1 



water for weeks. Leatherbacks also gen- 

 erate their own heat, keeping as much 

 as 40 Fahrenheit degrees warmer than 

 their surroundings. 



A sea turtle's senses, at least out of the 

 water, can sometimes seem out of kil- 

 ter — hatchlings, heading out to sea at 

 night, often confuse the lights of con- 

 dos and parking lots with the bright 

 horizon of open water. Yet once at sea, 

 the turtle is as flawless a navigator as an 

 airplane equipped with GPS. Leather- 

 backs fitted with satellite transmitters 

 have been tracked back and forth across 

 the Atlantic and the Pacific, and females 

 have returned, after an absence of more 

 than a decade, to the stretch of beach 

 where they were born. 



Like so many natural-history narra- 

 tives these days, Safina's is tinged with 

 anxiety. In some areas, particularly in 

 the Pacific, leatherback populations 

 have plummeted, ravaged by develop- 

 ment ashore and by unregulated fish- 

 ing at sea. Yet in the Atlantic, where 

 many shoreline communities have sea- 

 sonal regulations on lighting to ac- 

 commodate nesting turtles, and where 

 U.S. commercial shrimping nets are 

 now equipped with turtle escape de- 

 vices, leatherbacks seem to be flour- 

 ishing. Some populations are even 

 growing back exponentially. Sea tur- 

 tles have been around for a hundred 

 million years or so, and with a little 

 help from people like the ones in Sa- 

 fina's book, they may swim the seas for 

 hundreds of millions more. 



Sky in a Bottle 



by Peter Pcsic 

 MIT Press, 2006; $24. 95 



Why is the sky blue? That inno- 

 cent question is the point of de- 

 parture for Peter Pesic's magisterial his- 

 tory of light in the atmosphere. Its form 

 rec alls the encyclopedic monographs 

 of a century ago, focused on a single 

 point of erudition and replete with 

 footnotes in exotic tongues and arcane 

 alphabets. Pesic is a tutor and musician- 

 in-residence at the campus of St. John's 

 College in Santa Fe, New Mexico, 



whose curriculum emphasizes the 

 study of great books of the Western 

 tradition. So the mode of discourse the 

 author has chosen suits him just fine. 



But fortunately for the reader, the 

 scholarly Pesic has eschewed obfusca- 

 tion and produced a succinct and ap- 

 proachable intellectual history that 

 sheds light on the entire scientific en- 

 terprise. Sky in a Bottle begins, not sur- 

 prisingly, with the Greeks, who de- 

 bated all the fundamental elements of 

 the blue-sky problem: the structure of 

 the heavens, the phenomenon of color, 

 and the nature of vision itself. Does the 

 eye send out a stream of fire that "feels" 



Fanny Brennan, Sea Mail, 1987 



the objects it senses? Or do luminous 

 objects give off rays of some kind, 

 which convey color to the viewer? 



It was not until the late nineteenth 

 century that those questions were fully 

 resolved, and with them the question 

 about the color of the daytime sky. 

 Light, as it is understood today, is a 

 form of electromagnetic wave pro- 

 duced by the vibration of atoms. The 

 wavelength of light largely determines 

 the color we perceive. Sunlight, which 

 illuminates the sky, has a wide spec- 

 trum of wavelengths, but molecules of 

 air preferentially scatter the short, blue 

 wavelengths. By the same token, the 

 longer wavelengths — green, yellow, 

 red — tend to pass straight through the 

 air, so the color of the Sun we see is an 

 aggregate of those colors. So when we 

 look at the air instead of the Sun, we 

 see the scattered, blue light — the color 

 of the sky. 



Virtually every major philosopher 

 and experimentalist in Western science 

 contributed to this understanding. 

 They include household names such as 

 Newton — who pioneered the study of 

 the spectrum — as well as physicists 

 revered only by their own: Augustin- 



Jean Fresnel, Christiaan Huygens, 

 James Clerk Maxwell, and Thomas 

 Young. Leonardo da Vinci, the Re- 

 naissance polymath and artist, thought 

 deeply about how the atmosphere af- 

 fected colors. Indeed, Leonardo gets 

 the credit for the invention of "aerial 

 perspective," whereby the artist can 

 make the features in a landscape look 

 distant by adding some subtle bluish 

 tints. Leonardo's notebooks also doc- 

 ument an experiment to re-create the 

 azure of the sky in a bottle by shining 

 light through water-filled containers. 



Lesser known, but equally inven- 

 tive, was Horace-Benedict de Saus- 

 sure, an eighteenth-century Swiss nat- 

 uralist whose passion was mountain 

 climbing. To measure the blue of the 

 sky, which seemed to darken the 

 higher he went, de Saussure developed 

 a "cyanometer." Then he discovered 

 that a solution of copper sulfate and 

 ammonia could reproduce a jar of 

 heaven even more convincingly blue 

 than Leonardo's bottled sky. 



References to the blue of the sky 

 occur throughout Western art 

 and literature, and Pesic quotes from a 

 wide range of artists and writers — the 

 abstract painter Wassily Kandinsky, the 

 novelist Gustave Flaubert, the poet Wal- 

 lace Stevens, even Edgar Allan Poe — on 

 the baffling color of the heavens. 



Some may fault Pesic for casting his 

 net too widely, for mixing science, phi- 

 losophy, literature, and art with little 

 discrimination. Yet this little gem of a 

 book succeeds on a premise that echoes 

 the early debates of the Greeks about 

 the nature of light: that nature is more 

 than just physical forces acting on pas- 

 sive receivers. The blue of the sky owes 

 as much to mind as to matter. Pesic puts 

 it succinctly: "The sky as we see it al- 

 ways remains in the ultimate bottle: the 

 human brain." 



Laurence A. Marschall, author of 'The 

 Supernova Story, is W. K.T. Sahm Professor 

 of Physics at Gettysburg College in Pennsylva- 

 nia, and director of Project CLEA, which pro- 

 duces widely used simulation software for edu- 

 cation in astronomy. 



58 



NATURAL MIS I ory June 2006 



