How blue is your sky?

You’re standing at the summit of Mt. Blanc. Your legs are trembling in the snow, your cheeks and nose are red, and there are tears in your eyes. You stare out in wonder at the sky. “It’s so blue!” your companion shouts in delight. And it is. A deep, deep blue you’ve never seen before – almost black. Blue number 1, on the cyanometer you’ve invented. You’re the first person ever to have scientifically measured the blueness of the sky.

In 1787, after more than a decade of trying, Horace Bénédict de Saussure got to the top of Mt. Blanc. He wasn’t the first – two local Chamonix men beat him to the top two years earlier – but that should have come as no surprise. Way back in 1760, at 20 years old, the enthusiastic Genevan scientist and alpinist himself had promised a reward to anyone who could find a route to the top. The good people of Chamonix didn’t let him down. Even so, Saussure was no less amazed at the colour he discerned on his instrument.

Saussure was passionate about measurement. Everywhere he went, to every mountain he climbed, he hauled thermometers and barometers, hygrometers (for measuring humidity of the air) and magnetometers, electrometers and diaphanometers (to measure the clearness of the atmosphere), anemometers (to measure wind speed) and mountain eudiometers (for measuring gas volume) – many of which he invented or improved himself. And to the summit of Mt. Blanc, Saussure brought another of his own inventions to measure the blueness of the sky. He called it a cyanometer.

The first version of his device, which Saussure carried to the summit, is a simple piece of card with three columns of swatches, each column alternating between various blue tones (16 in all) and white. You can still see it on display at the Musée d’histoire des sciences in Saussure’s native Geneva. The card has yellowed with age, and the colours have faded to a palette of greys, but what stands out are still the darkest, blackest blues – including the number 1 blue that Saussure matched to the sky on the summit of the Mt. Blanc.

A year later, when Saussure spent 17 days with his son Nicholas-Théodore at the Col du Geant, 1000m below the summit, he debuted the second version of his cyanometer. It’s the more famous version, the one you’ll also see at the museum, and all over the internet, and with no wonder. The wheel of 53 colours is still strikingly vivid and beautiful. Dyed with Prussian blue, the world’s first synthetic pigment, invented earlier in Saussure’s century, the extended palette runs from white (labelled 0) to the same dark, dark blue (now 52, instead of 1) Saussure measured at the top.

While no longer used, the cyanometer was, not unlike paper litmus tests for measuring PH, a handy tool for empirically measuring and better understanding changeable natural phenomena. And, by comparing simultaneous measurements taken by his son and colleague in Chamonix and Geneva, Saussure correctly concluded that what made the sky light (rather than dark, as on Mt. Blanc) was the increased density of particles in the air.

While today Pantone publishes swatch books with hundreds of shades of blue, like PMS 5493C, Saussure had happily settled on fewer – first 16, then 52 – to measure the blue that most mattered to him: the skies above his beloved Swiss Alps.