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I have mentioned before the circumnavigation of Dolphin under Wallis, in
1766-8. That was the ship's second voyage round the World. Not a very
distinguished voyage, if you compare with Cook, but a quick one.

The main achievement was the discovery of Tahiti, where Dolphin remained
from 19 June to 27 July 1767, and which Wallis correctly considered to be
the ideal spot for observing the forthcoming Transit of Venus. He returned
to England in May 1768, in time to acquaint the Admiralty of his discovery,
before Cook left London that August.

The particular importance of that discovery was that both the latitude and
the longitude of Tahiti had been found, just what Cook needed to find his
way straight there. This he did by 12 April 1769, well in time for the
Transit on 3rd June. The longitude of Tahiti, as observed by the Dolphin,
was only half a degree out from its true value.

So how was the longitude of Tahiti measured? I've mentioned before, the
footnote on page 119, Vol.1, of Beaglehole's superb edition of Cook's
Journals (Hakluyt Society, 1968). That contains a quote from Wallis' Journal
(as yet unpublished as far as I'm aware) made on 20 August 1767, three weeks
after departing from Tahiti. Wallis attributes the measurement to his
purser, John Harrison, a mathematician, "thro whose means we took the
Longitude by taking the Distance of the Sun from the Moon and Working it
according to Dr Masculines Method which we did not understand". Matavai
Bay's longitude is given as 150 degrees West of Greenwich. A present-day
value is 149 deg 29' W.

That John Harrison was not, of course THE John Harrison, of chronometer
fame. Apparently, it was not unusual for a mathematician or astronomer to be
on a ship's books as a purser, when the voyage involved much navigation. A
purser's job was usually a clerical one.

The timing is interesting. Dolphin had left England in August 1766, when
Maskelyne was working on the very first Nautical Almanac, for 1767. That
wasn't published until December 1766, so it was very much a last-minute job.
So, unless he was privy to its pages in manuscrpt prior to printing,
Harrison had to do without its Lunar Distance tables, which would otherwise
have been just what he needed. Instead, he would have to go through the
long-winded calculations of the Moon position from Mayer's tables, Three
hours work or more for each, with endless opportunity for error.

However, I have just been reading a partial text of the Journal of George
Robertson, Dolphin's master, in "An account of the discovery of Tahiti" (ed
Oliver Warner), Folio Press, 1973. This is a shortened version of what can
be found in the Hakluyt Societ volume "The discovery of Tahiti", ed. Hugh
Carrington, 1948, which I haven't seen. And Robertson tells a different
story.

He doesn't mention any lunar distance observations as such. Instead, he
relates, for 24 July 1767- "This day I chanced to look to an ephemeris which
informed me that there was an eclipse of the Sun on the 25th inst., which
was said to be visible in Mexico and Peru, but not in any part of Europe. By
the same ephemiris it was said to be New Moon at Paris at Seven Hours Eight
minutes in the afternoon, for which reason I found it must be visible where
we was." This would be only a partial solar eclipse at Tahiti.

His words, referring to Paris, make it likely, as I see it, that the
ephemeris he refers to was the French "Connaissance des Temps".

It seems that there was a reflecting telescope aboard, to which a dark glass
could be applied. He and Harrison went ashore on the morning of the next
day, 25th by ship's reckoning. They had some difficulty in exact timing of
the immersion, but Robertson reckoned to get a precise time of emersion, at
the end, as 8h 01m 00sec, which would presumably be local apparent time (he
calls it log time) measured from local midnight. He also quotes it to end at
20 hours 1 minute on the 24th, astronomical time. The duration is quoted as
approx. 1hr 9m 10s, but not stated with great confidence because oif the
difficulties in timing the start. The moment of emersion was deduced by an
altitude of the Sun, taken as the eclipse ended, but Robertson doesn't quote
the measured altitude. The latitude would need to be known, for that
calculation, and for that Wallis quotes 17deg 30' S., a pretty accurate
value for Matavai Bay.

Robertson gives the magnification of his reflector as about 200x, and says
that with it he could see the mountanous profile of the Moon, against the
Sun as background, which sounds rather convincing.

Maskelyne also gave information about that solar eclipse in the Nautical
Almanac for 1767, which was of course unavailable on board Dolphin. He
states-
"July 25. Sun eclipsed, begins at Sun-rising in Lat. S. 19deg 16', Long
141deg 45' W. Ends at Sun-setting in lat. 3deg 23'S, Long 60deg 5' West.
Centrally eclipsed on the Merid. in Lat. 1deg 15' South". In the "phases of
the Moon" for July he quotes "New Moon 25d 6h 50m", which is presumably the
Greenwich-time equivalent of that Paris data.

However, something seems to be wrong here. The moments of new moon,  6h 50m
for Greenwich, and 7h 08m quoted by Robertson for Paris, differ by 18
minutes of time. However, the longitude difference between Greenwich and
Paris actually amounts to 9 minutes 20 sec, in time, so there's a big
discrepancy somewhere.

I wonder if Robertson or Harrison knew how to correct properly for Moon
parallax, which makes a big difference to observations of a solar eclipse.

I give all the relevant data that I have collected, in case anyone would
like to reconstruct those eclipse observations using a modern computer
ephemeris program, such as Starmap. One extra check that could be made is on
the 1767 Connaissance des Temps, and what time it gave for the moment of
that same new moon, if anyone has access to a copy..

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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