The Church's route to enlightenment

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The Sun in the Church: Cathedrals as Solar Observatories

by J. L. Heilbron Harvard University Press: 1999. $35, £21.95

Point of illumination: churches were an obvious place to install a meridiana.
The meridiana at the Duomo of Palermo: the Sun at noon in spring (in Taurus) or summer (in Virgo).

At first sight, the title of this book reminded me of similar titles by masterful storytellers, such as Willa Cather's Death Comes for the Archbishop and T. S. Eliot's Murder in the Cathedral. While it is certainly not in the traditions of fiction or fictionalized history, John Heilbron's book does tell a gripping story and with a splendid literary flair.

The story is about the early attempts to find where we are in a universe in which we think all the objects, except for ourselves, are moving. Since the search was a practical one, about how to live on the Earth, and since the Sun and the Moon are the most important objects in determining the rhythm of life on the Earth, the story is really about determining the position and motion of the Sun. But the author cleverly uses this theme to give literary unity to a much broader study in the history of science at the time when modern science was being born. By subtly inserting critical comments, the author evaluates the interactions of science in its gestation with the culture of those centuries and the repercussions that these interactions have had down to our own times. And so it becomes a story about people, and Heilbron tells it in a masterfully human way.

All his principal characters are focused upon the construction of the most effective instrument of the sixteenth and seventeenth centuries for measuring the position and motion of the Sun, the meridiana. The instrument is a very simple one. It consists of a hole in the south-facing wall of an edifice which would allow a beam of sunlight to fall on a line directed north–south (the local meridian) on the floor of the edifice. Since a large, dark enclosure was needed to observe the changing position of the sunbeam on the line, churches were the obvious places to install a meridiana. Churchmen encouraged and supported this because, among other practical matters, it would help in determining the calendar of Church feasts—especially Easter, which, as determined by the Gregorian calendar reform of 1582, falls on the first Sunday after the first full Moon after the vernal equinox.

Heilbron tells the fascinating story of how this was done in various churches, mainly in the Papal States, capturing with splendid irony the character of the principal personages involved. He conveys the spirit of the times and passes judgement on the larger issues that were brooding beneath the surface of the meridiane. It appears that Giovanni Domenico Cassini, a closet Copernican, is Heilbron's hero or, at least, the scientist most characteristic for him of that age. He is described brilliantly: “We are led to the unappealing, unromantic, counterintuitive conclusion that Cassini led the astronomers of his time without commitment to any cosmology or world system and that he lived amicably with the persistent traditionalists of Bologna and the convinced Copernicans of Paris.”

Heilbron's hallmark irony is found throughout the book. Copernicus, sent to Italy by his uncle, spent seven years “soaking up the sun and the Renaissance”. His “Greek was as accurate as the Alfonsine tables”. Johannes Kepler replaced the “hoary principles” of circular motion and constant velocity with “the products of exasperation, exhaustion, and genius”, namely, ellipses. Cardinal Gregorio Barbarigo, who promoted mathematics and the meridiane, “came within an ace of being elected pope and ended up a saint. … The famous blue-stocking of Bologna, Laura Bassi, received a licence to read Descartes and others' bad books sometime before 1740, although she suffered under the double disadvantage of being female and under age." My favourite is the picture caption: “A Jesuit telescope suspended by faith”.

Those who try to interpret this book as an apologia for the Roman Catholic Church (see The New York Times review, 19 October 1999) are grossly mistaken. Read carefully the first two sentences of the book. With his usual brilliant irony Heilbron attributes the Church's devotion to the science of the meridiane to “administration”. In fact, I cannot help but think that the clever title of the book is itself an ironic judgement, and a correct one too, on the Church of those times. The Sun is our great illuminator. Did it, upon its entry through the hole of the meridiana, illuminate the Church? The implied answer is obvious and, when the author discusses some of the more fundamental problems that the new science was creating for the Church, he makes it clear. He makes the correct judgement that “Urban VIII made a bad mistake in associating himself with Galileo's condemnation and with the characterization of heliocentrism as heretical in any sense”. Would that it had been a conclusion of the Galileo Commission, which instead put all of the blame on “some theologians”.

In his discussion of the commission's work, Heilbron is far too lenient. For instance, in the discourses of 31 October 1992, which apparently terminated its work, the commission concluded that Galileo was wrong because he would not accept that Copernicanism was hypothetical and because he had no proofs. There is an ambiguity involved in the use of the word “hypothesis” and Heilbron is well aware of it as he alludes to it frequently, especially in his treatment of Cassini. There are two distinctly different uses of the word: a mathematical expedient to predict celestial events, or an attempt to understand the true nature of the heavens. There is no doubt that Galileo understood his own investigations to be an attempt to understand the true nature of things. It is well known that he preferred to be known as a philosopher of nature rather than as a mathematician. It cannot be denied that he sought evidence to show that Copernicanism was really true and not just a mathematical expedient. He sought to find observational verification of it and, if not with total success, with more success than had been achieved by anyone up to his day. He can certainly not be accused of “betraying the very method of which he was the inspired founder”, one of the conclusions of the commission. Heilbron could have seized the opportunity to knock the commission on the knuckles a bit.

Also, he propagates a common error by claiming that Galileo's works were removed from the Index of works that Catholics should not read as a result of the Settele affair, in which a textbook on optics and Copernican astronomy went through a tortuous procedure before it received papal approval. The 1822 imprimatur on Settele's book did not refer to Galileo, nor to the sentence of 1633. It referred to the teaching of Copernicanism. And if it is claimed that the imprimatur implicitly reformed the sentence of 1633, why was that not made explicit? As a matter of fact, the works of Copernicus and Galileo remained on the Index until 1835, more than a decade after the Settele affair and even closer than Heilbron thinks to the discovery of stellar parallax, a “proof” of Copernicanism.

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Coyne, G. The Church's route to enlightenment. Nature 402, 579–580 (1999) doi:10.1038/45070

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