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    Re: Sidereal Hour Angle vs. Right Ascension
    From: Robert Eno
    Date: 2005 Aug 16, 14:08 -0400

    Excellent. Thank you very much Frank.
    But can you tell me why they switched from RA to SHA?
    I have two copies of the 1962 reprint of the 1958 Bowditch and consider it
    to be the best that they ever published, so it is interesting that you
    believe that 1958 marked the apex of celestial navigation. Certainly sextant
    technology reached a high.
    ----- Original Message -----
    From: "Frank Reed" 
    Sent: Tuesday, August 16, 2005 1:57 PM
    Subject: Re: Sidereal Hour Angle vs. Right Ascension
    > 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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