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    Re: Lunar distance measurement in ideal conditions: attainable accuracy.
    From: Bill Morris
    Date: 2013 Jan 15, 22:20 -0800


    Today I had technical difficulties with the tripod and with the bones of my neck. Eventually I gave up with the tripod and just shot two lots of three lunars, holding the sextant in my hand. Instead of the SNO-T, which kept giving me flashes of unshaded sun when viewing the sun directly, I used a C Plath dating from 1975, fitted with a Beck Kassel 6 x 30 prismatic monocular. Being bronze-framed, the sextant is a heavy beast. The big advantage of a monocular over the SNO-T's 6 x 30 'scope is the wide field of view, which makes acquiring the objects much easier.

    I took just one shot of the horizon for index error estimation, more closely mirroring normal practice at sea.

    I attach the results as Lunar 3.

    The first set of lunar distances gave a mean error of observation of 0' so, fortified by an evening meal and doubting myself, I did another set of three with a mean error of observation of -0.3' and a grand mean of the six shots of --0.15. I am not sure whether the error in longitude should have the same sign as the error in observation. The calculator does not give signs, so I left all values positive when calculating the means of longitude error.

    Unsurprisingly, the results are not as good as for an instrument rigidly held, with an index error determined from many (25 observations) rather than just one, but it seems that lunar distance observations made with a well-maintained and adjusted modern sextant are possible with a reasonable degree of accuracy.

    Bill Morris
    New Zealand
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    File: 121972.lunar-3.xlsx
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