Portrait of George Airy by James Pardon, 19th century, oil on canvas.
The paired Venus transits of 1874 and 1882 were opportunities to apply new astronomical techniques and to use the various planned expeditions to investigate other types of science, taking magnetic observations of the Earth and investigating regional natural history. For the 1874 transit the Astronomer Royal, Sir George Biddell Airy FRS (1801-1892), was given the task of co-ordinating the British side of an international effort. Under his direction, teams of scientists travelled to the Hawaiian Islands, including Honolulu; to Egypt; to the Islands of Rodriguez and Kerguelen in the Indian Ocean; and to New Zealand.
This was a scientific event that, for the first time, the public could follow almost as it happened. Rapid communication by telegraph and fast newspaper printing meant that observer’s accounts from all over the world would be read at the breakfast table. Successful results came quickly – although the remote Kerguelen observers, from British, American and German teams far to the south would be last to report.
Warren de la Rue FRS
The technique of telescope-based photography had been unavailable to previous transit observers and had only just reached a technological state of readiness in which it might be attempted in 1874. Warren de la Rue FRS (1815-1889) had pioneered the application of photographic techniques to sunspot observation at Kew Observatory in the years 1858-1872 – a method he termed the photoheliograph – and he had successfully observed solar prominences during an expedition to Spain to view the total eclipse of 1860. De la Rue advocated taking multiple images of the Sun and measuring the distance between to central points of Venus and the solar disc using a micrometer and Airy agreed.
Five heliometers were made and sent with each British contingent. But what had seemed a good idea became an expensive flop. As the prints were enlarged, it proved impossible to take accurate measurements, while distortions introduced by the optical system were frustratingly unpredictable. “The ardour of the Observers had been much cooled by the apparent general failure of the photographic principle” wrote Airy and nothing was published from the attempt.
