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Thanks to Henry Halboth and Bruce Stark for helpful responses to my
question about the existence of precise tables for predicting the moment of
sunset.

Scoresby would do his whaling in latitudes that would sometimes take him as
far North as 80deg, in seasons when the ice allowed it. But in each of the
sunset observations I'm investigating, Scoresby was on his way North, in
latitudes 62 to 65 North (near Shetland), up to 72 deg, but not yet in
Arctic circle midnight-sun waters.

Of course, once he is seeing a midnight Sun, there are no sunsets to
observe. And at dates close to that situation, when the Sun is setting, but
not going far below the horizon, the Sun-setting will be a slow and
long-drawn-out process, not leading to any accurate timing.

Henry asked-

"do you suppose he could have been making some use of Norie's Table XLV -
"For finding the time most advantageous for observing the altitude of a
celestial object, in order to obtain the apparent time" - by considering
sunset as the Sun's closest approach to the Prime Vertical and working
backwards by interpolation to arrive at a time."

I doubt that; the dates of the sunset observations I'm trying to sort out
are late March / early April, and in Northern latitudes, after the spring
equinox, the Sun has already passed through the Prime Vertical as it sets.
Also, looking at Table XLV, it doesn't distinguish between the Sun and
other celestial objects, so appears to make no allowance for semidiameter.
In which case, I presume, it's not intended to give results to any great
accuracy. In that respect, another possibility might be that if sunset
happened to be defined, for this purpose, as the Sun being bisected by the
horizon, then there would no longer be any need to allow for Sun's
semidiameter. Is that a possibility?

I thank Henry for such imaginative thinking, and beg him not to stop, if he
gets other such ideas.

As for Bruce's suggestion of Wakeley's "Mariner's Compass Rectified", I'll
search that out on my next library visit.

George.

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