A disc of light moves across the cathedral floor. The marble in its path lights up, revealing deeply colored swirls, rich with hues of burgundy, plum, caramel, and ochre. It is ancient rock, stained by terrestrial chemistry and by the infernal pressures of the inner Earth. Its surface is smooth and nearly reflective, testament to extraordinary craftsmanship but also to the effects of hundreds of years’ worth of penitent feet processing through the looming shadows of the church interior. The air smells of smoke and candle wax, and the occasional perfume of a passing tourist.
The source of this light is a hole punched through the roof of the church high above, elaborately accentuated by a brilliant halo of golden rays, painted to resemble the sun. The hole acts like a film projector. Daylight streams through, creating a narrow beam of illumination visible only in the presence of smoke or dust, as if something otherworldly has been forced into material form.
Seconds pass, minutes, an hour. Outside, the sun appears to arc slowly across the daytime sky; here, in response, the projected disc creeps inch by inch across the marble floor. At solar noon, when the sun has reached its highest point in the sky, the circle of light touches a long, straight line made of inlaid brass and copper, nearly 220 feet from end to end, or two-thirds the length of an American football field.
Although this line extends more than half the length of the cathedral floor, it seems to follow its own geometric logic: it is a long diagonal slash cutting between two columns, against the building’s floor plan, as if at odds with the structure that houses it.
Stranger still, on either side of this brass line, words and celestial images have been carved directly into the rock. There are the 12 signs of the zodiac interspersed amongst Roman numerals and references to solstices. There is Aquarius, the water bearer; Capricorn, with its confusing mix of shaggy horns and the coiled tail of a sea creature; Sagittarius, preparing to fire a magnificent bow and arrow; and the pouting fish of Pisces. At first glance, these symbols seem pagan, even sacrilegious, as if the astral remnants of an older belief system have somehow survived beneath the feet—and beyond the gaze—of daily worshippers.
Yet these symbols are not there to cast horoscopes, let alone spells. They are there for purposes of church administration and astronomical science. This cathedral, the Basilica di San Petronio in Bologna, Italy, also doubles as a solar observatory—at one point, one of the most accurate in the world—and these signs of the zodiac are part of an instrument for measuring solstices.
“I can say that I literally tripped over the subject,” science historian John L. Heilbron explains to me. Heilbron is emeritus professor of history at the University of California, Berkeley, as well as the holder of advanced degrees in both history and physics. Born on St. Patrick’s Day in 1934, he continues to lecture on topics ranging from “vortex rings” to the epistemology of Niels Bohr. In addition to a biography of astronomer Galileo Galilei, he is author of an excellent book called The Sun in the Church: Cathedrals as Solar Observatories, a study of what are known as “meridian lines.”
When we speak, Heilbron is spending a long weekend in Joshua Tree National Park. He is an avuncular presence over the phone line, revisiting points he had published two decades earlier as if he had written them yesterday.
“I wondered what this brass line was doing there in the floor, encased in marble,” he continues, “with all these numbers running up and down it, inside a Catholic church. I decided to look into it. It was really that simple.”
“The lesson, though,” he laughs, “is that when you’re inside a church you should look down as well as up.”
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.