Charles Green (astronomer) - Voyage of The Endeavour and The Transit of Venus

Voyage of The Endeavour and The Transit of Venus

Green joined Cook's first voyage of circumnavigation in 1768, accompanied by a servant. Green was one of two official astronomers appointed by the Royal Society to observe the transit; the other was Cook himself, who was a capable observer in his own right. Green was to receive 200 guineas from the Society for the appointment. (Cook was to receive 100 guineas for his role in making the observations.) The Society provided instruments for the expedition, including two reflector telescopes built by James Short, two clocks and an astronomical quadrant; these were supplemented by a telescope in the possession of Daniel Solander and another provided by the Navy.

The expedition arrived at Tahiti, the chosen site for the observation of the transit, on 11 April 1769, anchoring in Matuvai Bay on the north-western coast of the island two days later. A camp (Fort Venus) was established and the astronomical equipment was in place by the start of May. Of paramount importance for Green was the calculation of an accurate position of the location, which he calculated using the lunar distance method and by observation of the moons of Jupiter. The removal of the quadrant from the camp - either by the local Tahitians or by members of the expedition who traded it to the Tahitians - threatened to derail the enterprise, but it was soon recovered, albeit disassembled into pieces and with some damage, by the expedition botanist Joseph Banks, accompanied by Green.

The day of the transit, June 3, was a clear day and the transit was visible in its entirety. Although Cook had taken the precaution of sending two groups to outlying islands, the principal observation party remained at Fort Venus, where Green, Cook and (independently) Solander recorded the times of the transit. Cook noted in his log that the times recorded by the three observers for the times of contact differed significantly; this anomaly, now generally attributed to the black drop effect, would cast significant doubt, in the eyes of the Royal Society and Nevil Maskelyne, on the usefulness and value of the observations later.