And, there is a long, "popular" level article on leap seconds and GPS (prompted by the recent Motorola GPS receiver problem) in the 22 November issue of New Scientist now on newsstands. See also the article "GPS and Leap Seconds: Time to Change?" which appeared in my Innovation column in GPS World in the November 1999 issue. A PDF file of the article may be found here: <http://iraf.noao.edu/~seaman/leap/GPS-Nov99_Innov.pdf>. -- Richard Langley Professor of Geodesy and Precision Navigation and Contributing Editor, GPS World Magazine
On Mon, 1 Dec 2003, Woody Sullivan wrote: >This article will be of interest to those of you interested in those >weird leap seconds that sometimes get added to our clocks on New >Year's Eve due to the unpredictable changes in the Earth's rotation. >Ironically, amidst discussions of keeping time with the finest atomic >clocks, one factor in the debate is whether or not anyone cares about >(equally accurate) local solar time, as measured directly by a >sundial. > >++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ > >Leap Second Debate Heats Up [from "Physics Today", Oct. 2003, p. 34] > >Keep leap seconds, and glitches in telecommunications, navigation by >satellite, and legal marking of time could become more frequent and >serious. Lose them, and astronomers will have trouble pointing their >telescopes, and eventually the time of day will get out of sync with >Earth's rotation. > >At the crux of a debate about whether to continue inserting leap >seconds into Coordinated Universal Time (UTC), the standard in many >countries, is the unpredictability of Earth's rotation. Millennia-old >eclipse data show that Earth has long been slowing down. Nowadays, >Earth's rotation is measured by observing quasars with very long >baseline radio interferometry. The deceleration is chiefly due to the >tidal pull of the Moon; fluctuations on shorter time scales come >from, among other things, the oceans and atmosphere and core-mantle >interactions. > > > >A leap second every year or two has been the norm, though none has >been added since 1999, and timekeepers predict that more leap seconds >will be needed in the future. Without leap seconds, official time >will diverge from solar time by an estimated two minutes by the end >of this century; in 3000 years, the drift might be about eight hours. > >This month, a special rapporteur group (SRG) will submit its >recommendations on the leap second to the International >Telecommunication Union, where the matter will wend its way through >various committees before a final decision is reached--a process that >could take years. Says SRG chair Ron Beard, a physicist at the US >Naval Research Laboratory, "The issue is: Is relating to the solar >day of significance? And how does that weigh against the problems >that come with introducing integral seconds?" > >Sundials are the only clocks that mark time directly from Earth's rotation. >Before 1972, clocks were corrected with fractions of seconds and by >adjusting the length of the second. Since then, leap seconds have >been added to keep UTC within nine-tenths of a second of solar time >and synchronized with international atomic time, but with an >offset--currently 32 seconds. The atomic second is based on the >frequency of a hyperfine transition in cesium. > >But with technology relying increasingly on the precision of atomic >time, the leap second has come under scrutiny. Abolishing it would >have political, economic, legal, religious, and safety implications. >Says SRG secretary William Klepczynski, an astronomer and timekeeping >consultant, "There are all sorts of little technical problems that >you can see start to raise their heads in our society. Before they >become big problems, we should have one time for everybody." For >example, he says, bank recordings of interaction times run into >trouble when computers can't distinctly label a leap second. When >leap seconds were inserted in 1994 and 1997, he adds, the Russian >global positioning system, GLONASS, went off the air for several >hours. Mistakes in timekeeping could lead to flight accidents, says >Felicitas Arias, who heads the time section of the Bureau >International des Poids et Mesures near Paris. Leap seconds are "a >nuisance, and they could be a danger," she adds. > >The SRG hasn't reached consensus, says Dennis McCarthy, director of >time at the US Naval Observatory in Washington, DC. "But we came up >with a draft recommendation that asks for the leap second to go away, >arbitrarily in 2022--fifty years after its introduction." That should >give astronomers and others time to cope with software glitches, he >says. > >But some astronomers worry that leap seconds will be dropped sooner >and without adequate notice. "Some people would like never to insert >another leap second," says Steve Allen, a telescope programmer at the >University of California, Santa Cruz's Lick Observatory. "Telescope >pointing systems are one of the first things that would break." It's >like a Y2K analysis, he says. "You have to go through and figure out >where there are places that have assumptions that will become >untrue." It's hard to make a good cost estimate, he adds, "but it >could easily come to $10 000 to $100 000 per telescope." > >"Leap seconds may cause technological bugs, but getting rid of them >will cause bugs too," adds Robert Seaman, a programmer at the >National Optical Astronomy Observatory in Arizona. As an example, he >points to Motorola satellite receivers that, in a few weeks, are >expected to be confused about the day because their software can't >cope with such a long gap between leap seconds. Moreover, Seaman >says, "it doesn't make sense to worry [about leap seconds] right now. >There's no hurry. We have hundreds of years." > >Toni Feder >-- > >****************************************************************** >Prof. Woodruff T. Sullivan, III Center for Astrobiology & >Early Evolution >Dept. of Astronomy Box 351580 >Univ. of Washington tel. 206-543-7773 >Seattle, WA 98195 USA fax 206-685-0403 =============================================================================== Richard B. Langley E-mail: [EMAIL PROTECTED] Geodetic Research Laboratory Web: http://www.unb.ca/GGE/ Dept. of Geodesy and Geomatics Engineering Phone: +1 506 453-5142 University of New Brunswick Fax: +1 506 453-4943 Fredericton, N.B., Canada E3B 5A3 Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/ =============================================================================== -
