review by Albert Van Helden
The story of the revolution in astronomy that Nicholas Copernicus began in 1543 is now a well-established narrative: Copernicus put the sun in the cneter of the planetary orbits. Tycho Brahe provided accurate observations. Kepler made the orbits into ellipses. Galileo discovered new things in the solar system and eloquently argued for the Copernican System. And Newton tied the new astronomy together with the new physics. By this standard account, astronomy and cosmology were dangerous subjects in Italy after the trial of Galileo, and very few contributinos to Copernicanism came out of that part of the world. In The Sun in the Church, historial John Heilbron argues convincingly that these long-held interpretations are too simplistic and must be revised.
Although Heilbron is perhaps best known for his work on 20th-century science, he has also published widely on earlier times. His volume on electricity in the 17th and 18th centuries is a classic, and his new book will surely match it in importance. The Sun in the Church is a history of meridiane constructed in churches, "reverse" sundials that turned churches into giant pinhole cameras. In the 15th century, the humanist Paolo Del Pozzo Toscanelli installed a short north-south line close to the altar of Florence's great cathedral of Santa Maria del Fiore. Near the summer solstice, the sun's image, projected through a hole in a window inthe cathedral's lantern 90 m above the floor, crosses the line. Toscanelli hoped to use this meridiana to discover changes in the inclination of Earth's axis. A century later, the Dominican cosmographer Ignatio Danti constructed a small meridiana in Florence's Santa Maria Novella and then a bigger one in Bologna's San Petronio. This latter meridian line, after Giovanni Domenico Cassini's improvements in the 1650s, became the central insturment in the reform of precision astronomy during the 17th century.
Danti's original amim had been to determine the length of the tropical year to aid in the correction of the calendar. The Gregorian reform of the calendar in 1582 made this measurement less important, although some technical problems with determineing the date of Easter still remained. With Cassini, the improved meridiana of San Petronio became an instrument to solve practical astronomical problems. The accuracy of tables that predicted the positions of plantes had been improved greatly by Kepler's elliptical astornomy. By the middle of the 17th cnetury, when Cassini began his work in Bologna, the great barrier to further precisino in position measurments--and therefore predictive tables--lay in the correctinos required for refraction and solar parallax. Tycho Brahe had accepted the ancient value of 3 arc min for solar parallax and as a result arrived at different refraction values for th sun and the stars. By his death in 1630, Kepler had reduced solar parallax to about 1 arc min, but the problem with refraction remained. When Cassini tackled the proble, there were different tables of solar refraction for different seasons. He realized that if solar parallax was 12 arc sec or less these different tables collapsed into one. But he also had to correct existing refraction tables when he found that ignoring refraction above altitudes of 45 degrees resulted in a discrepacny of 2 arc min between the heights of the celestial pole measured by direct observation and by means of meridiana. The point is that the San Petronio meridiana (with a height over 27 m) allowed measurments accurate to perhaps 15 arc sec.
Indeed, properly alligned and leveled, meridian lines were the most accurate position-measuring instruments availalbe. Even though measuring arcs equiplled with telescopic sights came into use around 1670, these instruments could not compete in accuracy with the best meridiane for another cnetury. Meridiane were, therefore, central in determination of fundamental astronomical constants and corrections. For instance, the San Petronio meridiana--and others built on its plan--could measure the sun's diameter with sufficient accuracy to show that the eccentricity of the solar orbit (Earth's orbit, to us) needed to be halved from its traditional value. By the 1670s, Cassini had corrected Tycho Brahe's erroneous value for hte obliquity of the ecliptic. Cassini's successors went on to determine the variatinos in the obliquity to the arc second. Their measurments were confirmed by transite theodolites late in the 18th century.
Heilbron tells an important story, one that is not so much neglected as unknown amount historians of science. Even in histories of astronomy, there is usually only a passing reference to it. And one looks in vain through standard accounts for mention of meridian lines and their historical significance. Perhaps that is because the story entails very technical matters. Heilbron's approach is to deal with the basic geometry in the text and to treat the most difficult technical problems in appendices. The discussinos of geometry that remain int he text will still drastically slow the reading rate even for those who are not mathematically handicapped, but skipping the technical passages does not take away from the narrative. With wry humor, Heilbron breaks off one such demonstration, "Since the calculation will be obvious to trigonometers and tedious for everyone else, it will be enought to give the result."
The book's title is not just an allusion to these wonderful meridiane, it also refers to the role of the Catholic Church in technical astronomy. One the one hand, the Church's involvement in, and patronage of science did not stop with Galileo's condemnation. On the other hand, in its very places of worship devout astronomers made measuremetns that strengthened the Copernican "hypothesis" and finally made all other astronomical theories simpley irrelevant."