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    Re: Index Error
    From: George Huxtable
    Date: 2004 May 7, 15:30 +0100

    On 5 May, Gary Harkins commented on the inconsistencies Robert Eno was
    finding in his measurements of Sun diameter-
    >One mistake you are making is subtracting out the index correction.  Since
    >you are joining opposite limbs of the sun, it is the total distance that
    >you are
    >concerned about.  For example, if you had a really terrible index correction
    >of -1 degree you would have both limb sightings on the arc.  For determining
    >semi-diameter this would not matter as it is the total difference divided by
    >four.  Applying the index correction when you are trying to figure out the
    >semi-diameter not proper.  For normal sights it is of course necessary.
    Bill Noyce had earlier that day offered a similar analysis
    I think both of them were correct.
    The content of Gary's posting has attracted no attention, which has all
    been diverted to the controversial "signature" below it. There may be a
    lesson there.
    Another message from Gary has escaped comment, in the same way, and perhaps
    for the same reason. In a later posting he said-
    >Another source of error could be refraction.  When calculating the
    >semi-diameter you should do it holding the sextant horizontal.  This
    >eliminates the
    >refraction difference between the upper and lower limbs.
    In principle, what he says here is correct. But Robert Eno made it clear
    that he was referring to Sun altitudes above 30deg. At 30deg, the
    difference in normal-refraction between the upper and lower limbs of the
    Sun is no more than about 1.5 arc-seconds, and gets rapidly less as the
    altitude increases. So differential refraction is unlikely to have been a
    significant factor in Robert's problem.
    But we need to be careful about this matter, particularly at lower
    altitudes. We are all familiar with a setting Sun that is obviously
    distorted, bulging at the sides, flattened at the bottom. On such
    (frequent) occasions, layers of air at different temperatures are giving
    rise to such gross distortions. Attempts to allow for such local
    distortions using the temperature and pressure corrections in the almanac
    are quite futile. Often, similar effects can occur that are less severe and
    not apparent to the eye, but atill enough to upset sensitive altitude
    observations. Altitudes of 30deg and above will usually be immune from such
    Gary's suggestion, to use a sextant flat-on rather than upright for
    measuring Sun semidiameter, is good advice.
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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