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    Re: Index Error
    From: Trevor Kenchington
    Date: 2004 May 7, 12:52 +0000

    George Huxtable wrote:
    > On 5 May, Gary Harkins commented on the inconsistencies Robert Eno was
    > finding in his measurements of Sun diameter-
    > Bill Noyce had earlier that day offered a similar analysis
    > I think both of them were correct.
    So do I. And hence I do not see why this thread is continuing.
    George also wrote:
    > 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
    > effects.
    > Gary's suggestion, to use a sextant flat-on rather than upright for
    > measuring Sun semidiameter, is good advice.
    But the issue at hand was not a matter of measuring the Sun's
    semidiameter. It was determining index error. For that, it is sufficient
    to bring together the opposite limbs of the Sun, regardless of the
    apparent angular distance between them -- so long as that distance is
    the same when the two required measurements are made, which it will be
    if they are made immediately one after the other. There is no need to
    complicate the sights by holding the sextant at an unfamiliar angle.
    And that too has already been pointed out on this thread.
    Trevor Kenchington
    Trevor J. Kenchington PhD                         Gadus{at}iStar.ca
    Gadus Associates,                                 Office(902) 889-9250
    R.R.#1, Musquodoboit Harbour,                     Fax   (902) 889-9251
    Nova Scotia  B0J 2L0, CANADA                      Home  (902) 889-3555
                         Science Serving the Fisheries

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