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    Re: Moon's 4SD
    From: Bill B
    Date: 2006 Nov 02, 15:21 -0500

    I understand the mechanics.
    What I am getting at goes to a thread a while ago.  Vertical sun IE checks
    even with the sun over 60d elevation do not match horizontal IE checks.  4SD
    for vertical IE checks will be slightly over, 4 SD for horizontal checks at
    or under interpolated almanac 4SD values, so clearly not a "squished" sun.
    When I switch to the eye where the astigmatism is rotated approx. 90d of the
    other eye, the opposite happens.  I ran all sorts of tests, including
    inversion and substituting photo gels over the scope for shades.  Not the
    instrument's fault.
    The problem is in my eyes.  The point I am getting at here is that MY
    PERCIEVED IE (not the instrument's IE) changes as I rotate the sextant from
    vertical to horizontal as one might do with a lunar.  What my perceived IE
    is between vertical and horizontal is anyone's guess. In this case of lunars
    I would be better served using my horizontal IE value.
    That is why I favor an IE check from a line, (or point source if I could do
    do so well) rather than large spheres which my eye distort).  I am still
    working on how to reconcile the difference between vertical sun and horizon
    IE's to give me a working IC for sun and moon shots with a natural horizon.
    Put another way, if I had two identical stars equidistant from the moon's
    center, one below and one to the side, I will get two different angular
    measurements due to my eye distorting the sphere.
    > Bill,
    >> but have no clue what is happening
    >> when I tilt the sextant to do a moon IE
    >> check.
    > Why don't you just try, using the current nice weather
    > and good Moon in the evenings:-)
    > It is like the normal "swinging".
    > In one position of the sext, two images of the Moon
    > just touch, when you swing, they slightly separate.
    > This separation becomes smaller and smaller as the Moon
    > becomes fuller.
    > With 100% full moon, swinging has no effect.
    > Alex.

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