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    Dolphin 1767
    From: George Huxtable
    Date: 2007 Sep 19, 23:38 +0100

    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    (as yet unpublished as far as I'm aware) made on 20 August 1767, three weeks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---.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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