PD Smith

Why the sky is blue

Times Literary Supplement, January 25, 2008

Why the Sky is Blue: Discovering the Color of Life, by Götz Hoeppe (Princeton), 336pp, £18.95. ISBN: 978-0-691-12453-7

By PD Smith

One of the most memorable moments in Robert Musil’s disturbing novel about adolescent angst, Die Verwirrungen des Zöglings Törleß (1906; trans. Young Törleß), is when the troubled protagonist lies down in the grounds of his school and gazes up at the deep blue autumn sky. It is as if Törleß is seeing sky for the first time and he is shocked by its unfathomable depths:

“He felt it must be possible, if only one had a long, long ladder, to climb up and into it. But the further he penetrated, raising himself on this gaze, the further the blue, shining depth receded. And still it was as though some time it must be reached, as though by sheer gazing one must be able to stop it and hold it. The desire to do this became agonizingly intense.”

For Törleß, this encounter with the infinite comes to represent the ambiguity of experience and ultimately the inexpressible nature of reality. As Götz Hoeppe’s excellent history of our attempts to explain the blue of the sky shows, from moments of wonder like these, scientific theories grow. And, of course, not just science, but art has found inspiration in the blue above. Ruskin instructed his students to make careful observations of the sky. The “clear sky’s pure blue” (he said) was not just an inert hue, “but rather a profound, vibrating and transparent body of penetrating air”.

Leonardo da Vinci researched a book on light, shadow and colour, bringing his own acute observations together with 2,000 years of scholarship on the subject, including the ideas of Aristotle and the tenth-century, Basra-born pioneer of optics, Ali al-Hasan Ibn al-Haytham. Da Vinci’s research led him to conclude that “the blue air makes distant mountains appear blue” and he made this a rule of landscape painting: the progressive admixture of blue to give the sense of spatial recession. His blue remembered hills made a powerful impression on Goethe during his Italian journey and helped him frame his wonderfully original yet ultimately misguided critique of Newton’s account of light and colour. Despite Goethe’s best efforts, Newton’s notion that the air split white sunlight into rays of varying refrangibility (and thus different colours) remained the strongest explanation of the blueness of the sky until the early nineteenth century: “Most contemporary accounts just assumed that air or the particles contained in it reflect the refrangible (blue) light rays more than the less refrangible (red) rays, and left it at that”.

The Swiss geologist Horace-Bénédict de Saussure believed that the blue of the sky was due to moist particles in the air whose hue gave the air its colour. He invented a device in the 1760s called a “cyanometer” to measure the sky’s blue. This instrument consisted of a hand-painted colour circle calibrated with 52 shades of blue, ranging from white (“zero degrees”) to black (“51 degrees”). The putative air particles would, he estimated, measure 34 degrees on his cyanometer. Higher or lower readings were supposed to give an indication of how many particles were present in the air at a specific time and place.

On 20 June 1802, a German naturalist and a French botanist headed for “the summit of the world”. They were climbing Chimborazo, a steep pyramid of rocks and ice in what is now Ecuador, considered to be the highest mountain in the world since 1746. To scale it Alexander von Humboldt and Aimé Bonpland were dressed in leather shoes and frock coats. They didn’t even have gloves. Soon their hands were bloody from sharp rocks, their shoes were sodden and, as they ascended ever higher into the clouds, they began to suffer the effects of high altitude: “nausea, vertigo, difficulty breathing, and bleeding from the nose and gums”.

At noon on 23 June, the clouds parted and they were afforded – von Humboldt wrote in his diary – “a somber, magnificent view” of Chimborazo’s summit. Immediately, the intrepid explorer reached for his cyanometer. Von Humboldt stood as prescribed with the sun to his back and judged the degree of blueness on Saussure’s scale. A typical blue sky at midday at sea level measured 23 degrees. On Mont Blanc, Europe’s highest mountain, Saussure had recorded a blue of 39 degrees. “When the cloud cover over Chimborazo broke during that brief interval at noon on June 23, 1802, at an altitude never before reached by humans, Humboldt set a new record for the darkest blue ever seen: 46 degrees on the cyanometer”. As Hoeppe writes, the cyanometer embodies the Enlightenment’s “obsession with taking measurements of all the phenomena that could possibly be expressed in numbers”. By 1814, even Humboldt had acknowledged that its usefulness was limited; it was, he said, an “instrument that is still incomplete”.

In 1868, John Tyndall believed he had found evidence that “aequeous vapor” was the agent of colour production in the atmosphere. A mixture of hydrochloric acid vapour and air appeared blue in white light. “Never,” wrote Tyndall, “even in the skies of the Alps, have I seen a richer or purer blue than that attainable by a suitable disposition of the light falling upon the precipitated vapor.” Although the “Tyndall effect” is real, it was not in the end the long sought-after explanation of the sky’s colour. But thanks to Tyndall’s work, the ingenious hypotheses of da Vinci, Newton, Saussure, and many others were eventually supplanted by the explanation that is now accepted – light scattering. As Hoeppe says, “the sky is blue because air molecules scatter the sunlight!” According to the theory developed by Lord Rayleigh (John William Strutt) from 1871 to 1899, as light with short wavelengths (blue, violet) is more likely to be scattered than the longer wavelengths (orange, red), the scattered light is dominated by short wavelengths and so the sky above appears blue.

Why the Sky is Blue – first published in German as Blau: Die Farbe des Himmels (1999) – is a thorough and detailed history. Although the style is at times rather wooden, Hoeppe’s account of a complex subject is both lucid and engaging. He ends with a consideration of how the blueness of our planet is linked to the existence of life. Intriguingly, he suggests that the young Earth may not have had a blue sky at all. The lack of oxygen and the high levels of carbon dioxide in the atmosphere would have given the sky either a white or a yellow appearance. The relevance of this fact in an era of global warming and accelerating climate change leads to a somber and important conclusion: “contemplating the blue sky may be a first step toward understanding the atmosphere as a fragile and malleable realm that we must quickly learn to care for if we want to have a future on Earth, the best of all planets we know.”