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    Re: Index Error
    From: Frank Reed CT
    Date: 2004 May 5, 05:54 EDT
    Robert wrote:
    "Question for the list on some fundamentals and I add that I am somewhat embarrassed to ask such a question. "

    Hey, I wish I would ask more of the questions that I feel a little embarassed about. Being raised in Connecticut made me too shy and too paranoid about taking the risk to be wrong. Living in glorious Chicago taught me that taking the risk is the whole point... (but I still don't always do it).
    And you wrote:
    "A method I have long used for determining index error when taking practice observations on land is to use the sun's limbs by setting the sextant at 32' on arc, touching up the reflected and actual sun's limbs, recording the number, then doing the same off arc. One half the difference between the value on and off arc should give you your index error; the sign being the greater of the two values."

    Yes, it's probably the best method for getting index error, and it's described in some of the earliest navigation manuals c.1790s. I don't think you need the bit about presetting the sextant to 32 minutes on arc, but then again it does no harm. Also, this method will work with ANY object that has a clearly defined edge (see PS) and is more than a few miles away. In practice, there are very few such objects besides the Sun and the Moon. I suppose you could even apply it to a short star-star sight if the conditions were right...

    And you wrote:
    "I have found this to be a reasonably accurate method for determining index error; as long as the sun is at least 30 or more degrees above the horizon. "

    It's very accurate, but I don't see why you should worry about altitude. I think it will work even if the Sun is severely flattened by refraction close to the horizon as long as you do the on-off pair reasonably close together in time. Better yet, you could make the measurement "sideways" since the horizontal diameter of the Sun is not affected by refraction (at any remotely significant level).

    Of calculating the Sun's SD from the measurement, you wrote: 
    "I have seldom found this to be true. The result often differs from the tabulated value by up to 0.4' of arc. That's a lot."

    It is a lot. I don't see the mistake off-hand, but the technique should certainly work so something's amiss somewhere.
    Frank E. Reed
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois

    PS: A lot of celestial navigation depends on the fact that the Sun has a sharp, well-defined edge. And that's practically a miracle. The Sun, of course, has no literal surface. Very suddenly, it goes from being opaque to being transparent, and that's responsible for the light-emitting surface --the photosphere. It turns out that for stars like the Sun, this sharply defined surface (the Sun's limb, for a navigator) is created by an exotic ion. The Sun's visible edge is created by the H- ion, a hydrogen atom with an extra electron weakly bound to it. That such a thing even exists should be a surprise for almost anyone with a basic chemistry and/or physics background, and the fact that this odd ion is responsible for one of the most clearly visible features of our Sun, its "limb", is really quite extraordinary.
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