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    Re: Time-sight time conventions
    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.
    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."
    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."
    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.
    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."
    (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?"
    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.
    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!

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