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    Re: The Online Nautical Almanac
    From: Frank Reed CT
    Date: 2004 Jul 19, 23:12 EDT
    I wrote earlier:
    >if you wanted the positions of the stars, for a historical lunars analysis
    >maybe, in 1804, which would you recommend: Omar Reis's Online Nautical
    >Almanac (as modified), or Centennia Software's Online Nautical Almanac?

    George H replied:
    "I really couldn't say. I haven't investigated Centennia Software's Online Nautical Almanac."

    Sorry, George. I didn't mean to be obscure.
    (Centennia Software) = (HistoricalAtlas.com) = (me).

    In any case, let's take a specific example. For June 22, 1821, the position of Pollux is
    SHA = 246d 24.6'
    Dec = N28d 27.1'
    according to my Online Nautical Almanac.
    According to Omar Reis's Online Nautical Almanac (even after recent modifications), the position is
    SHA = 246d 23.9'
    Dec = N28d 28.2'.
    The distance between those two positions is about 1.3 minutes of arc --not a big deal for standard celestial navigation but a very large error if one were using this position as an input for a lunars calculation. Many other stars have similarly large errors in their positions.

    As I said earlier, the LARGEST problem with O.R.'s star positions arose because he failed to include proper motions. However, the positions are by no means correct yet. I can see at least two independent issues at work in his star positions, but I don't have time to puzzle out the source of these errors (this time around).

    Centennia Software's Online Nautical Almanac is accessible here:

    So far, my online almanac data tables and predicted lunars have passed every test I've thrown at them. They are accurate to a fraction of a second of arc in star positions and 1 second of arc (largest allowed error) in the positions of the Sun, Moon, and planets.

    Since I began keeping count on July 7, my Online Almanac & Predicted Lunars tools have been used 412 times. Everybody please let me know if you encounter any problems. I am working on yet another set of enhancements and hope to have them available soon.

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
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