2°) Historical background

By the seventeenth century, the development of astronomy had become crucial for the maritime trade of European colonial countries: the impossibility of taking correct directional bearings at sea resulted in many shipwrecks. Helping vessels to locate their longitude on the high seas was one of the main reasons for the creation of the observatories of Paris and Greenwich. Then, in 1714, the reward offered by the English Parliament for the solution to this problem was so huge as to make the bonuses of the other nations look derisory.

It was against this background that, in 1716, Sir Edmond Halley, the ‘comet’ man, invited all “curious” men to measure the Earth-Sun distance by using the transit of Venus. Aged 60, and suspecting he wouldn't last to witness the next transit, expected in 1761, he wrote a passionate admonition in which he proposed that an astronomer in the northern hemisphere observe the event in collaboration with another astronomer in the southern hemisphere. As indicated in Figure 2, each astronomer would observe Venus following a different trajectory across the sun’s disk (path aa' for A and path bb' for B): the interval e between the two chords would be proportional to the distance between the two men (AB) on Earth. It would then be possible to calculate e by timing the duration of each movement (about six hours), and thence the Earth-Sun distance. Halley promised a result with the excellent accuracy of 1/500.


Diagram 2 : Halley's plan for calculating the Earth-Sun distance from observations of the transit of Venus from the northen and southern emispheres

However, this procedure involved hazardous expeditions to far-away and required a multiplicity of foreign observation stations in order to ensure that such a unique moment was not wasted by the passing of some inauspicious cloud. Worst of all, the ‘world’ conflict, which commenced in 1756 for the control of the colonies (the Seven Years War), adding more complications for the missions in 1761, already under threat from piracy, shipwreck and scurvy.

But the scientists were not easily put off: courageously, they crossed the lines of belligerents and demanded the right to confer. The vessel carrying the English astronomers, Charles Mason and Jeremiah Dixon suffered a violent bombardment and heavy casualties; on the opposing side, English corsairs abducted the French scientist Pingré, and left him without supplies on Rodrigues island, whereas Le Gentil de La Galaisière, impeded by the English Navy, wandered for several years in the Indian Ocean before returning frustrated and empty-handed to Paris. At the time of the following transit, in 1769, misfortune descended once again upon scientists involved: Chappe d’Auteroche (France) lost his life in California and Charles Green, an English astronomer and member of James Cook’s expedition, died at sea. In his first journey, on the Endeavour, the famous English explorer’s prime duty was to observe the transit of Venus.

In the nineteenth century, the transit of Venus was the occasion for yet another scientific “first”: the founding of an all-female observatory. In 1882, taking advantage of the presence of some famous astronomers on their campus, girls at a school in South Africa built the first observatory ever created by women for women, a remarkable incursion into a domain hitherto dominated by men.