IT IS currently summer in the northern hemisphere, which may mean cloudless azure skies, some of the time at least. What makes the sky this colour?
For scientists, the answer is relatively straightforward: Rayleigh scattering. When white light from the sun reaches the earth it hits the gas molecules that make up the atmosphere. These molecules—mainly nitrogen and oxygen—are smaller than the wavelengths of light in the visible spectrum and so scatter the light. White light is made up of different wavelengths, which, since Isaac Newton’s experiments with prisms in the 17th century, we think of as a spectrum of seven different colours: red, orange, yellow, green, blue, indigo and violet. Light at the violet end of the spectrum travels in shorter, tighter waves, which are affected more by the molecules in the atmosphere than the longer, lower-frequency waves at the red end. This phenomenon is named after Lord Rayleigh, the British physicist who discovered it in the 19th century. The sky appears blue because shorter wavelengths are scattered more by the atmosphere than longer wavelengths; so the scattered sunlight that reaches our eyes when looking at the sky (rather than at the sun itself) is predominantly blue.
Once you have got your head around that, it might seem that the answer to the initial question—“Why is the sky blue?”—is rather simple. But there is a catch: not everyone would agree that the sky is blue. In 1858 William Gladstone, better known for being the Prime Minister of Britain four times during the 19th century, published a treatise on Homer. He noted, with astonishment, that the Greek poet did not once use the word blue. He used colour words rather oddly—he described the sea as “wine-dark”, iron as violet and honey as green. Further research showed that the Koran, the original Hebrew Bible, the Icelandic sagas and the Vedic hymns, written in India