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    Re: Advice concerning sextants
    From: Patrick Goold
    Date: 2010 Dec 29, 23:47 -0500
    Thanks to all who advised me  in the matter of sextants!   I finally acquired one, a Tamaya, after haunting ebay for months.  It appears to be in excellent condition and good adjustment.  It arrived on Christmas eve.  The weather has been rather poor for skywatching.  Today was the first clear day for a noon sight.   I was startled to discover that Portsmouth, Virginia is some 23 miles south of where the mapmakers seem to think it is!     I could have made an error, I suppose.  My boat being on the hard, I had to take the sight from my front garden and that required using an artificial horizon.  Many problems with this.  Wind disturbs the reflecting surface even with the glass covers in place.  The sky was a little hazy, so the direct image of the sun was much less distinct than the reflected one.   All the book reading and mathematics did not prepare me for the messiness of making the actual observations.   The effect of holding the sextant even slightly off the perpendicular is dramatic. It is going to take some practice to get good at this.   I find this quite fun!  Thanks again for the help!

    Best regards,
    Patrick

    On Mon, Feb 22, 2010 at 7:23 PM, Frank Reed <FrankReed@historicalatlas.com> wrote:

    Patrick,

    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).

    -FER

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    --
    Dr. Patrick Goold
    Department of Philosophy
    Virginia Wesleyan College
    Norfolk, VA 23502
    757 455 3357

    Charles Olson: "Love the World -- and stay inside it."

       
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