The Transit of Venus Enterprise in Victorian Britain (review)

Transits of Venus occur when the small planet passes between the earth and the sun. They are short, lasting at most about four hours. They are infrequent. And each is visible only from specific and often far-flung locales. The first transit of Venus that is known to have been seen (by one observer in England) occurred in 1639. Edmond Halley observed a transit of Mercury in 1677 and eventually realized that if astronomers could measure the duration of planetary transits, or time-specific aspects of transits, they could determine the distance between the earth and the sun, a figure considered to be the fundamental astronomical unit. Recognizing the value of this suggestion, Europeans mounted several enormously expensive expeditions to observe the Venus transits of 1761 and 1769.

Jessica Ratcliff focuses her attention on the British expeditions to observe the transit of 1874 (with a postscript on the transit of 1882). She writes from a history-of-science perspective but is mindful of matters of technology. Indeed, she explains clearly that the results of the transit observations may have been understood primarily as contributions to pure science, but most of the challenges of the project pertained to technology. Moreover, comparing the British expeditions with those mounted by France, Germany, Russia, and the United States, she concludes that the transit offered an important arena for international competition, and that the technological design of each observing program was a “national product” (p. 76).

The transit of Venus of 1874 occasioned the costliest British science project of the nineteenth century, and it was supported almost entirely by government funds funneled through the Admiralty. George Biddell Airy, the long-term and now elderly director of the Royal Observatory, Greenwich— a government-sponsored institution established for the determination of longitude and the improvement of navigation—was given responsibility for planning and organizing the several expeditions and making sense of the results obtained by observers sent to distant lands. One set of problems on his plate pertained to photography, a technology that may have been ubiquitous in Victorian culture but that had not yet shown that it was up to precision measurements. Each aspect of photography presented pros and cons. Which emulsion (collodion or daguerreotype) would capture the best images? Which would have the least distortion, or perhaps the most equal distortion in all directions? Which optical system should be used to focus the astronomical image onto the photographic plate? Determining how to model the transit so that observers could practice before the event was equally perplexing and critical. So too were decisions pertaining to such technologies as submarine telegraphy and precision chronometry that were used to determine the positions of the observing stations.

Ratcliff provides a particularly fascinating account of the challenges involved in making sense of observations that were, literally, all over the map. While trials with the model had led everyone involved to expect smooth sailing, the actual transit proved frustratingly complex. With regard to clocking the moment of internal contact, for instance, observers standing side by side might differ by as much as twenty or thirty seconds. When the observations finally returned to England, an astronomer or two at Greenwich faced the daunting task of interpretation. Should they disregard outlying observations as aberrations, or might outlying observations on one side compensate for those on the other? How should they understand observers who admitted relying on personal judgment? How far should they trust women, even if their observations were close to the norm? And so the project dragged on, with government calling for results and the aged Airy eager to retire.

In the end, despite all the new technologies employed and the care taken by observers from several nations, the astronomical results of the nineteenth-century transits of Venus were no better than those obtained in the late eighteenth century. But as presented by Ratcliff in a book that is remarkably informed, insightful, and accessible, the transits provide a window onto many important aspects of science and...