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    Re: Sidereal Hour Angle vs. Right Ascension
    From: Frank Reed CT
    Date: 2005 Aug 16, 17:07 EDT

    Robert Eno wrote:
    "But can you tell me why  they switched from RA to SHA?"
    
    A couple of reasons. First, as you already  noted, it was a nuisance dealing
    with an angular coordinate measured in hours.  Astronomy has been exceedingly
    conservative with its terminology and its units,  and "right ascension" (weird
    terminology to the modern ear) continues to be  listed in hours to this day.
    Second, it saved a step of calculation.  Fundamentally, you need to know the
    latitude and longitude where the given  celestial object is directly overhead
    for modern celestial navigation. The  declination is the latitude. The GHA is
    the longitude. We can calculate GHA with  several steps from the Sidereal Time
    and the RA (since SidT is the RA of the  obserer's zenith). Dec is easier
    since it changes very slowly. By tabulating  GHA/Dec directly for every hour of
    time, we save considerably on interpolation.
    
    For the stars, you have two choices: either select a small-ish number of
    stars and list their GHA for every hour much like the planets, or, as in the
    modern Nautical Almanac, you list GHA Aries (equivalent, indirectly, to Sidereal
    Time) and add one step of calculation for a longer list of stars. One final
    issue: SHA is measured in the opposite direction from RA. This converts a
    subtraction into an addition and, as we all know, adding angles is easier than
    subtracting them. A relic of the earlier use of RA in the almanacs is in the
    ordered list of navigational stars. They are in order by RA (at least they were
     before precession switched a couple) but in reverse order by SHA.
    
    By the  way, we *could* use almanacs listing RA for modern celestial, and
    they could be  somewhat smaller than the modern Nautical Almanac. But the trade-o
    ff in paper  was worth the savings in time on each sight.
    
    By the way, I mentioned the  late adoption of the GHA approach in the
    official British almanac (didn't happen  until 1950). But there were at least two
    popular unofficial sources for  astronomical almanac data (Brown's and Reed's
    nautical almanacs). In Reed's for  1947, the RA and GHA are both included but
    listed only for every 12 hours of  time. The explanation refers to the "Sidereal
    Time method" of calculation (using  RA) and the "GHA method".
    
    Two more data points: The 1950 German nautical  almanac (Nautisches Jahrbuch)
    tabulates RA only. The 1960 German almanac lists  GHA only.
    
    There's another way of thinking about "why" this occurred...  The reasons
    I've listed above are all fine, but there's also the question of  user/consumer
    demand. Bruce mentioned that the Navy (and world's militaries  generally) had
    to "train a lot of navigators in a hurry". During the war and  even more so
    after the war, this created a large pool of young navigators who  had relatively
    little attachment to the older methods and considerable practical  experience.
    These navigators were eager to be consulted, and their opinions no  doubt had
    a big impact on the practical improvements that led to the near  perfection
    of celestial navigation c.1958.
    
    -FER
    42.0N 87.7W, or 41.4N  72.1W.
    www.HistoricalAtlas.com/lunars
    
    
    

       
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