A Guide to “The Sun in the Church” by J.L. Heilbron

Meridian Line. S. Petronio, Bologna, Calter Photo Analemma at S. Petronio, Bologna Solar eclipse at the Meridian line at S. Maria degli Angeli in Rome by Mario Catamo
Meridian Line. S. Petronio, Bologna, Calter Photo Analemma at S. Petronio, Bologna Solar eclipse at the Meridian line at S.Maria degli Angeli in Rome by Mario Catamo

The Sun in the Church: Cathedrals as Solar Observatories by J.L. Heilbron is an extremely interesting book. Unfortunately, it is not easy to read. It contains both serious mathematics and serious history. The purpose of this page is to assist people who want to read it.

For a quick summary, start with either How the Church Aided 'Heretical' Astronomy (with graphics) or Churches as scientific instruments by J. L. Heilbron, Annual Invitation Lecture to the Scientific Instrument Society, Royal Institution, London, 6 December 1995, published in Universitas, Newsletter of the International Centre for The History of Universities and Science, no. 9, April 1996.

For help with the astronomy, please consult my student NG Yoke Leng's undergraduate research project A Mathematical Supplement to “The Sun in the Church, Cathedrals as Solar Observatories” by J.L. Heilbron. The project is a web page, but here is a printer friendly PDF version. Yoke Leng also did a project with me on Easter computations.

My student THAM Peck Fun did an honours project on Bisection Of The Eccentricity. We went a bit further (and corrected some minor errors in Yoke Leng's thesis). At least one error remains. On page 32 and 34 she says that "the smaller circle should be 1/15 that of the bigger circle." It should be sine of 23.5.

If you have a hard time keeping track of the different meridiana mentioned in the book, this table may be useful. Remember to check out the links to the web pages about the meridiana of some of the individual churches.

City Church Designer Year Page Figure and Plate
Florence S. M. Novella Danti 1674 P. 68 Fig. 2.28, 2.29
Santa Maria del Fiore (additional link) Toscanelli 1475 P. 70 Plate 1 (p. 70), 6
Ximenes 1755 P. 226 Fig. 2.30
Bologna San Petronio Danti 1576 P. 72 Fig. 2.31
Cassini 1655 P. 89 Fig. 3.4, 3.5, Plate 2 (p. 72), 4
Cassini 1695 P. 137
Zanotti 1776 P. 138
The Vatican Torre dei Venti Danti 1576 P. 79 Plate 3 (p. 79)
Rome Santa Maria degli Angeli Bianchini 1703 P. 147 Plate 5 (p. 156)
Paris Observatory Giovanni Domenico (Jean Dominique) Cassini 1671 P. 143 Fig. 3.1
Jacques Cassini 1729 P. 173
Saint Sulpice Lemonnier 1742 P. 220 Fig. 7.1, 7.3
Milan Duomo Cesaris 1786 P. 266
Palermo Duomo Piazzi 1801 P. 272 Plate 7 (p. 273)

Other links of interest.

Book reviews.

You may also enjoy the page for my course Heavenly Mathematics & Cultural Astronomy and Myths about the Copernican Revolution.

Helmer Aslaksen
Department of Mathematics
National University of Singapore

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