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    Re: The Observer’s Book on Astro-Nav igation
    From: Geoffrey Kolbe
    Date: 2016 Apr 3, 07:25 +0100


     Francis Upchurch  wrote:


    I think the flying boat sextant was a very small, but very accurate version of a normal sextant.

    ​I have one of these 'flying boat' sextants. They were marketed after the war as compact yachting sextants - Hughes probably had an embarrassing stock of them left over. As for being 'very accurate', they read to one MOA on the drum as does a normal size sextant​, but it has no vernier on the drum cursor to read to either 6 or 10 seconds, as a normal sextant would. So, it has the same precision as a box sextant which will read to one MOA on the vernier. The certificate on the inside of the lid states 0' minutes error for 30 and 60 degrees, but -1' for 90 degrees, so it is probably trustworthy to +/- 1 MOA. In this regard then, the flying boat sextant was no more accurate than the box sextant.

    As for using a box sextant - or any other sextant - one handed, the only way this is practically possible as far as I can see is to pre-set the sextant to a the calculated altitude of the sun for the expected position at a given time. At the appointed time, the sextant is lifted to the eye to see if the sun is co-incident with the horizon, as expected. If not, the error can be estimated from the known diameter of the sun (32 MOA) and the estimated position adjusted accordingly. The sextant could be laid in the lap and adjusted for the next scheduled sighting using (still) just one hand. With a box sextant, you can set it to whole degrees with reasonable accuracy without having to use the magnifying glass. The scheduled times for the sightings could be arranged so that the expected altitude reading would be in integer degrees.

    Geoffrey Kolbe

       
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