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Dear Robin,

I have looked at:www.fer3.com/arc/m2.aspx?i=113627 (second jpg) and now understand what is your query.

It seems to me that Janet Taylor has it wrong completely.
There is no proposition 20 book XII in the modern set of Euclids Elements which I looked up; but there is a prop 20 in book XI which deals with _solid geometry_ and solid angles...

prop 20 book XI: "If a solid angle is contained by three plane angles, then the sum of any two is greater than the remaining one".

Which might be causing the confusion in the text with Janet Taylor because spherical angles are not solid angles.
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In a spherical triangle, you can have, in a limit, two sides of 90 and one of 180 -which is a hemisphere.
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I have put the values for the spherical triangle in my calculator with HP as quoted, (now having amused myself recently by programming it for clearing lunar distances) and find the result to be: 103deg-0'-35".2
... not the 103deg-0'-16" as suggested by Janet Taylor in the text.
Perhaps my calculator comes up with slightly different value because it includes 'modern' refraction correction which might be different with what they used then.

Douglas Denny.
Chichester. England.
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