Horace Benedict De Saussure. What hue is the blue of the sky?
Many pass by it without noticing. Others just cast a glance at it. Some stop to mirror themselves on its shiny surface, to check their make-up or glasses. Then they walk on. But the blue monolith at the intersection between Dalmatinova Ulica and Slovenska Cesta, in Ljubljana, is more than a futuristic monument: it’s an extraordinary instrument.
Conquering Mont Blanc
Its history begins in the summer of 1760, when Horace-Benedict De Saussure was 20 years old. He had just graduated and was becoming one of the youngest professors at the University of Geneva. His scientific knowledge was so broad that we cannot easily define him: geologist, physicist, botanist, meteorologist, mountaineer, and explorer. Above all, De Saussure had a great obsession: conquering the summit of Mont Blanc.
Before him, no one has ever even envisioned such an accomplishment. Rather, local legends told the mountain was haunted by evil spirits, willing to pull into the abyss anyone who approached the summit. De Saussure didn’t believe those myths, as he didn’t consider himself the right man to undertake the ascent. He was a climber and knew his way on the mountains, but he was also aware of his limits. So he promised a generous reward to anyone so fool or reckless enough to lead the way. That summer, he just hiked a few minor peaks, a more difficult task than it might seem, because De Saussure also had another obsession.
Centimeters, hours, knots, pascals
Locals witnessed him climbing the steep ridges around Chamonix, carrying devices to measure everything: distance, temperature, weather, humidity, magnetic field. And if the proper instrument did not exist, he invented it. De Saussure is credited with the invention of a magnetometer, a diaphanometer, a eudiometer, an anemometer and, above all, the hair hygrometer, which measures humidity, exploiting the property of human hair to lengthen or shorten according to the amount of water vapor in the air.
During his walks, De Saussure was especially fascinated by a peculiar phenomenon, well known to those familiar with the mountains: as one climbs higher, the color of the sky becomes more and more intense. So the scientist cut out some sheets of paper in squares, painted them with 53 shades of Prussian Blue, and used them to note the gradations of the sky. A few years later, he refined the idea. He arranged the cards in a circle and invented the cyanometer: an instrument that measures the blue of the sky.
Under the sky, on top of the mountain.
In 1787, Horace-Benedict De Saussure was 47 years old, gazing spellbound at the landscape from the top of Mont Blanc. The previous year, two young men from Chamonix, Jacques Balmat and Michel Gabriel Paccard, driven by enthusiasm (and the lavish reward offered by the scientist), had reached the summit. De Saussure financed a new expedition to finally fulfill his dream. Escorted by Balmat, a servant and 18 assistants, he brought all his measuring instruments with him. He remained on the peak for four hours, during which he proved the highest mountain in Europe measured 2478 tese, or 4809.7m, missing by only one meter the height now considered official. He experimented on the boiling temperature of the water and, most importantly, noted in his notebook the blue of the sky: that day corresponded to 39 degrees on his cyanometer, the most intense blue ever measured until then. Years later, another scientist, Alexander Von Humboldt, will take a cyanometer to the Andean summit of Chimborazo, recording a blue of the 46th degree, the deepest blue sky ever detected on a mountaintop.
De Saussure believed these color variations were due to the number of particles suspended in the air. His intuition was not totally wrong. The sky’s blue is related to the scattering of light, an effect to which nitrogen and oxygen molecules contribute, but also to water vapour particles in the atmosphere. But when physicists Tyndall and Rayleigh confirmed this concept in the late 1800s, De Saussure’s cyanometer had long been forgotten.
Today a specimen of the original instrument used by the Swiss inventor is preserved at the museum of science in Geneva. Still, the romantic idea of measuring the sky’s blue never ceases to fascinate.
The monolith on Slovenska Cesta in Ljubljana is a contemporary version of Saussure’s cyanometer, created in 2009 by German artist Martin Bricelj Baraga. Around the hole at the top of the parallelepiped, one can see the 53 gradations that recall those of the original instrument. But the Ljubljana cyanometer is also software that takes photos of the sky, records its gradation, collects information on air pollution and publishes the data in real-time on cyanometer.net. If one day the sky will only have shades of grey, the data collected by the Ljubljana cyanometer will remind us that we were the ones who allowed it to happen.
Further reading:
- Andrea Sella. «Classic Kit: Saussure’s Cyanometer». Chemistry World
- Sarah Laskow. «A Scientist Invented the Cyanometer Just to Measure the Blueness of the Sky». Atlas Obscura
- Maria Savi-Lopez. Leggende delle Alpi. E. Loescher, 1889.
- «Why Is the Sky Blue?». NASA
