Heilbron’s book, published in 1999 by Harvard University Press, was the first major English-language study to take this advice seriously, exploring the origins, meaning, and transformation of these early astronomical instruments hidden in plain sight, disguised in the very architecture of European cathedrals. Bologna’s Basilica of San Petronio is not the only example of a meridian line, although it was considered to be the most accurate. Santa Maria degli Angeli in Rome has a beautifully realized and particularly grandiose example cutting through its nave; Saint-Sulpice in Paris hosts its own, as does Santa Maria del Fiore in Florence; a church tightly nestled in the packed streets of Fossombrone, Italy, bears a meridian line; the heavily worn remains of a line are still visible in the cloisters of England’s Durham Cathedral; and the duomos of Milan and Palermo also contain their own meridian lines. [...]
The very fact that there is a longstanding connection between astronomical observation and the Catholic Church would surprise many modern readers. If anything, the relationship between these institutions—that is, between the altar and the telescope, the cathedral and the meridian line—would appear to be antagonistic, even contradictory. After all, the Church rather infamously persecuted Galileo in the 17th century for suggesting that the Earth is not, in fact, at the center of the cosmos, and that, by extension, Church doctrine relating to God’s orderly plans for the world were inherently flawed. Galileo’s rejection as a heretic has become emblematic of the popular belief that there is an abyss separating religious faith from rational scientific inquiry. [...]
The stakes of getting the date right were unusually high, Heilbron writes. If the faithful were to worship Easter on the wrong Sunday, out of sync with the rest of Christendom, then their very souls could be at risk. This was not merely an academic concern: at the height of the Church’s calendar problem, in the second half of the 16th century, the eastern Church and the western Church were an incredible ten days out of sync with one another. This was only reconciled in 1582 when Pope Gregory XIII implemented what has become known as the Gregorian calendar reform.