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    Re: Index Error
    From: Robert Eno
    Date: 2004 May 7, 15:54 +0100

    Thank you to all for your insightful and informative answers. I would
    like to take one last kick at this dead horse to make sure that I am on
    solid ground. This particular aspect of my inquiring mind used to drive
    my instructors crazy. Pity the poor sod that has me as a student.
    The consensus is that the method of determining the index correction,
    which I earlier described, is sound. It also appears that my taking out
    index error so derived, before adding the off and on values (then
    dividing by four to obtain a semi-diameter) is incorrect.
    So let me get this straight:
    1. Any index error in the sextant will be reflected by one reading
    differing from the other; that is the upper limb observation may be
    greater than it should be, while conversely, the lower limb observation
    would be less than it should be. They should balance each other out.
    2. The sum total of the off and on readings divided by four should yield
    the correct semi-diameter. There is no need to factor in the index
    correction because one value should balance out the other. In otherwords,
    the index correction is already inherently factored in by virtue of the
    upper and lower limb values differing by X minutes of arc.
    Am I on track here gentlemen?
    >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
    >> that he was referring to Sun altitudes above 30deg. At 30deg, the
    >> difference in normal-refraction between the upper and lower limbs of
    >> 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
    >> rise to such gross distortions. Attempts to allow for such local
    >> distortions using the temperature and pressure corrections in the
    >> 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
    >> 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
    >                      http://home.istar.ca/~gadus
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