The Sun in the Church: Cathedrals as Solar Observatories (review)

The Sun in the Church: Cathedrals as Solar Observatories. By J. L. Heilbron. Cambridge, Mass.: Harvard University Press, 1999. Pp. ix+366; illustrations, figures, tables, appendixes, notes/references, bibliography, index. $35.

Pilgrims, local devotees, and resident ecclesiastics were once the principal users of the great churches of the Middle Ages and the Renaissance. Modern crowds enjoy their architecture and artistic furnishings. J. L. Heilbron's remarkable book draws our attention to church users of a very different kind: early modern astronomers measuring the solar path to correct the shift of the ancient Julian calendar, and incidentally to confirm or refute details of the cosmic systems of Ptolemy, Copernicus, Tycho Brahe, and Kepler. Between the fifteenth and eighteenth centuries they installed meridianae, astronomical instruments to survey solar inclinations and declinations, in several large Italian and French churches. The Sun in the Church tells their history in detail, alongside an exceptionally comprehensive and clear account of medieval and early modern astronomy.

Heilbron casts a remarkably wide net, contextualizing almost every technical problem in its social and religious setting. He seeks to clarify the controverted role of the Roman church in the development of astronomical science, especially after its disastrous silencing of Galileo. He portrays a trapped institution, generally supportive of science but in inner conflict, with many high officials sympathetic to heliocentric theories but forbidden from teaching them as true. The Almagestum novum, an astronomical encyclopedia published in 1651 by the Jesuit Giambattista Riccioli and his collaborator Francesco Grimaldi, exemplified this approach, deploying Copernicus's and Kepler's systems as useful mathematical hypotheses, fictions that did not challenge decrees of the Holy Office. Heilbron argues that the Roman church has not yet resolved fully this thorny issue, and predicts it will ultimately escape the dilemma by canonizing Galileo.

Much earlier, astronomers of the Christian Middle Ages faced the critical problem of computing far into the future the dates of Easter, celebrated on the first Sunday after the first full moon after the vernal equinox. This daunting task required coordinating irrational solar and lunar cycles, plus Jewish and Roman calendric traditions. Dionysius Exiguus, the Venerable Bede, Roger Bacon, and the Arabic astronomers of King Alfonso the Wise of Castile all tried and failed to produce exact calendric tables. After a few centuries, Easter shifted erratically by as much as a month. Heilbron's may be the most coherent history of the problem, encompassing the generalized Ptolemaic description of the heavens in Sacrobosco's 1220s treatise on the sphere. To solve the problem, accurate and lengthy measurement campaigns were needed. The astronomical instruments would have to be enormous, very stable, and durable. The solid masonry fabrics of great churches fitted that description. In 1475 Toscanelli built the first church meridiana using Brunelleschi's dome at the cathedral of Florence. In the late sixteenth century Duke Cosimo I de Medici proposed to reform the Julian calendar, and his cartographer Egnatio Danti built instruments at Santa Maria Novella in Florence and at San Petronio in Bologna. The Gregorian calendar reform of 1582 paid no attention to Danti's admittedly flawed results.

In 1655 Giovanni Domenico Cassini, who in 1669 would become the royal astronomer of Louis XIV in Paris, built a new meridiana at San Petronio. This exceptionally precise instrument was as critical to the advance of astronomy in the late seventeenth century as the great telescopes at Mounts Wilson and Palomar were in the twentieth century. Fitting it into San Petronio involved piercing an aisle vault to insert a metal plate with a small hole, and fitting a long, accurately leveled iron line on the floor along the noontime path of the solar image projected from the vault. The meridiana, of course, is skewed with respect to the church axis. It is beautifully executed, but clearly an odd intrusion into the building. Heilbron, who apparently has no architectural training, does not notice this, nor does he seem surprised by the fact that...