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On 4 Jul 2008 at 8:15, George Huxtable wrote:

> Perhaps, I'm starting to understand better what Ken Muldrew has been
> telling
> us about these Canadian surveyors' astronomical observations. Although
> Ken
> has clearly stated that Sun altitude was never measured, for the lunar,
> he
> is less specific about the Moon, but I will assume that the Moon was
> being
> treated in the same way.

Right. And while the sun's altitude is measured for a time sight to
accompany a sun-moon lunar, the moon's altitude in never measured for any
purpose (so far as I have seen in their journals).

> Let me summarise the matter in my own words, and then if I still have at
> wrong, Ken can correct me.
>
> To clear a lunar distance observation to get Greenwich time, the
> navigator
> needs the altitudes of the two bodies, for example Moon and Sun. But they
> don't need to be actually measured for that purpose; they can be
> calculated
> instead, from a rough assessment of the lat and long, well enough to
> arrive
> at a good estimate of Greenwich time, and hence a much better value for
> long. Because these navigators were travelling slowly over land, they
> always
> knew an approximate position, by reference to their previous position
> assessment. So they NEVER needed to measure Sun or Moon altitude (for the
> lunar) and never in fact did so. Their Sun and Moon altitudes, for the
> lunar, were always calculated instead..

Yes, that is correct.

> Having found a value for Greenwich time, so that their watch-error was
> then
> known, two altitudes were then needed, one near or at the meridian to
> define
> latitude, and one (a time-sight) as near as possible to East or West, to
> define longitude. But these altitude measurements played no part in the
> lunar distance calculation, and didn't need to be taken anywhere near the
> same moment as the lunar.

Latitude is used to calculate altitudes, and the watch error comes from
the time sight, so both of these altitude measurements are specifically
used in the calculation of Greenwich time. Because Thompson and Fidler had
such poor watches (ordinary verge pocket watches with Tompion regulators),
the time sight was done either immediately prior to or immediately
following the lunar (sometimes less that a minute separates the last
observation for the time sight (both Thompson and Fidler always take 3
altitudes) and the first observation for the lunar (Thompson usually takes
7 sights whereas Fidler always take exactly 10 sights for the lunar
distance)). The meridian altitude wasn't taken as close to the lunar but,
for example, Thompson would never use a latitude from noon sun for a star-
moon lunar; he would always take a meridian altitude of a star on that
particular night.

Ken Muldrew.

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