Universal questions in shades of blue

Sky in a Bottle
July 7, 2006

Why is the sky blue? Most elementary books give a simple explanation. Sunlight is a mixture of all the colours of the rainbow, from red (longest wavelength for visible light) to blue (shortest). Our air is at its most efficient in scattering the short wavelengths, so that the red and yellow rays come more or less straight through to us, while the blue part of the sunlight is smeared all over the sky.

This being so, it may at first seem strange to devote a whole book to the matter. But, as so often happens, things are not so clear-cut as might be thought, and Peter Pesic of St John's College in Santa Fe takes his readers down all sorts of unexpected avenues. Characters such as Leonardo da Vinci, Sir Isaac Newton, Lord Rayleigh, Albert Einstein and even Edgar Allen Poe flit through the pages, and interesting experiments are described - such as filling a bottle with water and then adding material to make the water sky blue.

We usually associate the colour blue with peace and goodwill, but this has not always been the universal view. For example, the Koran states that "demons have blue eyes, denoting their evil, infidel nature". Blue could have sinister connotations.

In 1862, Sir John Herschel, the leading astronomer of the time, wrote that the blueness of the sky was one of the "great standing enigmas" of meteorology, and in earlier days there were theories that seem strange now.

It was often believed that although air itself is pure white, the sky is blue because it "lets through the surrounding darkness".

The Italian scientist Ristoro d'Arezzo wrote that "if between the eye and the sky were blue-coloured air, all the stars would appear blue, which is not seen". And in the 18th century, the great mathematician Leonhard Euler speculated how terrible it would be if the sky were not blue, with the Sun surrounded by total darkness. He said: "This proximity of lustre insupportable and darkness the most profound must destroy the organs of vision, and quickly reduce us to total blindness." Yet this was precisely the view enjoyed by the Apollo astronauts on the surface of the moon, with no eye damage whatsoever. As Pesic points out, it is only in comparatively recent times that we have had any clear understanding of the nature of light.

Pesic goes on to bring the story up to date, and shows how we have tackled the problem that so puzzled men of the calibre of Herschel. This is not a book for the absolute beginner, because it does become quite technical when dealing with modern optical theories, but it is beautifully written and there are no mathematical formulae, so that it should have wide appeal.

There are detailed references that will be very useful to the serious student; the illustrations are adequate; and there is a good index.

This unusual book will be a worthy addition to any scientific library, public or personal.

Sir Patrick Moore is a fellow of the Royal Society and the author of more than 60 books, mainly on astronomy.

Sky in a Bottle

Author - Peter Pesic
Publisher - MIT Press
Pages - 262
Price - £16.95
ISBN - 0 262 16234 2

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