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    Re: Burton's "Nautical Tables, High Latitude Edition"
    From: Brad Morris
    Date: 2013 Feb 17, 12:26 -0500

    Hi John

    The longitude correction is interesting.  In Time Sight days, latitude was determined close to LAN and longitude was determined with the sun on either horizon.  So latitude was carried forward to the longitude reduction.  Similarly, the longitude is carried to the latitude determination, for the noon fix.

    From your description and what I read in Lecky's Wrinkles, the longitude determined previously is 'corrected' at the noon latitude sight.  If the latitude discovered at noon does not align with the latitude used for the longitude sight, we correct the longitude by use of the ABC tables and the magnitude of the latitude error.

    Seems like a terrific amount of calculation for a tiny longitude correction.  One must account for desired latitude changes, as not every ship sails a latitude line.  What about dither in latitude as one sails? 

    With the Time Sight determination of lat & lon, not every opportunity yields a result.  The can be periods of days in which neither the lat and lon can be determined.  How is one to apply the ABC tables now?

    Is there someone on list who has used these tables?   Have they faded into obscurity due to lack of relevance?

    Brad

    On Feb 17, 2013 6:14 AM, "John Brown" <jdb0302@yahoo.co.uk> wrote:

    Brad Morris wrote:

    "Can someone explain what the purpose(s) are of the ABC Tables? If its any help, A=Cot(HA)Tan(Lat) B=cosec(HA)Tan(dec) and C=Cot(Az)Sec(lat)"

    From Notes On The Tables in my 6th Edition of Burton's Nautical Tables:

    The ABC tables "provide one of the quickest and most convenient means of finding the azimuth; and also provide, with equal facility, great-circle courses and the 'Longitude Correction'."

    These tables were once in common use in British ships and are still contained in Reeds Astro Navigation Tables, published annually in the UK.

    "'C' (the combination of the values A and B) is the error of longitude corresponding to an error of one minute of latitude. It must...be multiplied by the total error of latitude to obtain the total error of longitude."

    A full description of their use follows in my copy of Burton's. Basically a noon estimated latitude obtained by course and distance run from (usually) an a.m. sun intercept terminal point was compared to the noon latitude by observation. The difference was multiplied by the 'C' correction to obtain the 'correction' to the estimated longitude. This is just the equivalent of a running fix by plotting.

    If anyone is interested, I can scan and post the full explanation.

    Burton's Tables do everything that Nories do, but in my opinion are superior in terms of layout and clarity.

    Regards to all

    John


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