Measuring the Universe

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In 1761 and again in 1769 a rare astronomical event took place: the planet Venus crossed the face of the Sun. This happens twice, with an eight year interval every 113 or 130 years. The eighteenth century transits were particularly challenging because it was known that the size of the solar system could be calculated from the observations. Undertaking the measurements would stretch every aspect of contemporary science.

In 1761 efforts were hindered by many factors. Britain was at war with France which meant British expeditions were attacked by French ships and vice versa. The weather was not helpful in many places and the exact location of observations on Earth was not known with sufficient accuracy. When the results were gathered, the distance from the Earth to the Sun ranged from 77 million to 96 million miles. The opportunity in 1769 was not to be missed.

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Astronomers also stressed the economical potential of the transit expeditions. Thomas Hornsby wrote in 1765: ‘It would be an object of much interest for a commercial nation to have a settlement in the Great Pacific Ocean’. The interests of the Royal Society and the nation happily coincided. A large sum of money was found, with King George III’s approval, to send Captain James Cook to the South Pacific and a team to the North Cape, off Norway. These expeditions were the first of many voyages which united scientific and political goals.

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The actual measurement made was the time that the planet Venus took to cross the Sun’s disc, and this would vary from different places on Earth. John Shelton, a leading London clockmaker made five regulators (particularly accurate long-case clocks with specialised dials) for the expeditions. The transits were timed by counting the ticks of the seconds pendulum. The expedition also took quadrants which were used to find the altitude of stars on the meridian line and hence the latitude. Both the regulator and quadrant at the Science Museum probably went to Norway.

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Accompanying Cook on board the Endeavour was an astronomer, Charles Green as well as Joseph Banks (who later developed Kew Gardens as a centre of botanical expertise) and Daniel Solander, a distinguished botanist. Cook also took leading artist Sydney Parkinson who made a record of the specimens found. The expedition combined astronomical measurement with surveying. After the transit of Venus, Cook circumnavigated New Zealand and surveyed the east coast of Australia, then known as New Holland.

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Astronomers had some problems with the ‘black drop’, when Venus appeared to stick to the edge of the Sun, but measurements were an improvement on those eight years earlier. Hundreds of amateurs and 151 official observers took part. The distance between the Sun and Earth was found to be between 92 and 96 million miles.