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Henry Halboth wrote-

>I have rather extensive examples of Longitude determination by sunset +
>sunrise which alternately use both upper and lower limbs at the time of
>occurrence. In any example that I have access to the resultant negative
>altitude is used in a conventional Time Sight solution. These examples
>are c. 1900 and make no reference to special tables and, to me at least,
>there seems to be little need for any such as the solution is relatively
>simple.

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Thanks, Henry.

Yes, I agree, there's no problem in calculating that hour-angle from
spherical trig. I can do it, and so, no doubt, could Scoresby, if he had
to.

It's just that he says specifically that he got it from a table that makes
me wonder what that table was.

I agree, too, that it's perfectly possible to use upper or lower Sun limb,
against the horizon, for a time-sight, or perhaps both alternately. I ask
Henry which of these a professional mariner would refer to as "the moment
of sunset".

George.

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