Bologna, San Petronio: Meridian of Giandomenico Cassini

Synchronizing Science and Religion? Why We Find Solar Observatories and Astronomical Features in Churches

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It is often assumed that science and faith are always at loggerheads with each other. This, however, is a common misconception, as there are numerous instances demonstrating the co-existence and co-operation between science and religion. One of these, for example, is the relationship between the Roman Catholic Church and astronomy, in particular, the former’s installation of astronomical features in churches, as well as their use of these sacred spaces as solar observatories.

Dome of Basilica Santa Maria degli Angeli e dei Martiri, Rome, Italy

Dome of Basilica Santa Maria degli Angeli e dei Martiri, Rome, Italy ( Public Domain )

Easter Calculations

For Catholics across the ages, the most important day of the year was Easter. Unlike Christmas, which is customarily celebrated on the 25 th of December, Easter is a moveable feast, which means that it would be celebrated on different dates each year. This was due to the fact that the correct prediction of this date depended (and still depends) on certain incredibly technical astronomical constants, including the length of a lunar month, i.e. 29.53059 days, and the length of a solar year, i.e. 365.2422 days. Traditionally, Easter is celebrated on the Sunday after the first full moon of spring. By the 12 th century, however, the normal ways used to predict the date of Easter had gone wrong, and the Catholic Church had to find other means of doing so. It may be added here that whilst the Gregorian calendrical reform of 1582 was instituted to address the question of the date of Easter, this problem still persisted.

It was imperative that Church authorities set the date of Easter years in advance, as this served to strengthen the Catholic Church’s power, and to ensure its unity. Therefore, astronomers were hired to work out the date of Easter. By pondering over old manuscripts and inventing devices that allowed them to observe the heavenly bodies, these astronomers not only successfully predicted the date of Easter years beforehand, but were also able to advance their science to greater heights. One of the evidences of this can be observed in churches, perhaps the last place that some would expect to find the presence of science. The most widely used international calendar, the Gregorian calendar was introduced by Pope Gregory XIII, as a more accurate tool to set the date of Easter than its predecessor, the Julian calendar.

Detail of by Pope Gregory XIII’s tomb by Camillo Rusconi (completed 1723); Antonio Lilio is genuflecting before the pope, presenting his printed calendar.

Detail of by Pope Gregory XIII ’s tomb by Camillo Rusconi (completed 1723); Antonio Lilio is genuflecting before the pope, presenting his printed calendar. ( CC BY-SA 3.0 )


In various parts of Europe, many churches and cathedrals have had astronomical features incorporated into them. One of the most visible of these is the meridian line. This was a device that enabled the astronomers of the time to measure precisely the progress of the Sun throughout the year. In other words, such cathedrals and churches in which meridian lines are found are in effect solar observatories. Some examples of places where meridian lines can be found include the Basilica of San Petronio in Bologna (Italy), the Basilica of Santa Maria degli Angeli e dei Martiri in Rome (Italy), the cloister of Durham Cathedral in Durham (England), and the Church of Saint-Sulpice in Paris (France).

The meridian line in the Basilica of Santa Maria degli Angeli e dei Martiri, Rome.

The meridian line in the Basilica of Santa Maria degli Angeli e dei Martiri, Rome. ( CC BY-SA 3.0 )

During that time, cathedrals and churches were the obvious choices for the construction of meridian lines for a couple of reasons. First of all, for a meridian line to function properly, a large, flat surface was needed, so that the meridian line could be drawn. Next, an open volume of unobstructed space was required, so that a precise beam of sunlight could shine through. Thirdly, a hole needed to be made in a ceiling high enough so that the beam of sunlight may be tracked for long distances, from one solstice to the next, and back again. Thus, cathedrals and churches were the ideal structures that the Church authorities had at their disposal for the construction of these meridian lines.

Meridian solar line of the Basilica Santa Maria degli Angeli e dei Martiri in Rome built by Francesco Bianchini (1702)

Meridian solar line of the Basilica Santa Maria degli Angeli e dei Martiri in Rome built by Francesco Bianchini (1702) ( CC BY-SA 3.0 )

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