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Transit of Venus1769 transit

Miniature of James Cook by Nathaniel Dance-Holland, 18th century.
When the Scottish physician James Lind FRS (1716-1794) volunteered his services to Astronomer Royal Nevil Maskelyne for transit observation duties at the beginning of 1769, Maskelyne’s written replies revealed the huge efforts being planned for expeditions in the Northern Transit area alone. Two astronomers, Jeremiah Dixon and William Bayly (1737?-1810) would be sent to Nordkapp in Norway. William Wales FRS (c.1734-1798) overwintered on the frozen Churchill River at Hudson’s Bay, North America to get his transit measurements, on observation stations designed by the Eddystone Lighthouse-building engineer John Smeaton FRS (1724-1794).

Wales would later teach at Christ’s Hospital School, where his tales of exotic voyaging adventure found the eager ears of Samuel Taylor Coleridge (1772-1834), resurfacing later in The Rime of the Ancient Mariner. The more prosaic side of travel was put to Lind: “Whether you still retain any fondness for this expedition...I don’t know. I understand that the Captain’s friends fear he would be incommoded by the addition of another gentleman, as he could only swing his cot in the Captain’s Cabbin.” Lind wisely stayed at home, viewing the transit from Edinburgh.
Astronomical quadrant believed to have been used by Captain James Cook during his observations of the transit of Venus in Tahiti, made by British instrument maker John Bird, 1767.
The Society’s southerly observations were entrusted to the expedition led by James Cook FRS (1728-1779) sailing with HMS Endeavour to Tahiti, in the Pacific Ocean. One of the most famous scientific voyages of all time encountered scepticism at Rio de Janeiro: “the account we gave of ourselves of being bound to the Southward to observe the Transit of Venus (a phenomenon they had not the least Idea of) appeared so very Strange to the narrow minded Portuguese that they thought it only an invented Story to cover some other design we must be upon...” James Cook and astronomer Charles Green (c.1734-1771) made a highly successful set of transit observations under clear Tahitian skies, somewhat overshadowed by the fame of their subsequent explorations of Australia and New Zealand. The information transmitted to the Royal Society was immensely valuable; tragically, it was to be Green’s legacy as he died of fever at sea as Endeavour sailed home in 1771.

Almost all of the transit observations had one thing in common, as Maskelyne commented from Greenwich: “The black protuberance was seen here by everyone”. As the transit began and ended, the shape of Venus appeared distorted into a teardrop. In Oxford, Samuel Horsley “was much astonished to find the shape of the black spot suddenly altered from a large segment of a circle...where the lower part, which still seemed the segment of a circle, is connected with the Sun’s limb, by a kind of ligament of darkness”. Fascinating as it was, it had a serious consequence. The solar parallax method depended upon knowing precisely when the transit commenced and ended. The distortion meant that no-one could be quite sure. Nevertheless, Earth-Sun distance calculations were tried: Samuel Horsley put it at “30008,4416 semidiameters of the Earth”.