1769 Transit


Conditions augured well for the renewed effort to measure the Sun-Earth via the transit; astronomers were better prepared thanks to their experience of the 1761 transit, international tensions had eased by 1769, and the parts of the Earth where the entire transit could be observed were more accessible than in 1761. Perhaps the most famous transit expedition was that of James Cook to Tahiti.

There were at least four observing sites in Canada. At Québec City, Samuel Holland (who had been Cook's instructor a few years earlier and was now the Surveyor-General for North America) began his preparations three months before the transit by finding his latitude using a Bird astronomical quadrant. Holland (1769) found his latitude (the mean of nine observations) to be 46º47'15". To get his longitude, he observed three immersions and two emersions of Jupiter's first satellite between March and June with a refracting telescope by Dollond. From the times of these phenomena, the Astronomer Royal, Nevil Maskelyne, deduced Holland's longitude to be 71º10'15". However, we know from the situation of Holland's house and modern maps that his longitude was 71º14'49" and his latitude just as he said. When it came to the actual transit, Holland used the same refractor and M. St. Germaine of the Quebec Seminary, observing with Holland, used one of the standard reflectors of 61 cm focus made by Short. The observers were only able to time the first contact, clouds preventing further timings.

Holland's assistant, Thomas Wright (Deputy Surveyor of the Northern District of America), observed the transit from Isle aux Coudres, about 90 km downstream from Quebec City. From several observations with "a brass Hadley's sextant of about 15 inches radius", Wright (1769) derived his latitude as 47º16'30" using several observations of two altitudes with the interval of time between them. (In 18th-19th century contexts "Hadley's sextant" was a generic term for a handheld device capable of measuring angles up to 120 degrees, invented by the naval officer John Campbell and the instrument maker John Bird ca. 1758. It was a development of John Hadley's octant of ca. 1730, which could measure angles up to 90 degrees). According to modern topographic maps the latitude of the north-west side of the island where Wright was located, is 47º 25'. Something seems to be wrong with his observations, and Newcomb (1891) noted, "The observer at this station seems to have been most unfortunate in his determination of his position, his printed computations being full of blunders."

Wright did not give his longitude, saying he would find it in the coming months, but from modern maps it appears to be about 70º24' and Newcomb figured it was 4h41m39s or 70º24'38". To observe the transit, Wright used a standard Gregorian reflector equipped with "two oblong smoaked glasses with different shades, made to slide in a groove fixed to my telescope." Maskelyne used Wright's observations and made slight corrections, finding for second contact 2h50m19s or 2h50m50s depending on whether the time was determined by the round appearance of Venus or by the breaking of the dark thread respectively. The observations at Quebec and Isle aux Coudres were also published in the Transactions of the American Philosophical Society in 1771 (Vol. 1, Appendix, p. 15).

The Hudson's Bay Company (which controlled much of what is now Canada), took an interest in the transit. This is not too surprising, since some of its London governors were also prominent in the Royal Society. As Ball and Dyck (1984) point out, the Company asked at least one of their chief factors, Ferdinand Jacobs at York Fort, to observe the transit. This he did "with a very good Telliscope Armed with a Smooted Glass next the Eye." Unfortunately, he only reported the times of contact to the minute and gave no information on whether he corrected his clock, so his results were of no use. A much better course of action was taken in the case of two astronomers sent out by the Society to Fort Prince of Wales at Churchill, MB. With the support of the Company, instructions from the Astronomer Royal, equipment from the Royal Society, and favoured by good weather, they made the only complete timing of any transit of Venus ever observed in Canada. William Wales (1734-1798) and Joseph Dymond (1746-1796) were two young Yorkshire men who had worked under the Astronomer Royal, Nevil Maskelyne, at Greenwich, and he recommended them to the Royal Society for the transit expedition. The Society appointed them and arranged with the Hudson's Bay Company for their transportation and accommodation. Because ice usually chokes Hudson Strait until June of each year, they had to arrive nearly a year ahead of time, in the summer of 1768. These observers and their accomplishments have been the subject of many articles, including a very recent one by Griffin-Short (2003; now see Rosenfeld 2012). One aspect of the Wales and Dymond expedition that does not seem to have been noticed elsewhere (other than in Harry Wolf's study of 1959) is that they took with them a prefabricated wooden observatory designed by the great British engineer, John Smeaton. This is mentioned in the minutes of the Council of the Royal Society in March and April, 1768, and briefly in Wales' (1770) descriptive journal and in the Churchill post journal.

Unfortunately, no illustration of the observatory is extant among the Smeaton drawings at the Royal Society. Rosenfeld has reconstructed its possible external appearance.

Wales and Dymond (1769) made hundreds of astronomical observations during their long year at Churchill. They used a Bird astronomical quadrant to find their clock correction just as Maskelyne had outlined in his INSTRUCTIONS Relative to the Observation of the ensuing Transit of the Planet VENUS over the Sun's Disk, on the 3d of June 1769 in The Nautical Almanac for 1769, and to take nearly two hundred meridian altitudes to find their latitude. All their observations included thermometer and barometer readings so that refraction could be taken into account. Though Wales was puzzled by the large scatter in the values he got for latitude, careful examination now shows that the best values were obtained using the Sun near summer solstice (when it was as high as it gets) and using stars whose declinations were well-known (e.g. a Persei) and which transited at zenith distance less than 10º, in other words where refraction was minimized. If Wales had restricted himself to these situations, he could have found his latitude within 2" of its modern value (58º47'48"). Using standard Gregorian reflectors made by Short, Wales and Dymond also observed six occultations of stars by the Moon and one immersion of Jupiter's first satellite. These would have been useful in determining the longitude of Churchill, though it appears from Maskelyne's (1771) report that observations at Greenwich, or some other established observatory, would also have been needed to serve as a basis of comparison. Results from the Ephemeris Generator show that the Churchill occultation observations were remarkably accurate. If the events at the Moon's bright limb are excluded as well as the one event which was nearly a grazing occultation, their timing errors of the remaining five events were -7, 0, -8, 0, 0, seconds. Recalling that in 2 seconds of time, the Moon moves about 1 arc-second, and that the error in longitude would therefore be 30" for every 2 seconds error in the time of the occultation, it seems safe to say that the longitude of Fort Prince of Wales could have been determined within 1 minute of arc. Dymond and Wales did not calculate or even state their longitude but Hornsby (1772) who used their observations along with others to calculate a mean solar parallax of 8.78", gave the longitude of the Fort Prince of Wales just 19" (or about 300 land metres) short of the modern value (94º12'41"). Hornsby did not say how he calculated this. Nevil Maskelyne also noted Wales' and Dymond's results in the Transactions of the American Philosophical Society in 1771 (Vol. 1, Appendix, p. 4).

To observe the transit itself, Wales and Dymond had the two Short telescopes, one with a split objective micrometer attached to it. This was used to measure the distance between the limbs of Venus and the Sun about fifty times as the transit progressed. It was also used to measure the angular diameters of Venus and the Sun which can now be used to check the micrometer's accuracy. It had a mean error of ±2".

An interesting personal sidelight is that Wales had married Mary Green in 1765. She was a sister of Charles Green who was the astronomer on James Cook's expedition to observe the transit at Tahiti. Wales was the astronomer on Cook's second and third voyages. About 1780, Wales became mathematical master at Christ's Hospital in London where he was a popular teacher of navigation for the rest of his life. He wrote a number of papers and books, mainly relating to navigation and was appointed to the Board of Longitude in 1795.

Nothing is known of Dymond's career after 1769. Thanks to the initiative of Peter Jedicke, asteroid 15045 was named Walesdymond in honour of these two dedicated observers.


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