**How do you determine the colour of the sky? In the 18th century, Horace Bénédict de Saussure climbed Mont Blanc to explore this question.** Wikimedia

The blue of the sky…

In August 1787, Genevan naturalist Horace Bénédict de Saussure climbed Mont Blanc with the aim of answering a seemingly childish question: why is the sky blue?

Katrin Brunner

Katrin Brunner is a self-employed journalist specialising in history and chronicler of Niederweningen.

Horace Bénédict de Saussure was born in Conches on 17 February 1740. Was blue his favourite colour? We don’t know. But in addition to numerous other questions of physics and observations of nature, he was also interested in the fact that the sky appeared bluer in the mountains. De Saussure was fascinated by the mountains from an early age. His passion for scientific subjects such as geology, botany and philosophy probably owed a lot to the influence of his two uncles, Albrecht von Haller and Charles Bonnet. Both were well-known and renowned naturalists and scientists.

**Portrait of Albrecht Haller, ca. 1830.** Swiss National Museum

**Charles Bonnet in a formal portrait dating from 1778.** Swiss National Museum

The locals called the mountain range around Mont Blanc “Les montagnes maudites”, the cursed or accursed mountains. The sky gets darker and darker the higher you climb; at some point, it is said, you are swallowed up by the blackness. But Horace Bénédict de Saussure wanted to determine the colour of the sky in a measurable form: “It is a well-known fact for all who have reached the top of high mountains that the sky there appears a darker blue than on the plains. But as the concepts refer more or less to indeterminate perceptions of which only traces remain in an often deceptive imagination, I sought a means of obtaining, so to speak, a sample of the Mont Blanc sky, or at least of the colour that this sky showed to me.”

**Looking up at the sky: Horace Bénédict de Saussure.** Wikimedia

This quest – and the aim of conducting further scientific experiments – prompted him, together with an entourage of 19 porters, to venture onto the roof of the Alps. He wasn’t the first person to climb the white mountain. A year earlier, Michel-Gabriel Paccard and Jacques Balmat from Chamonix had been the first to conquer the summit. De Saussure wasn’t concerned with making the first ascent. In the approximately four and a half hours it took the group to climb to the summit, he conducted various studies to define air pressure, humidity and the blue of the sky.

**Watercolour of de Saussure’s ascent of Mont Blanc, 1787.** Wikimedia

For this latter task, he invented the “cyanometer”. In a first version of this innovation, he glued 16 strips of paper onto a disk. The Geneva scientist dyed the individual strips of paper in different shades of blue which he had obtained by crushing the “Prussian blue” pigment. Further shades of blue were later added to the colour scale. Horace Bénédict de Saussure concluded that the differences in the colour of the sky had to be related to the humidity and transparency of the air. Alpine guides told him they had seen the stars even during the day. De Saussure believed them. A year later, in 1788, he stood 3,356 metres up on the peak of the Col du Géant; his son Nicolas Théodore was in the valley in Chamonix, and his friend Jean Senebier, a pastor and a fellow naturalist, was in Geneva. All three carried out their measurements simultaneously. In the results the trio obtained, Horace Bénédict de Saussure saw confirmation of his theory that atmospheric humidity influences the blue of the sky.

**Horace Bénédict de Saussure’s cyanometer, 1788.** Wikimedia

Sunlight is composed of the spectral colours – red, orange, yellow, green and blue – which we also see in a rainbow. When they enter the Earth’s atmosphere, the individual colours are scattered by gas molecules in the air, such as oxygen and nitrogen, at different wavelength speeds. In contrast to the long-wave red, the short-wave and faster blue encounters these molecules more often. However, our eye only sees the spectral colours when they are reflected by something – such as, for example, a gas molecule. At midday, the colour waves have a shorter path from the sun to us. It’s longer in the morning and evening. But this fact gives even the slower colour waves the chance to encounter and be reflected by the gas molecules in the atmosphere. This can result in red sunrises or red sunsets. None of this was yet known to Horace Bénédict de Saussure. Nonetheless, his research laid the foundations for scientists such as Alexander von Humboldt and Albert Einstein.

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