My book choices often seem to lead from one to another, and in this case it was Mason and Dixon who set me off on the trail of the 1761 and 1769 transits of Venus.
The transits were important because they would allow the measurement of the distance between the Sun and the Earth by comparing measurements of the transit time from different parts of the Earth whose locations could also be measured – a fact which had been predicted and advertised by Edmond Halley almost 50 years earlier. Closer to the time, a French astronomer called Joseph-Nicolas Delisle took charge of the operation, with international cooperation – and competition – seeing observers sent to St Helena, the Cape of Good Hope (having failed to make it to Bencoolen), Rodrigues, India, Scandinavia, Siberia, St Petersburg and Newfoundland – only the end of the transit being visible across most of Europe, and none of it across most of North America.
None of the records were as accurate as the organisers and observers had hoped – as well as troubles with clouds, accurate timekeeping, disagreements and onlookers crowding the observatories, the entrance and exit on Venus on the face of the sun were not the clear events which had been hoped – the planet seemed to change shape and ‘stick’ to the edge of the sun, as well as being surrounded by a shimmering halo that was the first definite sign that Venus had an atmosphere similar to Earth’s.
With 8 years between pairs of transits, the observers now had time to try to fix these problems before their second attempt. It was this transit which saw James Cook dispatched to Tahiti – it having been realised that the greatest contrast in timings would be between northern Scandinavia and the South Seas – and it also saw both Russia and North America eager to play a greater part in scientific endeavours. It was these measurements which led to a measurement of the distance of the sun accurate to within 3%, and the various travels required for the measurements also led to discoveries in botany, mapping and other sciences.
The only problem with this book was that there were just so many people to keep track of – at times I found myself longing for a straightforward account of the French, or Russian, or British attempts, rather than jumping between them all. But in general the book was a lovely mix of science and voyages, and I really enjoyed it.