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I had referred to the section on sextants by Andrew David (ed), in vol 2, 
"Voyage of the Resolution and Adventure" of "Charts and Coastal views of 
Captain Cook's voyages". I mentioned that  David's account  "reports that 
Wales and Bayley , the astronomers of the expedition, measured Sun lunars 
of up to 155º!"

Brad comments, sagely-
"With the usual caution to not observe objects below 10 degrees due to 
refraction, we can then assign the greatest inter-object angle observe-able 
at 160 degrees.  Why?  Because 160 + 10 + 10 = 180. In other words, the 
lunar you wrote about is near to the extreme of possible measurement!  Even 
if we let both objects rise to equal altitudes, neither is above 13 
degrees.  Clearly, this is a near full moon - sun lunar.

I would like to hear more about this lunar.  Were the altitudes computed or 
observed?  Were the Nautical Alamac tables of the day providing angular 
values in this range?  How well did the computation work for them?  How 
does it compare to Bruce Starks result?  Frank Reed's result?  Do the 
equations hold at this extreme or do they break down?

My curiosity has been engaged!"

===================

Mine too. I would like to learn a bit more, and have resolved to look up 
Wales and Bayly on my next visit to the Bodleian.

Unfortunately, David is not very forthcoming. He writes, on page xx of his 
introduction, "Both Wales and Bayly used Dollond's sextant to measure 
angles up to 155º between the Moon and the Sun on a number of occasions". 
However, on page xxv, he provides more detailed references to the relevant 
work, which is-
Wales W, and Bayly, W. "The original astronomical observations, made in the 
course of a voyage towards the South Pole, and Round the World." London 
1777. I haven't discovered a digitised version that's available (to me). If 
anyone else manages, I would be keen to know about it.

David wrote on page xxv- ""Wales also observed angles exceeding 120º 
between the Sun and the Moon on 21 occasions... These observations once 
again involved Wales in lengthy computations since distances between the 
moon and the sun exceeding 120ºwere not listed in the Nautical Almanac." He 
adds- "Bayly too observed angular distances exceeding 120º between sun and 
moon on a number of occasions, his greatest angle being 155º 13' 7" 
obtained on 4 August 1773 when in 20º 49' S".  That date should allow Brad 
to investigate the circumstances, which as he says  must have been within a 
couple of days of full moon, one side or the other.  Bayly was astronomer, 
not in Cook's ship, but with  Furneaux in "Adventure".

He mentions that the Dollond modification (for backsights) "was not ... 
suitable for observing angles in excess of 120º between the Moon and one of 
the fixed stars due to the difficulty of keeping a small and relatively 
faint object such as a star within the field of vision". He goes on to say 
"...lunar distances between the Moon and the fixed stars were not tabulated 
in the Nautical Almanac for angles greater than about 105º and usually they 
did not exceed 90º.

I think David misses the point here. There's really no shortage of target 
stars around the ecliptic, so once the lunar distance between Moon and star 
got inconveniently large, the observer could always switch to another star 
which was more suitable. However, there is only one Sun, so if a navigator 
wished to use Sun lunars with a very gibbous Moon, there was no choice but 
to try to measure such big Moon-Sun angles. Since those very early days, I 
know of no other attempts to observe lunars exceeding 120º..

George

contact George Huxtable, at  george@hux.me.uk
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

   

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