Models, Metaphors, and the Transit of Venus at Victorian Greenwich

Abstract

During the nineteenth century, the Royal Observatory Greenwich underwent a remarkable transformation, which has been described by historians as like an industrial revolution in observatory science. George Biddell Airy (Astronomer Royal from 1835 to 1881) was a leading figure in this period. The effort to measure the distance to the sun (solar parallax) using observations of the 1874 transit of Venus was one of the most ambitious enterprises of his career. In many ways the transit program became a proving ground for the new methods of “industrial” astronomy that had been developing over the previous half-century. In order for the plan to succeed, multiple geographically-separated observations of a particular moment in the transit’s passage would have to be made. The problem was that no one alive had ever witnessed a transit or knew what this moment would look like. The response was to build simulation transits of Venus-mechanical models-to be used for experimentation and training. The design of these models was driven in part by calculations and in part by the astronomical reports from previous transits of Venus in the eighteenth century. Some of these reports evoked metaphorical descriptions of a “black drop effect” at the crucial moment to be observed. When it was found that some models also produced a black drop effect, the phenomenon came to be thought of by Airy and his assistants as real and reliable. This belief would turn out to be false.