The Catholic Historical Review 88.4 (2002) 779-780
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The Sun in the Church: Cathedrals as Solar Observatories. By John L. Heilbron. (Cambridge, Massachusetts: Harvard University Press. 1999. Pp. ix, 355; 8 plates. $35.00 clothbound; $18.95 paperback.)
A close examination of the relationship between Catholic ecclesiastical patronage and the development of the natural sciences in Italian cities during thecentury after Galileo has long been missing. John Heilbron's investigation of the evolution of cathedrals or Duomos as significant solar observatories contributes impressively to that study. Heilbron reveals that Church leaders, motivated chiefly by a need for improved calendrics, financially assisted greater achievements than previously thought in observational astronomy and mixed mathematics in Italian cities in the late seventeenth century. Records from Italian cathedrals in Bologna, Florence, and Rome, along with St. Sulpice in Paris, are the major sources for Heilbron. His book combines ecclesiastical history and the history of science with astronomy, mixed mathematics, and architecture.
Heilbron first reviews germane developments in astronomy from Ptolemy to Copernicus, Kepler, and Galileo, finding "shameful" the Church's treatment of Galileo. Yet Heilbron scarcely considers studies of Galileo after 1990, such as Mario Biagioli's, and does not explore the influence of the Thirty Years' War upon the Papacy. He then recounts the scandals over calculating the date of [End Page 779] Easter from antiquity and Pope Gregory's calendar reform in 1582 supported by meridiane results on equinoxes. The size of cathedrals and their likeness to a camera obscura made them a powerful locus for meridiane, properly placed holes in cathedral roofs that determined the position of a rod on the floor as it marked the movement of the stuttering light from the sun's transit during the year. A solar year is complete when the line crosses itself. But the length of the solar year is 365.2422 days and obtaining a precise approximation was challenging.
Heilbron concentrates on astronomical observations and computations conducted at Santa Maria Novella in Florence and San Petronio in Bologna. Prominent among the savants whose work he investigates are the Dominican Egnatio Danti and Giovanni Domenico Cassini. With patronage from churchmen and the Medicis, Danti constructed a meridiana in mid-sixteenth-century S. Maria Novella and ordered the building of new instruments, including an armillary sphere and astrolabe. Skill in using these and trigonometric computations and conics enabled him to calculate more accurately solstices and equinoxes. Confirming the extensive mathematics in Heilbron's book takes time. In the 1640's Cassini, a professor of mathematics at the University of Bologna, had thegrand meridiana in San Petronio constructed that became the model for others. Cassini clarified the nature of comets and Saturn's rings, supported Keplerian astronomy, which church censors opposed, and devised a method todetermine longitude at sea. But Colbert drew Cassini to the Paris Observatory. Book banning and church censorship of heliocentrism did not bar Cassini's research, and their influence grew weaker. In the 1720's, for example, the Inquisition only delayed Gabriele Manfredi's heliocentric tracts in Bologna and did not prohibit his later support of stellar aberration based on Copernican astronomy.
As Heilbron makes clear, Italian cathedrals with carefully designed solar observatories and improving telescopes supported many accomplishments in observational astronomy and mixed mathematics in the century after Galileo. That suggests a more nuanced mentality toward Copernican astronomy on the part of the Papacy and Catholic censors, leading by the 1750's to receptivity. Apparently, only after the appearance of the new royal observatories in Paris, Greenwich, Berlin, and St. Petersburg were Italian efforts in astronomy surpassed.
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