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    Re: Were there secret techniques in the 17th and 18th Century
    From: Paul Hirose
    Date: 2018 Oct 18, 11:41 -0700

    During WW2 some Japanese sight reduction publications were secret.
    Bowditch (1984) says, "Japanese H.O. Pub. No. 351, Celestial Navigation
    Observation Table, was published in 1940-42, in seven volumes for
    latitudes 0°-70°. The original printing was classified 'secret.' The
    tables are similar to British Air Pub. 1618, with several differences...
    Altitudes, including refraction at 4000 meters, are tabulated to a
    minimum value of 2°."
    
    "Japanese H.O. Pub. No. 603, Simplified Celestial Observation Table, was
    published in 1943. This publication is virtually the same as Pub. No.
    351, except that eight additional stars are given, all farther south
    than those of Pub. No. 351."
    
    "Altitude and Azimuth Almanac was published by the Japanese Hydrographic
    Office, beginning in 1944. Originally, this was a secret publication.
    Several different versions were printed, and there were some
    modifications after the first editions. In each, however, the functions
    of almanac and sight reduction tables were combined. For each of several
    specific locations, the altitude and azimuth of one or more celestial
    bodies are tabulated for the date and time, usually at ten-minute
    intervals. In the earlier editions, the locations selected were
    important points in the western Pacific. From this practice, these
    publications are sometimes called 'destination tables.' Later editions
    used positions differing in latitude by 5°. These tables provided a
    quick solution for observations made at the tabulated times. On a
    worldwide basis such a system would involve a very voluminous tabulation
    each year, or cumbersome corrections."
    

       
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