On Saturday the sixth of June 1761, the gentleman farmer and amateur meteorologist Isaac Fletcher gathered with two of his friends at his comfortable house at Mosser. A small hamlet which straggles along the lower slopes of the fells, it has wide views across a sedge-filled valley towards Pardshaw, with its recently built Quaker Meeting House, below the rock on which George Fox had preached several times. Fletcher was part of the coterie of enquiring Quaker minds in this western enclave of noncomformity, as was another of the three gathered early on that June morning: Elihu Robinson from Eaglesfield, only twenty-six years old, but already noted for his quick mind, his interest in the world. It was shortly after dawn, and the day promised to be clear. “The sun then appeared large with a thin black cloud over it, which soon went off as the sun rose higher in the horizon.” wrote Fletcher in his diary “The image of the sun & the planet Venus as a black spot appeared very plain upon a white cloth put up for that purpose. The path of the planet over the sun's disk was easy to delineate and very curious to behold, tho' clouds sometimes interfered.”
It was the first time that the transit of Venus had been studied with such scientific precision. With the ability to accurately predict this rarest of astronomical phenomena, scientists across the world were rigging their white cloths and paper screens, hoping to record in detail the time of the planet's passing across the surface of the sun. With the application of some basic trigonometry, they would be able to estimate the distance from the earth to the sun, as though our place in the heavens could be measured.
The Royal Society in London had prepared for years. They had dispatched expeditions to South-east Asia and the south Atlantic, the furthest reaches of the world which would be friendly towards British explorers. To St Helena went Nevil Maskelyne, later to become Astronomer Royal. To Sumatra, they sent the surveyor Charles Mason, with his assistant Jeremiah Dixon. The two made it as far as South Africa in time, and observed the transit from the Cape of Good Hope. Two years later, the pair went together to America, to map the frontiers of the newly-developing states, demarking a line which still bears their name.
Transits occur eight years apart; like London buses, you wait 105 years then two come at once. By 1769, the Royal Society were determined to gather accurate data. They dispatched a young up-and-coming naval Captain, James Cook, to the south Pacific in HMS Endeavour, taking with him a naturalist, Joseph Banks. The expedition was away for three years, establishing an astronomical base on Tahiti, Point Venus, before moving south, encountering lands never before visited by Europeans, a Terra Australis of strange fauna and unfamiliar people. It was a voyage that would make Banks' name as a naturalist; he was fêted on his return, became the brilliant star of the London Salons.
In 1772, the year after Cook and Banks returned, Nevil Maskelyne proposed to the Royal Society that the unspent funds from the Transit of Venus expeditions could be used for new work to determine the density of the earth. The site they selected, a perfect-shaped mountain in central Scotland, gave the name to the Schiehallion Experiment; but it also resulted in Maskelyne's assistant, Charles Hutton, devising the technique of contour lines as a new means of mapping slopes. The Transit of Venus became a catalyst for exploration and discovery, a rare event from which the ripples of science could spread in the enlightenment, a dark speck on the surface of our sunny ignorance.
Venus has always been the morning and evening star; the brightest light in the heavens. It is named after the Roman goddess of love and beauty, as though we could never imagine so perfect an earthly thing. The Ancient Greeks believed it to be two separate stars which would appear at either morning or night: Phosphorus and Hesperus, the Light Bringer and the son of the Dawn Goddess, Lucifer and Vesper. From a distance, it appears beautiful, silvery-blue, freshly polished. For centuries we could imagine it to be an idyll, a romantic remove. Since the first landings in the sixties, we now know its atmosphere is almost pure carbon dioxide, tainted with traces of sulphuric acid, its surface scarred with the weals of volcanoes, an imagined hell, a warning of how things may yet become; a warning of how knowledge becomes a thing we can never un-know, a return from an ideal, the fading of distant beauty.
Tomorrow, Venus passes once again across the face of the sun, a path as perfect and predictable as a ball thrown into the air, curving through the sky on an arc governed by gravity and time. Once again, the telescopes of the world will be following its course, this time excited by the information it might reveal about exoplanets, or those that might exist beyond our own solar system. This centennial event has become a repository for our hopes and dreams of a world beyond our own, a tiny point of focus for the sharpest scientific minds of our age, a single point of darkness crossing the landscape of a bright, flaring world.