The Sun in the Church: Cathedrals as Solar Observatories.

John L. Heilbron. The Sun in the Church: Cathedrals as Solar Observatories. Cambridge, MA: Harvard University Press, 1999. x + 8 pls. + 366 pp. $35. ISBN: 0-674-85433-0.

This excellent book adds a welcome complexity to the historiography of astronomy in the years after Galileo's abjuration allegedly brought Italian astronomy to its knees. Heilbron's title refers specifically to the precise projections of the celestial meridian on a spate of church floors during this period (and well into the nineteenth century). Through an aperture high on a south-facing wall or roof, a measurable image of the sun daily crossed these finely graduated lines at local noon. More generally, the title hints at productive relations between astronomy and Roman Catholicism during a period often presumed to mark their nadir. For Heilbron, the Galileo affair is but a wrinkle in the centuries-long ecclesiastical patronage of astronomy, which culminates -- counterintuitively -- in the "big science" of the seventeenth- and eighteenth-century meridiane. His detailed case history of large-scale astronomical instrumentation thus substantiates a large-scale generalization that should inform every discussion of the Galileo affair.

To the reader who sees meridiane as glorified sundials, Heilbron demonstrates in detail their scientific sophistication. Paolo Toscanelli started the trend with his fifteenth-century meridiana in the Florentine duomo, but its heyday occurred in the decades after Galileo. Astronomer-craftsmen like Giambattista Riccioli, Giovanni Domenico Cassini, and their colleagues revamped older meridiane or built splendid new ones to exacting specifications in San Petronio (Bologna) and Saint Sulpice (Paris), and Santa Maria degli Angeli (Rome), among several others. These "heliometers" were the most precise astronomical tools available; in expert hands, they made signal contributions to astronomy, and indirectly to cosmology. Thus they refined such crucial parameters as the length of the year and settled the millennium-long problem of the varying obliquity of the ecliptic. They also generated data crucial for telescopic accuracy, most notably the thickness of the earth's atmosphere and the variation in its refraction, th e key astronomical problem in the 1 660s. When Cassini used the San Petronio meridiana ca. 1655 to measure the eccentricity of the Sun's apparent orbit, the results favored the relative dimensions of Kepler's ellipse, with side effects for Copernican cosmology.

The glamour of grand theory often draws the eye away from such challenging attempts to solve tightly circumscribed empirical problems. But Heilbron here highlights this demi-monde of "normal science" and its juggling of assumptions, measurements, techniques, theory, and calculation. Strikingly, not every one of these practitioners was interested in cosmology and, of those who were, not everyone was a Copernican. But all were, to varying degrees, obsessed by precise measurement. Riccioli and his associates counted almost 200,000 beats to determine empirically the length of a one-second pendulum at the latitude of Bologna. Measurement advanced astronomy largely oblivious to cosmological controversy, while unintentionally aiding updated versions of Copernicanism.

Heilbron's book also reinterprets the relations of science and religion in the shadow of the Galileo affair. The novelty of his argument is neither that religion can stimulate astronomy -- a well-established claim for both the Babylonian and Islamic contexts -- nor that ecclesiastical patronage encouraged learning, including astronomy, throughout the Middle Ages and Renaissance. It is rather that the Church signally fertilized astronomy in an era when most historians portray the two as antagonists. Indeed, research conducted with these highly precise, church-based meridiane unwittingly and gradually prepared the acceptance of Copernican cosmology in its emended seventeenth-century forms.

While Heilbron overstates the extent to which a "problem of administration" (the date of Easter) drove the Church's patronage of astronomy after 1200, he understands the subtle interplay of institutional logic, bureaucratic action and inaction, and ideological concerns in the decisions of both regulators and regulated. And when he considers the professions of belief in terrestrial immobility, he appreciates both their sincerity around 1650 (many historians, anachronistically, do not) and their hollowness fifty years later. As empirical evidence and theoretical coherence multiplied on the Copernicans, some high ecclesiastics not only winked, but unsuccessfully tried to reassure the faithful: upon receiving a Copernican armillary sphere from the cardinal-president of the Congregation of the Index in the early eighteenth century, the Bolognese Academy of Sciences nervously commissioned Ptolemaic and Tychonic counterparts for it. After this slow start, the rehabilitation of Galileo has now gathered such momentum that Heilbron predicts canonization within the century.

Although the publisher is courting the "general interest historian" (evidently repelled by real footnotes), Heilbron's ideal reader should enjoy trigonometry, geometry and the occasional challenge of awkward notation or an elliptical explanation. But even buyers insensitive to such pleasures will appreciate the witty prose of the argument and the elegant design of this important book.