# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Navigation without Leap Seconds**

**From:**Richard B. Langley

**Date:**2008 Apr 22, 09:30 -0300

Fig. 1 in the article on leap seconds for which I posted a URL a few days ago shows delta-T between 1600 and 2000 on a linear scale. -- Richard Langley On Tue, 22 Apr 2008, George Huxtable wrote: > >Geoffrey Kolbe and I seem to agree well, now, about the difficulties in >predicting future changes in delta-T. > >However, his posting ended with- > >| George's Delta T = 0 for the epoch of 1900 for a current value of 38 >| seconds for Delta T seems to be adding to the chaos which he feared with >| different time systems. In such literature as I have read on the subject, >a >| value of 24.349 is used for the 1900 epoch, with a current value of about >| 65 seconds. See, for example, ftp://maia.usno.navy.mil/ser7/deltat.preds I >| would be interested to know where George got his value for Delta T. > >Sorry about that. > >Geoffrey asks where I got that "current value of 38 seconds for Delta T" >from, and I am rather ashamed to say that I simply pulled it out of my >memory, and got it quite wrong. I should have known better. A current value >for delta-t, for 2008, will be somewhere round 65 seconds, not 38 seconds, >just as he says. > >However, Geoffrey and I seem to disagree seriously about its earlier value, >around 1900, which he states to be 24.349 seconds. I'm not sure where that >comes from, but not from the web page that he quotes, which deals only with >recent years. I have in front of me the "Explanatory Supplement to the >Ephemeris", 1974 amended impression , which gives, in table 3.1, delta-t >values until 1972. The value for the year 1900.5 is -3.90 sec, and values >were negative from 1869 to 1902, reaching an extreme value of -8.23 sec in >1892. If the deceleration of the Earth was steady, then delta-t would follow >a parabolic curve, but it's vastly perturbed by those short-term >fluctuations. There may well have been recent reassessments of the >historical changes in delta-t, but surely not enough to account for the big >discrepancy between that table and the number that Geoffrey quotes. > >Between the two of us, we seem to have added rather a lot to the chaos that >I warned against... > >By the way, if anyone chooses to look up that volume, they should be careful >about the graph on the facing page to the table, fig 3.2, which purports to >show "general trends of delta-T, 1660-1972". This was taken from a paper >intended to show up other matters, in which much readjustment of the numbers >had taken place. In a later edition, this was replaced by a version of fig >3.2 that was completely revised, and now accords with the numbers in the >table. > >A completely revised "Explanatory supplement" appeared in 1992, edited by >Seidelmann, excellent in many ways, but this doesn't tabulate delta-t >changes, and its graph is on a compressed log scale, from which it's >impossible to extract numerical values. > >George. > >contact George Huxtable at george@huxtable.u-net.com >or at +44 1865 820222 (from UK, 01865 820222) >or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. > > >> > =============================================================================== Richard B. Langley E-mail: lang@unb.ca 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/ =============================================================================== --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---