The 1769 Transit of Venus

In 1716 Edmund Halley, namesake of comets, figured how to calculate the distance between the earth and sun. Twice every 120 years, Venus passes between the bodies as a black spot gliding across the Sun’s surface. By measuring the transit’s duration at different angles from different points on Earth, one could, in theory, calculate the distance between Earth and Venus, and consequently the sun, through parallax and trigonometry.

Halley did not live to see his work complete. In 1761, when the next transit came, astronomers observed from Russia, Mauritius and South Africa.  The Seven Years War hampered efforts, however. Guillaume le Gentile, for example, failed to observe the transit from Pondicherry, India after the British seized the city and closed their doors.

When the next transit came on June 4th 1769, Europe was at peace. In a far greater effort, the continent’s scientific authorities dispatched astronomers to 76 points across the earth.

Jean Baptiste Chappe d’Auteroche, who had observed the 1761 transit from Siberia, led a French-Spanish team through the Mexican wilderness to Baja California. He succeeded, but 26 of 28, Chappe included, died from dysentery and yellow fever not long after.

Le Gentile chose Manila for the 1769 transit but was again rebuffed, this time by the Spanish, forcing the French astronomer to observe Venus from Pondicherry (now back in French hands) instead. Le Gentile waited in his observatory but come June 4th, poor weather clouded his telescope. It was sunny in Manila.

The British Royal Society sought to observe Venus from Tahiti, an island 20 miles wide in the distant South Pacific. Seasoned explorer James Cook received two missions, his first – to find Tahiti, befriend the locals, build an observatory and record the transit. The second, from the Royal Navy, was contained in a sealed envelope to remain unopened until the first was complete. Astronomers, botanists and anthropologists accompanied him – the expedition was both scientific and colonial.

Scurvy was the bane of 18th-century sailors. On an eight-month journey like Cook’s, it would typically kill half those aboard. Working from a recent hypothesis based on a folk remedy, Cook forced his men to eat oranges and lashed those who didn’t. It worked – citrus proved a cure. Captains have ensured their sailors eat plenty of fruit and vegetables ever since. The ‘scourge of the seas’ was defeated. Later science linked scurvy to vitamin C deficiency – before Cook, sailors ate mainly biscuits and jerky.

Cook’s View of the Transit of VenusDespite the effort, observations of Venus’s 1769 transit were inconclusive. The ‘black drop effect’ marred accurate measurements, an issue not resolved until the 20th century. It did, however, have widespread effects. Cook’s second mission was to explore the Pacific’s rumoured southern continent which Abel Tasman had sighted in 1642. From Tahiti, Cook sailed to and mapped New Zealand, and from there Australia.   Though a boon for geography and the natural sciences, the Cook expedition paved the way for British colonisation of the region, displacement of its inhabitants, and genocide of the Tasmanians.

Sources: Yuval Noah Harari – Sapiens: A Brief History of Humankind, NASA, Sky and Telescope.

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