Meet the cyanometer: an 18th-century device solely built to measure the blueness of the sky

The cyanomètre. Credit: Bibliothèque de Genève, Switzerland.

When 18th-century Swiss scientist Horace-Bénédict de Saussure was in his twenties, he had one epic goal in mind: to climb the summit of Mont Blanc, the highest mountain in the Alps. However, ever the scientific mind, he didn’t want to complete his quest without objectively recording the color of the sky from the highest peak in Europe. So in 1789, Saussure invented a tool for measuring blueness — a circle of blue hues called the cyanomètre.

The scientific method hinges on observation and measurement. In ancient times, the human body was at the forefront of measurement — the width of a foot and the distance of a step were all accepted measurements. Nowadays, we have sophisticated instruments that objectively measure anything from temperature to time. For Saussure, measuring the sky’s blueness was also important.

When Saussure was a youth, the summit of Mont Blanc was still unclaimed. The Swiss researcher, who became a professor at the Academy of Geneva at just 22 years old, dreamed of being among the first to climb its tops. Unfortunately, his efforts always came up short. So the young scientist, who was particularly fascinated with the geology and botany of the Alps, publicly offered a reward to anyone who could prove they reached the top of the treacherous summit.

On August 8, 1786, twenty-seven years after Saussure issued his challenge, the grand prize was claimed by two mountaineers, Jacques Balmat and Michel Paccard.  “I had reached the goal where no one had as yet been,” Balmat said at the time, “not even the eagle nor the chamois.”

Portrait of Horace-Bénédict de Saussure, in the Library at Geneva. Credit: Bibliothèque de Genève.

Later, Saussure and other mountain climbers noticed that the sky became increasingly blue higher up in altitude. As he prepared for new attempts to climb Mont Blanc, the Swiss scientist decided he wanted to measure the color of the sky.

Over the course of several years, Saussure refined his idea into a tool called the cyanomètre, a circular array of 53 shaded sections printed on a piece of paper. That’s it.

An early cyanometer made by Saussure from 1790. Credit: Biodiversity Library.

It may sound like a silly invention, but just because it’s simple, doesn’t mean it’s useless. On the contrary, when Saussure finally claimed his own ascent of Mont Blanc in 1787, just one year after Balmat and Paccard, he measured that the sky corresponded to 39th-degree blue.

Later, famed geographer Alexander von Humboldt took the cyanometer to expeditions across the Atlantic, where he measured the blueness of the sky above summits in the Caribbean, the Canary Islands, and all over South America. The darkest sky Humboldt recorded was over Mount Chimborazo, the highest peak in Ecuador, at 46th degree of blue.

Saussure’s theory was that the sky changed color with altitude due to the amount of water vapor in the atmosphere. In the 1860s, John Tyndall showed that light scattering was the reason why the sky is blue in the day but red at sunset.

Saussure invented other instruments for measuring meteorological phenomena, including the diaphanometer for judging the clarity of the atmosphere, the anemometer for measuring wind speed and direction, and the mountain eudiometer, a device that measures the change in the volume of a gas mixture.

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