A brilliant comet, with a tail stretching across the sky, can be a magnificent sight. Some comets seen during the 19th century were bright enough to cast shadows, and only last year the latest spectacular visitor, Hale-Bopp, caused immense interest among scientists and non-scientists. Today we know a great deal about comets, but in earlier times they are regarded with dread. They were sent to give divine warning about disasters ahead - disasters that might be natural, man-made or both. This view prevailed until well into the 17th century, and echoes of it persist even now. Panics associated with Halley's comet occurred in 1910, and to a lesser extent at its last return in 1986.
In this book, Sara Schechner Genuth, of the Smithsonian Institution, sets out to trace the story of our understanding of comets from very early times up to the period of Isaac Newton and Edmond Halley. It is a story that extends into many spheres of investigation, from alchemy and astrology to superstition, divination, politics, religion and, eventually, pure science. All of these branches of research are intertwined, and here they are brought together in a way that does not seem to have been attempted before, at least not in such detail.
As is pointed out in the first chapters, astrology played a major role in ancient and medieval times, and virtually all early astronomers were also astrologers, so that it was quite natural for comets to be associated with the deaths of leaders and rulers. Remember the lines in Shakespeare's Julius Caesar: "When beggars die, there are no comets seen; the heavens themselves blaze forth the death of princes." In ancient Rome there was little doubt that Caesar's soul was indeed carried aloft by a bright comet. It was Halley's comet that shone down over England soon before the overthrow of the Saxon kingdom at the battle of Hastings. Halley's comet was again in the public eye in 1456, when the Turks were threatening Europe, and the pope, Calixtus III, preached against the comet as an agent of the devil. Then, in a chapter of the book titled "The decline of cometary divination", we learn how educated people began to veer away from "vulgar" almanacs and beliefs and to have less faith in the power of the astrologers. The process was slow, but it was at least steady.
Comets were still regarded with apprehension, and there was a fear that a direct collision might have dire effects upon the earth. The flimsy nature of a comet was certainly not appreciated, even by Newton, who went so far as to write that "The vapours which arise from the Sun, the fixed stars, and the tails of comets, may I pass gradually into the form of salts, and sulphurs, and tinctures, and mud and clay, and sand, and stones, and coral, and other terrestrial substances." Halley was the first to show that comets move around the sun, but he also believed that a direct blow from the bright comet of 1680 would have caused a major change in the earth's orbit. There was also William Whiston, Newton's successor as Lucasian professor at Cambridge, who argued that at 11am on Thursday, November , 2348bc, the earth had passed through the tail of a comet and been doused with water, which, needless to say, accounted for Noah's flood. Whiston also held that the world would end as the result of a cometary impact.
Ideas changed mainly because of the spread of education and the increased knowledge of astronomical science. Comets came to be regarded as cosmologically important, as indeed they are; they can give us vital clues about the origin and evolution of the solar system. Yet, as is brought out in the middle section of the book, these advances were still contemporary with old superstitions. In the 17th century, George Cheyne concluded that "these frightful Bodies are the Ministers of Divine Justice, and in their visits, send us Benign or Noxious Vapours, according to the Design of Providence". In 1750 Thomas Wright, author of the famous "Original Theory" of the universe, believed that comets were fiery bodies spewed out from stellar volcanoes. It was only in 1805 that the French mathematician Laplace was able to show that comets were of negligible mass by planetary standards.
All of these various angles are described in this book, and the text is written so clearly that it can be followed even by the reader with no previous knowledge of science. It is also well illustrated. However, perhaps its main strength lies in its value to the serious student. A tremendous amount of research has been undertaken, and the list of references covers almost 100 pages. This means that there is also a great deal of information about the social, political and religious views during the period covered.
There is a brief appendix, suitably titled "Recent resurgence of cometary catastrophism", that is highly relevant to what has gone before. For example, in the 18th century Pierre Maupertuis wondered whether comets might contain "alien fluids" that could infect the air and water of a planet, thereby to some extent anticipating the views expressed today by Fred Hoyle and Chandra Wickramasinghe. Whether or not the dinosaurs were wiped out as the result of a cometary or meteoritic impact is still a matter for debate, but that we could suffer a major hit is not in doubt, and there have been official discussions of ways in which we could deal with the situation if the potential impactor were discovered in time.
Comet literature is extensive, but to the best of my knowledge no previous work has quite the overall grasp of this one. There are some books dealing with restricted subjects that seem certain to become standard references for many years to come. This is one, and it should be in every serious scientific library.
Patrick Moore is an astronomer who has specialised in studies of the moon. He is a past president, British Astronomical Association.
THE TIMES 7Jjune 26J1998 history of science books 25 Sky light: The Comet of 1811 (engraving by H. R. Cook after Abraham Pether) as seen at daybreak on October 15, from Otterbourne Hill, near Winchester
Comets, Popular Culture and the Birth of Modern Cosmology
Author - Sara Schechner Genuth
ISBN - 0 691 01150 8
Publisher - Princeton University Press
Price - £32.50
Pages - 365