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