![How do you determine the colour of the sky? In the 18th century, Horace Bénédict de Saussure climbed Mont Blanc to explore this question.](https://blog.nationalmuseum.ch/app/uploads/horace-titelbild-300x225.jpg)
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?
![Portrait of Albrecht Haller, ca. 1830.](https://blog.nationalmuseum.ch/app/uploads/albrecht-von-haller-233x300.jpg)
![Charles Bonnet in a formal portrait dating from 1778.](https://blog.nationalmuseum.ch/app/uploads/charles-bonnet-246x300.jpg)
![Looking up at the sky: Horace Bénédict de Saussure.](https://blog.nationalmuseum.ch/app/uploads/saussure-252x300.jpg)
![Watercolour of de Saussure’s ascent of Mont Blanc, 1787.](https://blog.nationalmuseum.ch/app/uploads/besteigung-mont-blanc-300x212.jpg)
![Horace Bénédict de Saussure’s cyanometer, 1788.](https://blog.nationalmuseum.ch/app/uploads/cyanometer-300x292.jpg)
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?