The 61st star of the Swan

With my recently-acquired binoculars, I was able to locate 61 Cygni. It’s not a spectacular sight, but it has a lot of history: it was the first star whose distance was determined with reasonable accuracy, by Friedrich Bessel in 1838. Before reading up on this I hadn’t actually been aware of Bessel as an astronomer; both from undergrad calculus and tinkering with software sound synthesis I was familiar with Bessel functions. (Which it turns out were discovered, or invented, depending on whether you lean Platonist or constructivist, by Daniel Bernoulli.) Bessel functions can be used to describe the vibrations of a cylinder, hence of the head of a drum.

The whole story of Bessel’s observations, with historical background, is given a jaunty treatment in Alan Hirshfeld’s Parallax: The Race to Measure the Cosmos. At the same time, coincidentally, I also had out from the library Andrea Wulf’s Chasing Venus: The Race to Measure the Heavens, about the 18th-century observations of the transit of Venus that allowed the size of an Astronomical Unit to be calculated for the first time. I have to look askance at this trend in the titling, or subtitling anyway, of science books, but despite that I enjoyed both of these. My own observations of the recent transit of Venus consisted entirely of a few seconds of looking at it through a pair of solar glasses that were passed back the line-up to look through a telescope at the Washburn Observatory; but it was cheering to see all the people who had gathered for it, and to feel a sense — however tenuous — of connection to the people who sailed for months to take a few hours of measurements.

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