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    Re: Refraction for beginners
    From: Frank Reed CT
    Date: 2004 Jul 9, 20:58 EDT
    Bill also wrote:
    "Would also guess that the parallax change etc. by moving closer or farther
    from the Earth's center because of tides is minimal, but might encounter
    that and other problems with a mountaintop bubble shoot and HO229."

    The parallax change is negligible since mountain heights are a very small fraction of the Earth's radius. By contrast, mountain heights are a significant fraction of the "scale height" of the atmosphere.

    And added:
    "After all, the cover of my HO229 does say, "Sight Reduction Tables for
    Marine Navigation." Sounds like HO249 (which I have not explored yet) is the
    way to go for mountaintop bubble shoots."

    This distinction isn't really so much about altitude (although 249 does cutomarily include the height above sea level correction table). It's more about speed of computation and expectations of accuracy. Vessels on the ocean travel relatively slowly while high-altitude jets travel quite fast (dozens of times faster). Since you're moving so fast in a plane, a celestial fix has to be obtained very quickly. Hence HO 249 focused on a very small number of stars with precalculated Hc values for those only. A line of position could be plotted quite quickly. In addition, the tables are less accurate than 229 since a position accurate to a tenth of a mile is much less likely to be useful when you're travelling 500 miles per hour. But HO249 has been sometimes very popular with marine navigators in small craft, too, since it is extremely easy to use (smaller books too). Its disadvantages include a smaller selection of objects available for use and the fact that it has to be updated now and then since the stars' positions are gradually changing.

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