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    Re: Time-sight time conventions
    From: Paul Hirose
    Date: 2018 May 20, 00:05 -0700

    On 2018-05-17 11:15, Ed Popko wrote:
    > Look towards the bottom in the final steps.  The example shows Apparent Time 
    at the Ship to be 3:56:42 and Greenwich time 7:45:27. Given the date above, 
    1880, we know we are dealing with GMT Civil Time or 12:00:00 at noon.
    
    I don't think so. Didn't the Almanac in that era follow the astronomical
    time convention? I.e., GMT was 12 hours behind Greenwich Civil time. In
    a controversial step adjustment, GMT changed to the civil time
    convention at the beginning of 1925.
    
    In the example from Lecky, ship time is 1880 June 25, "about 4 p.m." At
    the observation, the chronometer reads 7:47:57 after correction for
    rate. Longitude by DR is 57 12 W, so GMT (modern style) is about 4 hours
    ahead of local time. Thus chronometer time corrected for rate is
    19:47:57 GMT modern style, or 07:47:57 GMT old style, June 25 in both cases.
    
    At that time I get these values from the USNO MICA software:
    
    14 05 14.2 Greenwich apparent sidereal time
      6 19 47.0 Sun apparent right ascension
    ----------
      7 45 27.2 Greenwich apparent solar time (old style)
    
    That matches the "Apparent Time at Greenwich" near the bottom of the
    page in Lecky. Since I calculated solar time in the old style, the
    12-hour adjustment required by the modern style isn't necessary. In
    other words, old style apparent solar time is simply the GHA of the Sun.
    

       
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