The Sun in the Church: Cathedrals as Solar Observatories, by J.L. Heilbron
Astronomers who wanted to lay down a marker to watch the movement of the sun through the year needed a large, open, but secure space to do it. Cathedrals and other large churches answered the need. The author never explains how this got started, but from the sixteenth to the eighteenth centuries, meridian lines were carefully laid down in churches. This required painstaking placement of a hole high in the wall to let in the sunlight, as well as exact placement of the meridian line. The traditional orientation of churches along an east-west line, with a north-south transept, had something to do with the choice of churches to house this work, though the churches were not built with astronomy in mind, at least not with making observations. Building a church to face the rising sun as an aspiration in worship would not quite meet the astronomer’s need for precision. The meridian line might have to run up a pillar. Meridians were placed in Rome and Venice, Padua and Milan, Paris and Marseilles. They can be seen today if you pay a visit.
For church purposes, the meridian lines helped in measuring the year to define movable feasts; identifying dawn, noon, and midnight, to fix the times of divine office and of feasting and fasting; and specifiying the occurrence of twilight. Civic purposes included regulation of clocks. For astronomers, the meridian lines could be used to measure the lengths of the day and the year; to measure the declination and apparent diameter of thee sun, the motion of the North Star; the obliquity of the ecliptic; the latitude; and the right ascension of stars and planets.
All this began around the time that Galileo was tried. The book is useful to show the Church’s sponsorship of real practical astronomy. Until the nineteenth century, Italians set their watches by the sound of the church bell. The book ranges all over the place–from the change in the calendar, fixing the date of Easter, developments in telescopes, and the standardization of clocks and time zones. There is a great deal of math in the text that belongs in appendices. There are some color photos as well as old drawings and diagrams to illustrate the meridian lines.