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    Re: Sidereal Hour Angle vs. Right Ascension
    From: Bruce Stark
    Date: 2005 Aug 16, 14:24 EDT
    Robert,

    I'm pretty sure sidereal hour angle wasn't used in the American Nautical Almanac before 1950. But, for all I know, it was in the Air Almanac from the beginning of that publication.

    You're not alone in preferring SHA to right ascension. SHA fits the Greenwich hour angle approach to celestial navigation, which is the only approach anyone understands now. As you know, SHA is RA turned backward, to increase from east to west, and given in arc, rather than time. It would have been as awkward for the nineteenth-century navigators as RA is for us.

    Since sidereal hour angle was designed to work with Greenwich hour angles, the idea of GHA had to come first. I believe the GHA approach to celestial navigation was developed by P. V. H. Weems, as a way to simplify air navigation. It first got a toehold in the American Nautical Almanac in 1934. Here's a quote from the 1934 Preface. Notice the mention of air navigation. 

    "The object of this volume is to provide the navigator, including the aerial navigator, with a compact publication containing all of the ephemeris material essential to the solution of problems of navigational position. The material is similar to that contained in the American Nautical Almanac for 1933, but tables have been added giving the Greenwich hour angle of the Sun, Venus, Mars, Jupiter, Saturn, and 55 stars."

    No SHA yet. Those small extra tables gave positions in GHA. And they were given only once for each month! Tabulating the GHA of 55 stars at short intervals would have taken a huge amount of space. The solution to that problem was Sidereal hour angle. SHA Aries can be given for each hour, and there's no need to give the SHAs of stars at short intervals. Their positions are almost constant. It's a convenient system, and takes little space.

    My guess is, the big shift to GHA in surface navigation didn't come until WW II. The Navy had to train a lot of navigators in a hurry. For that, GHA was the way to go. Besides being easy to teach, it was a perfect fit for the altitude-intercept approach to position finding. That's my theory, anyway.

    Bruce
       
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