My plans to retrace Captain Cook's unfinished voyage have been postponed a year while I work on the next Marine Diesel Basics book and get my new boat SV Oceandrifter ready for sea.
The official purpose of Cook’s first voyage around the world was to observe the Transit of Venus in Tahiti in order to aid astronomical calculations, specifically the calculation of a ship’s longitude (Harrison’s clock was tested in 1736 but not accepted until 1773) and the Earth’s distance from the Sun. This official purpose was also a cover for Cook’s additional orders to explore for the “great southern continent” which was supposed to exist in the South Pacific.
The 1769 Transit of Venus attracted a lot of scientific interest and many American and other European scientists set up observatories, and their results were keenly awaited. The French Government gave Cook safe passage in recognition that he was “out on enterprises of service to all mankind”. The British Royal Navy had extended the same recognition to a French astronomer in 1761. The Transit occurs when Venus passes across the face of the sun (appearing as a black dot). A transit occurs in paired cycles, 8 years part every 243 years. Each transit can last almost seven hours.
Tahiti had been “discovered” in June 1767 by the British explorer Samuel Wallis and the island were chosen for the Transit observation because its exact location in the Pacific Ocean was known. (John Gore, who served with Wallis, was sent back to Tahiti with Cook.)
Cook’s HMS Bark Endeavour reached Tahiti in April 1769 and after initial contacts with islanders and some exploring, Cook prepared for the Transit. Wikipedia reports:
Cook decided to set up the Venus transit observatory on shore. He required a completely stable platform which the ship could not provide and plenty of space to work with. The location of the observatory would be known as “Fort Venus.” A sandy spit on the northeast end of Matavai Bay, named Point Venus by Cook, was chosen for the site. They began building Fort Venus two days after they arrived. They marked a perimeter and construction began. It had earthworks on three of its sides adjacent to deep channels. Wood was gathered to construct palisades that topped the earthworks. Casks from the ship were filled with wet sand and used for stability. The east side of the fort faced the river. Mounted guns were brought in from the ship. A gateway was assembled and within this fortification, fifty-four tents were pitched which housed the crew, scientists, and officers as well as the observatory, blacksmith equipment and a kitchen. Cook sent a party led by Zachary Hickes to a point on the east coast of the island for additional observations.
Unfortunately, the Royal Society was disappointed with the data collected from the transit, though it was subsequently found
that other observers elsewhere in the world had had the same problems with a black drop or “shadow” trailing Venus making precise observations very challenging. The Royal Society blamed the astronomer Green, who’d (conveniently) died on the voyage back to England.
The next transits of Venus will be on 10–11 December 2117, and 8 December 2125.