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    Re: Nautical astronomy was different
    From: Trevor Kenchington
    Date: 2004 Oct 22, 17:23 +0000

    Ken Muldrew wrote:
    
    > The way I read the passage from Bowditch that Bruce cites is that the
    > navigator of old regulates the chronometer by bringing it into unison with
    > local apparent time. The error in the mechanism is immaterial since LAT is
    > constantly changing due to the movement of the navigator. Since the local
    > time is only known approximately anyway, there is no need to worry over
    > the running of the mechanism itself (within limits). Presumably this is a
    > different chronometer than the one being used to track Greenwich time,
    > being used just as a common watch was before chronometers were available.
    
    
    But a chronometer is a chronometer, not a watch to be re-set daily.
    
    What I understand Bowditch to have meant is that Time (unless otherwise
    qualified) means local time -- LMT, not LAT, as is clear from his words.
    The chronometer (after any corrections for its rate of going) tracks GMT
    but that is not Time. The difference between the chronometer's corrected
    indication and LMT is then an error in its representation of Time, from
    which error one can calculate longitude at 15 degrees to the hour.
    
    To a modern navigator, of course, Time (the true, fundamental time) is
    GMT, as shown by the chronometer (after whatever correction for its rate
    of going). Zone time is just a convenient way of reconciling GMT to
    local daylight hours, while local time is something involved in figuring
    out the GMT of meridian passage and the like. We wouldn't think of, for
    example, keeping a logbook in local time as Bowditch's contemporaries
    did. But if we know GMT and longitude, figuring out LMT is trivial.
    
    And that, I think, is Bruce's point: Where we use Time=GMT plus
    longitude to find LMT when we need it, an earlier generation of
    navigators used Time=LMT (or even LAT) plus the difference between LMT
    and GMT to find longitude. Which serves to explain why a time sight is
    called a time sight not a longitude sight: It was used to find Time when
    Time was considered to mean local time.
    
    
    Trevor Kenchington
    
    
    --
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
    
                         Science Serving the Fisheries
                          http://home.istar.ca/~gadus
    
    
    

       
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