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    Re: Sidereal Hour Angle vs. Right Ascension
    From: Frank Reed CT
    Date: 2005 Aug 16, 13:57 EDT

    Robert Eno asked:
    "Can anyone tell me at what  point in time navigators moved from using right
    ascension to sidereal hour angle  for reckoning celestial coordinates?"
    
    In 1933 an experimental American  Air Almanac was published which used
    GHA/SHA instead of RA (or so they say --I  haven't seen it). Though this early air
    almanac was not continued, some of its  features were incorporated into the big
    revision of the American Nautical  Almanac in 1934. In this year, the
    appendix on calculating geocentric lunar  distances was finally dropped along with
    some other flotsam, and for the first  time ocean navigators found GHA in
    parallel with RA. Those who wanted to could  now use GHA, while conservative
    navigators could stick with Right  Ascension.
    
    In the late 1930s and early 1940s, various countries began  regular
    publication of air almanacs and these apparently used GHA/SHA  exclusively with short
    intervals between tabulated values (e.g. GHA given every  ten minutes). This
    allowed "eyeball interpolation" and shortened the work by a  few minutes per
    sight. For air navigation every second counts.
    
    The  American Nautical Almanac was again revised in 1950 and from this date
    it  broadly resembles the modern Nautical Almanac with GHA/SHA tables and no
    more  Right Ascension. This is also the year that the cardboard orange cover
    appears.
    
    Meanwhile across the Atlantic, the "Abridged Nautical Almanac",  which was
    the standard almanac for British mariners, was not revised during this  whole
    period. In 1953 there was a major revision of the AbNA which finally  replaced
    RA with GHA/SHA. This year also marked the beginning of serious  attempts to
    create a common, unified almanac. After considerable negotation the  American
    Nautical Almanac and the (British) Abridged Nautical Almanac were  unified in
    1958, mostly along the lines of the American almanac. The titles  remained
    separate until 1960 when the astronomers' almanac finally lost its  nautical title
    and the combined mariners' almanac could recover the logical  title: The
    Nautical Almanac.
    
    Modern celestial navigation reached its apex  in 1958. The new, drastically
    revised Bowditch was released that year. And the  Nautical Almanac achieved its
    final, modern form. Apart from the inclusion of  sight reduction tables
    starting in 1989, there have been almost no changes to  the almanac since the
    pivotal year 1958.
    
    -FER
    42.0N 87.7W, or 41.4N  72.1W.
    www.HistoricalAtlas.com/lunars
    
    
    

       
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