The Mountain Man Who Measured the Sky’s Brilliance

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In 1787, Swiss scientist Horace Bénédict de Saussure was one of the first to reach the summit of Mont Blanc, the highest peak in Western Europe. But he didn’t climb nearly 16,000 feet simply to marvel at the panoramic view of the snow-capped Alps: he came equipped with scientific equipment designed for this daring climb. Among his tools was a cyanometer, invented by Saussure to accurately measure the blue hue of the sky.

de Saussure, born on this day in 1740, was fascinated by the geology and flora of the Alps. In 1759 he graduated from the Collège de Genève, and a few years later he became a professor of natural philosophy at just 22 years old.

He was one of the first researchers to suggest that naturalists should collect detailed measurements in the field, rather than only collecting specimens. This fascination with the untapped data around him led him to a strange invention.

During his mountain expeditions, de Saussure noticed that the sky took on a more intense blue color at higher altitudes. He developed an instrument to measure this phenomenon: the cyanometer, a disk with pieces of paper dyed in different shades of blue. In August 1787, de Saussure began his ascent of Mont Blanc with the help of 19 porters and guide Jacques Balmat, who had become the first to reach this summit a year earlier.

At the summit, the vibrant blue he observed proved almost too blue for the cyanometer. It also measured atmospheric pressure, humidity and peak static electricity, among other data. de Saussure attributed these variable blues observed during hikes to the humidity and transparency of the air at different altitudes.

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A year later, he held the cyanometer skyward at the Col du Géant, about 11,000 feet above sea level, while his son did the same in the Chamonix valley below, with a friend in Geneva. For de Saussure, these measurements offered strong proof of his hypothesis. It was “the dawn of quantitative and coordinated meteorology,” writes chemist Andrea Sella for World of chemistry in 2010.

It took scientists about a century to truly understand the mechanism behind these contrasting blue hues: experiments by Irish physicist and mountaineer John Tyndall in the 1860s revealed that sunlight scatters particles in the upper atmosphere, giving the sky its color. And we know today that at higher altitudes, there are fewer molecules that repel light. This causes the darker shades seen by de Saussure during his ascension, and also explains why space is pitch black.

As the cyanometer fell out of fashion, de Saussure invented other instruments still used by scientists today. These include a type of hygrometer that measures humidity with a human hair. When relative humidity changes, organic substances like hair expand and contract. He also created what is believed to be the first solar cooker and helped popularize the term geology over its predecessor, geognosy – the new iteration certainly sounds much better.

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Main image: Wellcome Collection / Wikimedia Commons

• Molly Glick

  Published on February 17, 2026

  Molly Glick is the newsletter editor of Nautilus.