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    Re: Reliable Index Correction to a Tenth Minute of Arc; WAS Longitude by lunar altitudes
    From: Bill Morris
    Date: 2010 Feb 17, 12:20 -0800

    Brad

    You wrote:

    "As to the recalled difficulty with different telescopes, I would suggest that this implies a lack of parallelism between the plane of the arc of the instrument and the long axis of the telescope. "

    I think I am correct in saying that as the sextant reading approaches zero, so does the error from mis-collimation reduce. At a reading of 10 degrees, for example, Simms gives the error as only about 5 seconds for a tilt of a whole degree (an amount that should be noticeable to most people).

    I suggest that those who believe they can correctly estimate their index error to a tenth of a minute on the basis of a single estimate deceive themselves. I would invite them to estimate it thirty times in a row and if they can get the same result within a tenth of a minute each time, I will be among the first to congratulate their instrument.

    Kind regards

    Bill Morris
    Pukenui
    New Zealand
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