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Huygens, the (slightly) nearsighted astronomer

By Luke86 | Astrofacts | 16 Mar 2023

Extraordinary astronomer of the seventeenth century, Christiaan Huygens was also a telescope designer. But his instruments were not always up to the standards of the time. According to Alexander Pietrow, a researcher at the Leibniz Institute for Astrophysics Potsdam, the reason could be a slight myopia, which prevented Huygens from improving his designs beyond a certain threshold.


He suffered from mild myopia. In diopters: -1.5. This is the diagnosis signed on Notes and Records of the Royal Society Journal of the History of Science by Alexander Pietrow, a researcher at the Leibniz Institute for Astrophysics Potsdam (Aip), in Germany, for an exceptional patient: the Dutch mathematician and astronomer Christiaan Huygens. Back in time, as he identifies and measures myopia in a person who lived in the 17th century. And backward for the process followed, a sort of reverse engineering: the diagnosis is, in fact, formulated from a defect - or rather, from a realization not up to the knowledge of the time - in the telescopes he made.

"It is probably the very first posthumous eye prescription, and made for someone who lived 330 years ago!" Pietrow notes, retracing the steps of his discovery on the Aip website. To formulate it, Pietrow reviewed the tables developed by Huygen and his brother for telescope design. These tables were accompanied by equations - as stated in Pietrow's paper - that closely approximated the tables and could be used when the latter were not at hand. One table, in particular, caught Pietrow's attention, created when Huygens was about 50 years old and apparently based on a single optimized telescope - through trial and error - of which the astronomer then used the parameters to build telescopes with the desired magnification.

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However, the parameters are far from optimal. And indeed, the telescopes that the Dutch scientist designed using these equations - despite enabling fundamental discoveries, including that of Titan - do not hold up in terms of sharpness compared to the best refractors of the time. Hence the hypothesis that the limiting factor in building his telescopes could be his own eyes: Huygens' designs could not be optimal because beyond a certain threshold, the scientist could not perceive any improvement.

If there is really this visual defect at the origin of Huygens' telescopes, one wonders how it is possible that the greatest mechanic of the seventeenth century - as Huygens has been called - did not find the time to build himself a pair of glasses between one telescope and the other. A tool that, for example, even his father used. According to Pietrow, the answer probably lies in the fact that Huygens had a mild myopia, especially for the time. For his contemporaries, not being surrounded like us by screens and signals of all kinds and not having to drive, myopia - below a certain threshold - was not a defect to be treated, as it is today. "My theory," Pietrow concludes, "is that since Huygens - unlike his father - did not need glasses in everyday life, he probably did not think of them when he made the telescopes. Thus unconsciously incorporating this visual defect into his designs."

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