Hello everyone,
I'm responding to the following very interesting query about lower-limb bubble sextant shots.
> Date: Sun, 9 Nov 2008 11:17:38 -0800
> From: bruce.hamilton@shaw.ca
> To: NavList@fer3.com
> Subject: [NavList 6475] Re: The Moon and bubble sextant
> Thanks Mike that worked well, and I tried another method that I would
> like to know if anyone else has had luck with. I put the lower limb in
> the center of the bubble. (My AP is my actual position on earth.)First
> result the LOP .3 NM from AP second try was 1.1 NM from AP. My luck
> factor was very high last night! I wish I had taken the a-12 out too.
I have three Mk IXs (I haven't tried any other type -- yet!) and I routinely use lower limb technique, with both sun and moon, and get better results with this than than with putting the whole disc inside the bubble.
I don't know about the 'A' series sextants, but one thing that helps on the Mk IX is that, during daylight illumination, the bubble shows a small central, circular core which can help you 'place' the lower (or upper) limb of the sun. (You don't get the same effect with night-time illumination, when the moon is often shot. In any case, my moon shots are hardly ever as accurate as my sun shots, which, with extreme care, and probably some luck, are often somewhere within 1 NM. Provided, of course, that the sextant is firmly placed on a windowsill.)
Earlier this year, I had email and phone contact with 25+ ex-RAF navigators who had trained on the Mk IX. Many used these instruments extensively in service. All but one said that they always followed their training and official documentation and put both discs in the centre of the bubble. However, one man said he preferred the lower limb, especially when he used the Mk IXBM, with telescopic eyepiece. And intrestingly, when Francis Chichester wrote his
Complete Observers Astro-Navigation (London: George Allen & Unwin, 1940), aimed at RAF navigators in training and dealing in some detail with the Mk IX, one of his worked examples (Vol. 2, p. 71) explicitly included an upper-limb moon observation. So some navs probably used them!
Kind regards
Ian Payne
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