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    Re: Advice concerning sextants
    From: Frank Reed
    Date: 2010 Feb 22, 16:23 -0800


    You wrote:
    "It is with some trepidation that I post the jejune request of a neophyte to celestial navigation for help concerning sextants."

    Trepidate not, O Neophyte... :-)

    Please understand that while there are some rather rarefied discussions that come up on NavList, most of us very much enjoy the basics, and no questions are too basic! And we won't roll our eyes and tell you to go "read the FAQ file" (we don't have one, and I personally think that's a good thing for this very reason).

    You asked:
    " What is the best buy for a beginner who wants one that is usable for actual navigation but who will probably never depend on it for that? Are the Davis plastic sextants acceptable?"

    Yes. Definitely. You could always get a plastic sextant to start with and if you decide to go for a more serious, more expensive sextant later, you can sell the plastic one on ebay and recover most of your original expense. It has been my experience that Davis plastic sextants sell for about $75-$100 in good condition on ebay, and that price seems very stable. There does not appear to be any substantial depreciation --used is used-- unless the instrument is actually damaged.

    But consider this: you can get a very good metal sextant, which is unquestionably superior as a measuring instrument and also just feels great and looks great for about $250-$350 on ebay. So if you think this will be a lasting interest, you may just want to save up a few more ducats and explore that market. Just make sure that the item you're looking at is a real sextant and not a reproduction. Most of the "sextants" for sale on ebay are non-functional reproductions. If it's polished metal all over, it's probably not a real sextant.

    The main difference between a plastic sextant and a metal sextant is that plastic is thermally unstable. In other words, as the temperature changes by a degree or two, the index error of the sextant might change by a minute of arc corresponding to an error of a nautical mile typically in position fixes. By contrast, a good metal sextant will generally not change its adjustment unless you bang on it.

    And you wrote:
    "What are the chances that one bought used (ebay has many) is still accurate? "

    Very good actually. With few exceptions, all sextant "errors" can be adjusted out. The instrument was designed from the very beginning, nearly 300 years ago, so that a careful observer could reset it to take accurate sights even if it had been knocked around or mis-handled. You'll want to learn the various steps required to set the mirrors perpendicular and, most importantly, how to measure index correction (not to a tenth of a minute of arc, which has been discussed recently on NavList and takes some very careful work, but just to half a minute or so, which is relatively easy).


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