A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Time-sight time conventions
From: Paul Hirose
Date: 2018 May 21, 22:16 -0700
From: Paul Hirose
Date: 2018 May 21, 22:16 -0700
On 2018-05-20 0:15, I wrote: > > In > a controversial step adjustment, GMT changed to the civil time > convention at the beginning of 1925. The idea had been around for decades. A resolution at the 1884 Washington meridian conference expressed "the hope that as soon as may be practicable the astronomical and nautical days will be arranged everywhere to begin at mean midnight." Astronomers generally were opposed. "Two of our highest astronomical authorities have recently pronounced their verdicts upon the decision of the Washington Prime Meridian Conference with respect to the astronomical day, and in each case the verdict is an adverse one." The astronomers were Foerster and Newcomb. http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1885Obs.....8...81.&db_key=AST&classic=YES Asaph Hall (discoverer of the moons of Mars) at the USNO took a poll of astronomers connected with the observatory. All were against any change. "I submit that the opinion of the above astroomers on this question is of greater value than that of the admirals, generals, and diplomats who so largely formed the Conference that assembled in Washinton in the autumn of 1884." http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1886Obs.....9..161H&classic=YES Some astronomers saw the other side of the argument, however. An 1895 report by the French Bureau of Longitude said, "It is obviously inconvenient for the astronomer to change the date in his observing book in the midst of a night's observations; it is legitimate to fear that he might even often forget to do this, and that the errors which might result would be difficult subsequently to discover and to correct. This inconvenience, however, has to be faced already under the present system in the case of observations of the Sun, and, as these are the most common observations at sea, sailors have constantly now to face the same inconvenience which appears so terrifying to astronomers. "The Bureau of Longitudes is in agreement, in principle, with the reform proposed by the Canadian Institute for the change of the commencement of the astronomical day." http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1917Obs....40..323.&classic=YES I believe a 1917 letter by eminent astronomers Dyson and Turner, published in "The Observatory," was the seed that finally caused the re-definition of GMT. http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1917Obs....40..301D&classic=YES This British movement was noticed in America. In 1918, Publications of the Astronomical Society of the Pacific said, "For several decades a proposal to have the astronomical day begin at midnight has received serious consideration. Following the lead of the Royal Astronomical Society of London, the American Astronomical Society in 1917 appointed a committee to consider and report upon the proposal... There appear to be no seriously inconvenient consequences of the proposed change, even in the debated matter of the Julian day reckoning. It will merely be necessary to remember that 0.5 day should be added to every Julian date prior to 1925 when comparing observing times before 1925 with observing times following January 1, 1925. "The opinion of practical navigators as to the desirability of the change appears to be almost unanimous in its favor." http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1918PASP...30..358C&classic=YES (In the end, a change to the Julian date reckoning didn't occur. Thus the fractional portion of a JD still rolls over at noon UT.) Apparently the times were ripe, since the new GMT appeared in the 1925 almanacs. But even after the date of the big change was set, there were letters wondering if it could be stopped: "What do we gain by this change except greatly increased risk of error and further confusion to future investigators? The introduction of the egregious 'Daylight Saving Time' has already caused such confusion in the time-keeping records all over the world that on would have thought that it was all the more essential that the standard system represented by G.M.T. should be continued. We were told that there was a pressing demand from the seamen for this change, but rumour says that the sailor-men now say they wanted no change. Has anyone ever heard of a ship being lost because the navigator forgot that the astronomical day began at noon?" http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1921Obs....44..123T&classic=YES Dyson (the Astronomer Royal) took a pragmatic view: "When, a year or two ago, the use of time commencing at midnight was decided upon for the Nautical Almanac from 1925 onwards, I was in favour of marking the change by a use of different symbols, such as G.T., but am now reconciled to the use of the old form G.M.T. ... The fact must be admitted that the phrase Greenwich Mean Time to non-astronomical people means time commencing at midnight. It is no use astronomers protesting that Greenwich Mean Time commences at noon. They are only considered pedantic. On this account the best way to secure uniformity in time-reckoning is frankly to adopt the popular usage from 1925, and to add the word (Civil) to the letters G.M.T. where there is any chance of a mistake. http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?bibcode=1921Obs....44..158D&classic=YES I have looked in the literature for post-1925 complaints about the new GMT, but the naysayers seem to have fallen silent. And I have not noticed any proposals to return to the old way!