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    Re: Sextant certificates
    From: Fred Hebard
    Date: 2006 Jan 31, 11:13 -0500

    Jim,
    
    I didn't mean to imply that sextant certificates have no use.  I
    don't know about Ken Gebhart.  The certificates that show corrections
    every 10 or 15 degrees should cover most of the extremes of arc
    irregularities.  I would be more wary of the Hughes certificates that
    covered only every 30 degrees of arc.  Part of what Ken Gebhart was
    saying was that the measurements to develop the certificates were
    accurate to no more than 0.1 or 0.2 arcminutes, although I expect
    this could be improved with appropriate instrumentation.  This is
    also the point of the statements on Cassens + Plath certificates that
    the instrument is suitable for use, without specifying numbers
    
    Most modern sextants have errors of less than 1 arcminute, which is
    within the margin of error of position determinations from large
    vessels (about plus/minus one nautical mile).  Thus certificates are
    not especially necessary for modern navigation.  Older sextants and
    octants might have had much larger errors; for those, I'm sure the
    certificates were useful, especially for identifying inferior
    instruments and manufacturers.
    
    Additionally, if one were to drop one's sextant, it would be
    reassuring to have it recalibrated.  Just ordinary handling must have
    introduced some changes; one occasionally can see evidence for this
    on Ebay when multiple certificates are shown: there usually are
    changes in calibration.  I'm be surprised if the Russian, British and
    American Navies all maintained elaborate facilities for calibrating
    sextants and reissuing certificates if there were no use for same.
    
    For lunars, sextant certificates certainly would increase accuracy,
    as Bruce Stark pointed out.  We also have one list member, Doug
    Royer, who used to be fairly accurate in his sextant work, perhaps
    closer than one nautical mile much of the time, enough so that he
    would win the pool for closest position determination while underway
    on large vessels.  If his sextant had had arc errors (I don't believe
    his did), I'm sure the certificate might have helped him win the pool!
    
    Fred Hebard
    
    On Jan 30, 2006, at 3:44 PM, Jim Hickey wrote:
    
    > Thanks all for the feedback.
    >
    > My suspicions confirmed.
    >
    > Seems certificates represent a sense of formality without real
    > practical use.
    >
    > I find it odd that an instrument maker would use this sort of
    > convention. Considering the comments below, one could argue you are
    > actually better off to ignore the correction.
    >
    > -----Original Message-----
    > From: Fred Hebard 
    > To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
    > Sent: Sat, 28 Jan 2006 12:28:14 -0500
    > Subject: Re: Sextant certificates
    >
    > On Jan 27, 2006, at 9:45 PM, Ken Gebhart wrote:
    > > Jim,
    > >
    > > Here is my take sextant certificates. ....
    >
    > > 1. In addition to the regular 10 deg correction values, correction
    > > values for each in between degree (which you are not shown) can go
    > > quite far from expected values. For example on one sextant, for
    > > 40, 50, and 60 degrees, we have corrections of +10, +9, and +2
    > > secs respectively. Not bad. But at 47 and 54 degs we have a
    > > correction of 0! So, you can see it is not a straight line
    > > variation between the 10 deg values.
    >
    > Thanks for sharing this with us. The one place where I have seen
    > data like these is for late-model Husun sextants available for sale
    > on eBay, where a photo of a British Admiralty certificate is
    > inclu! ded. The certificates include a continuous curve, although I
    > don't know whether that represents continuous data or interpolation
    > between set measurement points. On these certificates you can see
    > the errors wandering considerably between the standard correction
    > points.
    >
    > > So, if you combine all of these uncertainties, it makes the
    > > accuracy certificate something to be taken with a grain of salt. I
    > > have a SNO-T sextant with a certificate that says straight zeros
    > > across the board. Of course this is ridiculous. Whenever I see a
    > > claim by a manufacturer of less than +/- 15 sec. I believe they are
    > > making a marketing statement rather than a true one.
    >
    > The larger Husuns were the "luxury" models compared to the "Mate."
    > Oftentimes these would show zero or near zero corrections, both on
    > National Physical Laboratory and on Husun! certificates, whereas the
    > Husun certificates issued with "Mates" usually show errors of up to
    > 50 seconds. Because of this, I believe the certificates for the
    > large Husuns may not have been lying too awfully much.
    >
    > An optical physicist described a few years ago on the Yahoo sextant
    > list his testing of his Astra sextant. He said it was very good.
    >
    > Fred Hebard.
    >
    
    
    

       
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