Meridian Lines

Wilmari Claasen and Alessia Tami (University of Zurich) 

Portrait of Cassini and depiction of his meridian line in San Petronio, 1695. Image source: Cassini, Giovanni Domenico. La meridiana del tempio di S. Petronio tirata, e preparata per le osseruazioni astronomiche l’anno 1655. Riuista, e restaurata l’anno 1695. Di Gio: Domenico Cassini. In Bologna: Per l’erede di Vittorio Benacci, 1695, [Leaf 4 verso].

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As well as contributing to elegant building design and sunlit interiors, architectural design theory involving the strategic use of sunlight could serve astronomical purposes. The Catholic Church was interested in meridian lines and often sponsored their construction since they could be used to calculate the date of Easter more accurately. Meridian lines were mapped onto the floor inside large dark buildings, often cathedrals, and ran from south to north. Sunlight would enter through a hole in the roof and illuminate the meridian lines. In this way, astronomers could use the interior space to tell the time and make calculations about the position of the Sun at noon.

Cathedrals lent themselves to the construction of meridian lines due to their massive scale and cavernous dark interiors. Some examples of buildings with meridian lines include the Santa Maria Novella in Florence (1574, incomplete), the Oratorian College in Marseille (1636), and the Basilica San Petronio in Bologna (1655). The meridian line in San Petronio forms part of the oldest cathedral observatory and was installed by the Italian and French astronomer Giovanni Domenico Cassini, a feat some thought impossible due to the layout of the basilica. Cassini succeeded in installing a meridian line over two hundred feet long, which just barely managed not to cross paths with any of the interior pillars.

Floor plan of San Petronio showing the meridian line. Image source: Cassini, Giovanni Domenico. La meridiana del tempio di S. Petronio tirata, e preparata per le osseruazioni astronomiche l’anno 1655. Riuista, e restaurata l’anno 1695. Di Gio: Domenico Cassini. In Bologna: Per l’erede di Vittorio Benacci, 1695, [Plate inserted after p. 75]. 

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It was, however, quite difficult to construct meridian lines that allowed for precise observations. If surfaces were not aligned in exactly the right position, then measurements and observations made using the line would no longer be accurate. The floor had to be exactly level, for instance, and the hole in the roof through which sunlight entered had to be at a precise height. Even if one managed to build a perfect meridian line, buildings often settle over time, causing elements such as the height of the hole to shift and throw off the accuracy of the measurements. Nevertheless, the meridian line in San Petronio led to astronomical observations that supported Johannes Kepler’s discovery of elliptical orbits, making a major contribution to ongoing debates surrounding the ordering of the solar system.

Click here to learn more about Kepler.

Click here to learn more about Cassini and his meridian line.