09:31 -0700 From: Steve Allen To: Leap Second Discussion List Subject: UT and the IAU Message-ID: <20030510090931.GA17917@ucolick.org> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.4.1i Sender: owner-leapsecs@ROM.USNO.NAVY.MIL Precedence: list X-Virus: Clean Status: RO Content-Length: 4382 Lines: 81 In 1925 the publications of nautical almanacs and ephemerides made a major change. They started counting the hours of the day as beginning at midnight (as with most civil time) instead of at noon (as had been the astronomical practice). Before 1925 the American Ephemeris and Nautical Almanac (AENA) had called time starting at noon GMT, and starting with 1925 (thru 1952) it called time starting at midnight Greenwich Civil Time. The British almanac could not be persuaded not to apply the term GMT to the new reckoning of hours starting at midnight despite the high likelihood of confusion of the meaning of GMT. In 1925 the IAU Temporary Committee on Time approved the following text: Astronomers have not yet arrived at sufficient agreement upon a single terminology which it is desirable to use for time, and are not in a position to lay down a rule on the subject. It is desirable that astronomers should state exactly which time they are using. In 1928 IAU Commission 4 (Ephemerides) adopted the following text: The terms Greenwich Civil Time (G.C.T.), Weltzeit (W.Z.) and Universal Time (U.T.) denote time measured from Greenwich Mean Midnight, and are not ambiguous. I believe that this is the original definition of UT. I am unaware of any significant usage of WZ, and in the AENA GCT was replaced by UT in 1953. (In the mean time most of the world except for old astronomers has forgotten that there ever was an ambiguous usage of GMT, and the British interpretation of GMT=UT is broadly believed.) The discovery of seasonal variations in earth rotation did not occur until 1936/1937, so the expectation in 1928 was that UT would be suitable both as a measure of angle ("time-of-day") and as a measure of dynamical time. Within a decade it was clear that earth rotation was not a good clock. The IAU began to take steps to distinguish between rotation and time, and Ephemeris Time (ET) came into being. Beginning in 1960 the almanacs started publishing ephemerides using ET. The various types of UT were treated as measures of angle, and this conforms with the original 1928 definition that indicates UT is measured from midnight. At that time most civil time broadcasts were providing UT2, and throughout the 1960s the rate of UT2 was adjusted in a way that precluded its interpretation as a uniform timescale. UT in all its forms was a measure of "time-of-day", not time. The definition of UTC as atomic time with leap seconds to keep in step with UT was codified in 1970, implemented in 1972, and the rules were refined over the next few years. So, inasmuch as the IAU controls the definition of types of "Universal Time", the IAU has always held that UT means angle. As such, all forms of UT have always required input from astronomers. Early determinations of UT were optical measurements from transit instruments, but by 1970 IAU Commission 31 (Time) remarked that the "exotic systems such as VLBI" were proving important. In 1973 a joint session of IAU commissions 19, 31, and 24 recommended that "multi-national and global networks of stations" be established and "operated for a long enough time" to evaluate their results and that "additional technical developments be undertaken" to improve their accuracy. Naturally this required money. During the 1980s the deployment of Mark III VLBI correlators was greatly accelerated by the influx of NASA dollars as part of the "Mission to Earth". VLBI provides information on all sorts of esoteric astronomical objects and geophysical effects. Nevertheless, it was always safe to explain to a congressman that these dollars to VLBI were directly contributing to the ability to tell what time it is. "Time-of-day" is a concretely relevant datum that anyone on the street can grok. If UTC, or even merely civil time, ceases to rely on astronomical input, will astronomers be less able to justify why legislators should spend tax monies on "the next generation of Mark III correlators"? Will the IAU quietly allow the re-definition of a form of Universal Time in such a fashion that it diverges from all past usage and also kills a cash cow? -- Steve Allen UCO/Lick Observatory Santa Cruz, CA 95064 sla@ucolick.org Voice: +1 831 459 3046 http://www.ucolick.org/~sla PGP: 1024/E46978C5 F6 78 D1 10 62 94 8F 2E 49 89 0E FE 26 B4 14 93