A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: IC Payne
Date: 2008 Nov 17, 10:48 +0000
Bill Morris ('engineer') has alerted me to your message about aircraft sextants, and I'm replying to your 2nd question as it relates to the Mk IX series of sextants.
First, a word of introduction. When I first became interested in restoring and using the Mk IX series, I emailed the RAF association to ask if any ex-navigators would be willing to help with the finer points of using them. I'd expected one or two replies, but in the event I got over 30, from all over the UK! Most had trained after WWII, but a small but significant number of wartime navs replied, by email, letter or phone with their experiences and information. All 30+ navs who replied had had training (at nav school, often in Canada) on the Mk IX series. Each nav had a log book in which they had to complete a certain number of shots on the ground and in the air and record the reading, together with the error (if any). Results in these tests varied very widely as to accuracy both on the ground and in the air.
Now to answer your question about use in the air. Most of the navs who responded said that, though some of them enjoyed astro and using the sextant, astro was their least favourite method because it represented so much work in the air, where time passed so quickly and there was so much else to do. Furthermore, wartime Bomber Command navs hardly ever used the Mk IX series on operations, partly for obvious reasons (e.g. pilots were reluctant to fly straight and level while sights were taken) and partly because, especially as the war progressed, easier and more reliable nav aids became available, such as H2S. I don't think that a single wartime nav who replied to me (none of whom served earlier than 1944) ever used a sextant on operations, though it formed part of their kit and hung in the aircraft's astrodome in case it was needed.
One area where it was indispensible, though, was in Transport and Coastal Commands, and the navs who served in them had the most extensive experience of using it. One man told me they couldn't have managed without it.
The sextant was suspended from the centre of the astrodome and the operator would have his shoulders pushed against the edge of it to give a steady observation. Describing the models with clockwork averager, one post-war nav wrote to me: 'the mechanism just continues running once you have pulled the trigger, thus averaging the angle observed over a minute or two minute run. We kept the object being observed in the bubble by use of the Fine Tuning, degree & second controls. For the timing of the observation (or ‘shot’ as we called it) we would take the time at the start of the averaging observation and add one minute if using the two minute run or 30 seconds if using the one minute run. The averaging device would not be necessary on the ground if the sextant was on a tripod or other firm base. When ever we practised using the MKIX on the ground at Nav school we always used the averaging device'.
One of the things that surprised me was that the Mk IX was not limited to wartime use, but was used for training (and to some extent in service) afterwards, until superseded by the periscopic sextant. Even the original Mk IX (which had a six-shot totalizer that had to be operated manually and was superseded from 1941 by the more familiar IXA with clockwork averager at the front) was used for ground shot practice: one of my Mk IXs, made in 1940, has a 1950 calibration certificate, but after 1941 the Mk IXA (and later variants) was always used in the air. I have a 1943 MkIXBM (with choice of ordinary or telescopic eyepiece) that was in training use as late as 1962.
If you want to pursue the use of the MKIX in WWII, I can recommend Francis Chichester's Observers Astronavigation, published in four parts 1940 and intended for nav training during the war. It has chapters devoted not only to the Mk IX instrument itself, but also advice about observations and a discussion of the sorts of accuracy to be expected. There are 2nd hand copies about. Understandably, these instruments are much easier to use on the ground (I can often get a result somewhere within 1 NM if the bubble is kept perfectly still) than in the air (where Chichester talks about '7-mile accuracy', though greater accuracy was possible). I'm told they were hard to use in the air because the effect of plane vibrations, coriolis effect and other forces on the bubble, and the accuracy depended on keeping the object centred in the bubble whose equator marked the horizon. The Mk IX is superb for backyard astro, 'on the ground'.
Does this help? Please ask if you have any more specific questions.
PS a note on cases. The Mk IX boxes I've seen are of two basic types, mostly of bakelite though roughly the same shape. The original Mk IX (without averager dome) had a smaller, more compact case into which the instrument fitted very snugly. The later models, with protruding clockwark dome, had a case which, though essentially the same shape, was taller with space in the lid to accept the dome and winding key. Some of the latter types even had a shallow, raised metal dome inserted in the lid to accommodate the averager. Several navs commented to me that the 5 Kg cases banged against legs and shins when carried out to the plane; but if you remember to hold it so that the tapered part of the lid is facing down, closest to the leg, this doesn't happen!
Subject: [NavList 6559] Sextant boxes.
Date: Sat, 15 Nov 2008 20:31:31 +0000
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