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Frank asks an interesting question-


"On May 11, 1788, William Bligh aboard the Bounty in the South Atlantic
writes in his logbook:
"Got a sight of the Sun exactly at Noon by the TK which I believe to be
within a mile of the truth" (in case it isn't clear, he's probably saying
that he believes that the resulting latitude is within a mile of the
truth).

And again on August 5, 1788, In the Indian Ocean, we have:
"Lat Obs is from an Alt exactly at Noon by T Keeper"

"TK" and "T Keeper" are just the chronometer, of course, which was known as
a "Time Keeper" until c.1800. But what's his navigational theory here?? It
strikes me as a bit unlikely. He's using the chronometer to determine the
instant of noon (maybe in cloudy weather with only brief glimpes of the
Sun). Of course he can calculate the time of local noon if he has the GMT
from the chronometer and a very good idea of his longitude from some other
source --and no he didn't have any other source on these dates except DR.
Was he just confused? Theories and speculation welcome. :)"


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Response from George-

No, I don't think Bligh was confused.

Bligh was recording, in the log, each day, Bounty's longitude by
timekeeper. That would have been done by a morning or afternoon sight, when
the Sun was rising or falling quickly; a routine practice. He gives no
details, in the log, about those observations, in the transcription I'm
looking at here. But those observations must have been made.

Let's assume, on the morning of the 11th, several hours before noon, Bligh
measured a Sun altitude. To go with that, he also needed a good idea of his
latitude at that moment; which could have come, via dead reckoning, from
the previous noon, or (better) from the meridian altitude of a star at dawn
twilight. He could also use a bit of hindsight, based on back-tracking from
that later noon observation on the 11th. Anyway, we can take it that
mariners of those days always had a good notion of their latitude, except
after long periods of cloudy weather.

Knowing the Sun's declination on that date, together with altitude and
latitude at that moment in the morning, it was a standard calculation to
derive local (apparent) time. So Bligh then knew exactly how long he had to
wait until local apparent noon. It needed no knowledge of longitude, though
it did call for an estimate, by DR, of the Easting / Westing made by the
vessel, since the moment of the morning observation. If his initial
longitude was wrong, because the timekeeper was in error, that didn't
matter. All the timekeeper was doing was providing the correct interval
from that morning observation, to noon.

Before (and for a long time after) timekeepers became available, a
mariner's noon procedure was to record the Sun's maximum altitude, at
whatever moment that turned out to be. There would be small error, because
that moment depended slightly on any North-South component of ship's speed,
as we have discussed at some length on Navlist. However, at the low speeds
of those days, and at low latitudes, such effects were negligible. The
disadvantage was that it called for a prolonged observation, to determine
the maximum, over a period when the navigator may well have had better
things to be doing, and when intermittent clouds may be obscuring the Sun.
Bligh's technique appears to be the prediction of the correct noon moment
by timekeeper, and taking a Sun shot then, without waiting to check whether
it was maximum altitude or not. Indeed, there's a lot of leeway available
in that observation, because of the slow changes around noon. Bligh's
approach was as precise, and efficient, as was possible. I don't know
whether it was introduced by Bligh; it may well have been standard practice
of the time.

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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