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    Re: Lunars for exploration: western China in 1870
    From: Frank Reed
    Date: 2012 Oct 6, 13:27 -0700

    I wrote previously:
    " I am attaching a page image with the details in case anyone wants to work up these lunars with modern tools. Does the accuracy hold, or was it a lucky accident??"

    Sorry, at first glance I thought that the page included more details of the observations. It gives the resulting longitudes for two sets of lunars, one by three observations of the angular distance of the Moon from the Sun and the other by five observations of the distance from Altair (alpha Aquilae). There is still something to consider with respect to accuracy though. The computer, William Ellis, has averaged the two sets which differ by 38.5 minutes of longitude. That's where the very accurate longitude 77d 14.75' comes from: 50% for the Sun lunars, 50% for the Altair lunars. He has averaged them with equal weights. If he had averaged them with weights based on the number of observations in each set (a more valid procedure?), then the longitude would have been 77d 19.5', almost five minutes east of the published position. So just on that basis, it's pretty clear that the exceptionally accurate result was just good luck. Myself, I would actually give more weight to lunars using the Sun, pushing the longitude five or ten miles west. In any case, +/-10 minutes sounds to me like reasonable error bounds on this result.


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