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    Re: Index Error
    From: Bill Noyce
    Date: 2004 May 5, 10:35 -0400

    I don't think you should be performing this step:
     62.6 - 0.7 = 61.9
    You observed two diameters.  Let's correct each of them
    separately for the derived index error:
      32.0 - 0.7 = 31.3
      30.6 + 0.7 = 31.3
    Therefore your measurements say twice the diameter is 62.6,
    which is the same as you get by simply adding the two raw
    observations together.
    Thus, your measured semidiameter is 15.65 -- a bit closer,
    but still not exact.
    If you measure the SD to be too small, it could be explained
    by flattening due to refraction.  Basically, the altitude of
    the lower limb is less than the altitude of the upper limb,
    so their refraction corrections would be different.  But the
    difference should be no more than 0.05' for reasonable altitudes.
    If you measure the SD to be too large, it could be explained
    by the illusion that brighter objects appear larger -- is this
    called illumination?
            -- Bill
    -----Original Message-----
    From: Navigation Mailing List
    [mailto:NAVIGATION-L{at}LISTSERV.WEBKAHUNA.COM] On Behalf Of Robert Eno
    Sent: Wednesday, May 05, 2004 4:57 AM
    Subject: Re: Index Error
    Good point but this was taken into account. For example, yesterday, I
    came home for lunch and couldn't resist taking a quick noon sun shot.
    Index error calculations were as follows:
    32.'0 on  30.'6 off
    Difference = 1.4
    1.4/2 = 0.7
    IC = -0.7
    Now let's take the next confirmatory step:
     32.'0 on + 30.'6 off = 62.6
     62.6 - 0.7 = 61.9
    61.9/4 = 15.5 (rounded up)
    Actual semi diameter = 15.9
    add'l error = 0.4'
    In calculating my noon latitude, I used an IC of -0.7', ignoring the
    other mystery error. My noon latitude differed from actual by 0.1'.
    Pretty close and one could say: "fluke", but I have been getting this
    kind of consistent accuracy with my sextant.
    So the question is, where in the hell does this "other" error come from
    and if I wanted to factor it in to my IC, how do I go about doing this?
    Theoretically, the sum of my off and on readings should have been 63.6
    but I got 62.6.  I should know all of this by now but even someone with
    experience needs to go back to the fundamentals every once in a while.
    I am baffled.  Additional comments welcome.
    >According to the 2004 nautical almanac the semi-diameter of the sun
    varies between 16.3' at perihelion to 15.8' at aphelion. Or the diameter
    varies between 31.6' and 32.6', for a difference of 1.0' or 2 X 0.5'
    >Could this be the source of the error?
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