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    Re: Use of Marine Sextants in Aircraft
    From: Brad Morris
    Date: 2018 May 9, 14:21 -0400
    Hello David

    There may be two issues with the airship shadow principle.  The principle supposes that the airship is essentially parallel to the surface upon which the shadow is cast.

    In the first instance, we can see that if the airship is not parallel to the surface of the water, then the shadow length will obviously be changed as a function of the tilt of the frame of the airship.  With the ponderously slow rates of motion due to the rather large inertial mass, it may not have been as simple as the navigator telling the pilot to "hold it level".  The pilot could tell the navigator when it was level so perhaps the navigator could get around this issue.

    Suppose the airship shadow is now cast onto land (not water).  The length of the shadow is now compromised by the terrain.  Airships didn't just cross water, they crossed land as well.  The shadow as an altimeter principle will suffer greatly under this circumstance.  

    The principle does have undoubted merit, and these minor objections may be easily obviated, yet they remain.



    On May 8, 2018 5:03 AM, "David Pike" <NoReply_DavidPike@fer3.com> wrote:

    Lars you wrote concering the Hughes Aircraft Marine Sextant: What was the intended use in RAF for this small sextant? 

    In the early days it was common to use a marine sextant for air navigation.  It was used in the R34 airship, by Alcock & Brown, by Coutinho & Cabral, and in the Graf Zeppelin airship.  The only problems were: it might be night time, the horizon might be obscured by cloud or mountains, or you might have difficulty knowing your exact height to calculate dip, because in mid-ocean you might not know the surface pressure to apply to your pressure altimeter.  The advantage of a marine sextant as opposed to a bubble sextant was that you didn’t have to worry about acceleration errors.  Anecdotally, Coutinho only used the famous bubble attachment for his marine sextant when the natural horizon wasn’t available.  I read somewhere, but I can’t remember where, that on the approach to St Paul’s Rocks, he did have a natural horizon, so he didn’t need use his bubble attachment.  Also anecdotally, the crew of the R34 airship calculated dip by checking their height over the sea by using their marine sextant to measure the angle subtended by their shadow, but that takes a bit of getting your mind around deciding if it’s a 2D or 3D problem.   

    Maritime aircraft usually flew at low altitude, and in some parts of the world in perfect sunlight, so it made sense to have a marine sextant available.  It might also have been used while fleet shadowing to measure ‘distance off’ to keep out of range from a capital ship of known dimensions (e.g. at 3nm a 300’long ship would subtend approximately one degree, 300/6080x3, 1 in 60);  and for a flying boat, when on the sea to check position, or horizontally when taking soundings close to land. DaveP

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