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    Re: Circular sextant?
    From: Peter Monta
    Date: 2017 Jul 29, 23:50 -0700
    Hi Bill,

    It's a reflecting circle.  Just as you say, it looks like a sextant, and it has the same operating principle.  The advantage is that it's self-calibrating, because the circular scale can be arbitrarily rotated and redundant measurements taken, permitting the target angle and the scale errors to be jointly estimated.  It's bulkier for a given radius, though, and I guess somewhat more expensive too.

    "Reflecting circle" on images.google.com pulls up a number of nice examples.  I think the name of Borda is associated with the device, but I'm sure others here know more of the history.

    Cheers,
    Peter

       
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