NavList:

Francis

I suppose that the results of the 1957 investigation into the accuracy of astronomical observations at sea represent a 'best guess', given the limitations of the data.  Has there ever been a comparable study?  Henry's 2009 post has interesting insights into alternative ways of judging accuracy.

However, the 0.7' of arc 'result' is entirely feasible.  I can say this having seen my own and many other star fix cocked hats on ship's plotting sheets.

The comparison with obtainable results on 30-40 ft yachts is of course unfair, as I also know from first hand experience.  I carry a Freiberger yacht sextant and a Davis Mk 3 on my 36 footer, but they now spend more time in their lockers than my fishing gear, which also produces statistically dodgy results.

SR methods are are always interesting to me.  I note your enthusiasm for slide rules.  I own a few of these and my problem is tiny, closely spaced numbers and 'slippage' - made worse on the boat by cold, wet hands.  However, I do occasionally play with my 10" Blundell Harling Academy 504, marine navigation model, which features in JC Podmore's book The Slide Rule for Sea and Air Navigation. Still available from the publishers, in brand new hardback, at the astonishingly low price of £2.76, plus postage. Many people seem to have difficulty extracting numbers from tables and adding them up, which I find relatively easy.

I am now trying to find my copy of Francis Chichester's Gypsy Moth Circles the World, in which I seem to remember he remarks on having forgotten the cosine-haversine formula, which he needed at some point. Perhaps too much past reliance on his Bygrave manipulations?

Thread drift warning light comes on...

Cheers

John