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Why Catholics Built Secret Astronomical Features Into Churches to Help Save Souls

After centuries of war, Catholicism and science reconciled over meridian lines.

Giovanni Domenico Cassini's design for the meridian line in the Basilica di San Petronio in Bologna.
Giovanni Domenico Cassini's design for the meridian line in the Basilica di San Petronio in Bologna. Linda Hall Library of Science, Engineering and Technology

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.


High above the heads of church visitors, almost invisible in the arches of the  cathedral, a tiny hole lets in a beam of sunlight. This beam is what allows the  meridian line to function.
High above the heads of church visitors, almost invisible in the arches of the cathedral, a tiny hole lets in a beam of sunlight. This beam is what allows the meridian line to function. Geoff Manaugh

“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.

The zodiac sign for Cancer on the meridian line in the Basilica Santa Maria degli Angeli e dei Martiri in Rome.
The zodiac sign for Cancer on the meridian line in the Basilica Santa Maria degli Angeli e dei Martiri in Rome. Jean-Pol GRANDMONT/CC BY-SA 3.0

In the process of researching the phenomenon, Heilbron uncovered a surprising story of cooperation not only between religion and science, but between precision astronomical observation and Catholic liturgy, between architectural design and the Christian calendar. Direct, even enthusiastic collaboration, uniting esoteric science with canonical religious belief, lay at the core of this hidden story.

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.

This was not always the case, however. As Heilbron points out in his book, the Catholic Church supported astronomy for more than six centuries, from medieval times to the Enlightenment. This sponsorship continues into the 21st century, as we’ll see—although the meridian lines themselves had another, less lofty purpose entirely.


Giovanni Domenico Cassini's floorplan of San Petronio, showing the meridian line.
Giovanni Domenico Cassini’s floorplan of San Petronio, showing the meridian line. Internet Archive/Public Domain

Easter, a Christian holiday commemorating Jesus Christ’s resurrection from the dead, is defined not only by Church liturgy but also by astronomical circumstance. As specified by the First Council of Nicaea, Easter is not a straightforward anniversary, always recurring on the same date from now until eternity. Instead, it is to be celebrated on the first Sunday after the first full moon after the spring equinox—an occasion already laden with astronomical significance. The equinox, after all, is a day when time is split equally into 12 hours each of light and darkness, of illumination and obscurity.

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.

Gregorian reform eliminated, at a stroke and literally overnight, ten entire days from the western European calendar. People going to bed on October 4th, 1582, when the reform was implemented, would have woken up the next morning to find it was October 15th. Although this disorienting reform was intended specifically to put the calendar back on track for reaching the next spring equinox on March 21st, March 21st is not always the true, astronomical spring equinox. To determine exactly when the equinox would be, in the future—and, thus, when Easter should properly be celebrated—a more subtle and astronomically precise tool of measurement was required. A meridian line.

A portrait of Giovanni Domenico Cassini.
A portrait of Giovanni Domenico Cassini. Public Domain

Understanding the structure and rhythm of the cosmos through direct scientific observation was thus not antagonistic to Christian worship at all. It was an essential expression of human piety: an earnest attempt to synchronize human religious activity with the divine and invisible clockwork of the universe. And thanks to this vast astronomical device embedded in the floor of a cathedral, Easter could now be determined not just with a quick glance, but with unquestionable precision.

The first meridian line at Bologna was installed by an artisan and Dominican cartographer named Egnazio Danti in 1575; when the church was enlarged many years later, in 1653, however, a wall central to Danti’s instrument was displaced, fatally undermining its intended function. Almost immediately, Jesuit astronomer and engineer Giovanni Domenico Cassini was brought in to repair, extend, and substantially improve upon Danti’s work. Cassini later became director of the renowned Paris Observatory and shares credit for discovering Jupiter’s famous “red spot.” More recently, his name was borrowed for a high-profile NASA satellite mission launched in October 1997 to photograph the moons of Saturn.

Here on Earth, in 1655, Cassini began work on the meridian line that we still see today in Bologna. With the exception of repairs performed in 1695 and again in 1776, the device remains true to Cassini’s 17th-century design.


To help preserve the meridian line—but also to draw more attention to it—the  church has roped the line off from the feet of daily visitors. Just visible here, in a  plaque in the marble floor, are the zodiac signs of Pisces and Scorpio. Both  appear near the spring and autumn equinoxes, respectively.
To help preserve the meridian line—but also to draw more attention to it—the church has roped the line off from the feet of daily visitors. Just visible here, in a plaque in the marble floor, are the zodiac signs of Pisces and Scorpio. Both appear near the spring and autumn equinoxes, respectively. Geoff Manaugh

The Basilica di San Petronio tries to make the most of its meridian line. Although the line itself is now roped off and partially encased in plexiglass, these barriers have the effect of drawing even more attention to this strange diagonal slash across the marble. A small explanatory pamphlet is offered for sale near the cathedral entrance. Written by Giovanni Paltrinieri, a local historian, it comes in multiple languages; the English version was translated by none other than historian John L. Heilbron.

If it still seems surprising that a cathedral would house an astronomical instrument, consider the unusual spatial circumstances such a device would require. You need a large, flat surface on which a meridian line can be drawn. You need an open volume of unobstructed space through which a precise beam of sunlight can shine. You need a hole in a ceiling high enough so that this beam can track hundreds of feet, from one solstice to the next and back again. “The most convenient such buildings were cathedrals,” Heilbron writes.

The way meridian lines operate is both surprisingly complex and quite easy to grasp. As the sun tracks from north to south on its annual migration between the summer and winter solstices, its image on the cathedral floor also shifts, moving slowly along the meridian line. Halfway between the solstices, of course, are the spring and autumn equinoxes. Once the position of the solar circle indicates the spring equinox, believers must simply wait for the next full moon; the first Sunday after that full moon will be the proper date of Easter.

The winter solstice—Solstitium Hybernum—is marked on the cathedral’s marble  floor, accompanied by the mythological figure of Capricorn, one of the 12 signs of  the zodiac.
The winter solstice—Solstitium Hybernum—is marked on the cathedral’s marble floor, accompanied by the mythological figure of Capricorn, one of the 12 signs of the zodiac. Geoff Manaugh

So far, so good. Constructing a well-functioning meridian was no simple matter, on the other hand, and there were many ways it could go wrong. In order to support those cavernous church interiors, for example, huge columns are required; those columns are large and orderly placed, but they also invariably complicate the possibility of placing a straight line uninterrupted across the cathedral floor. In Bologna, for example, the meridian line butts up against—and is partially absorbed by—a column. Other architectural details, such as cornices, can also get in the way, blocking the beams of sunlight so essential to the meridian’s function. Heilbron points out, for example, that, in Rome’s Santa Maria degli Angeli, an architrave—or door lintel—had to be partially removed to ensure that sunlight could reach the meridian line.

This vision—of selectively dismantling the interiors of European cathedrals, one architrave at a time, in order to transform them into finely tuned scientific observatories—is extraordinary, as if, beneath all the masonry and ritual, with just a slight movement of specific details, powerful astronomical tools lie hidden. Beneath the pulpit, a planetarium.

From the point of view of Church doctrine, however, a larger problem was beginning to emerge. Perhaps these instruments were too precise. The observations they enabled began to reveal evidence not that the Earth was stationary, pinned at the center of Creation, but, on the contrary, that the Earth was mobile, circling in a dizzying choreography around the sun. The cosmic model endorsed by the Church was wrong, in other words, and proof of this wrongness was revealed every season by instruments built into the very floors of some of Europe’s finest cathedrals.

Well within sight of the church pews, a moving beam of sunlight suggested that the cosmos was altogether stranger than Christian theology had allowed itself to imagine. In Heilbron’s words, Catholic officials had not expected “that their cathedral would provide information about the heavens opposed to the teachings of their church.”


A detail from the frontispiece of Cassini's book <em>La meridiana del tempio di S. Petronio</em>, showing the sun shining into San Petronio and onto the meridian line.
A detail from the frontispiece of Cassini’s book La meridiana del tempio di S. Petronio, showing the sun shining into San Petronio and onto the meridian line. Internet Archive/Public Domain

Paul Mueller is a member of the Society of Jesus and administrative vice director of the Vatican Observatory. Raised in the American Midwest, he became a Jesuit after receiving a Bachelor of Science in physics from Boston University. From there, Mueller went on to an intimidating succession of advanced degrees in philosophy, divinity, sacred theology, and physics. He currently splits the year between the papal complex at Castel Gandolfo outside Rome and the Vatican Observatory in Arizona.

The Church, after all, is still actively engaged with astronomical research. Its sponsorship did not end with meridian lines, but has continued well into the 21st century with, among other things, the Vatican Observatory’s high-tech facility located in the remote darkness of Mount Graham, near Tucson. There, a dedicated team of Catholic astronomers—studying such topics as stellar evolution, the atmospheres of exoplanets, and processes of galactic formation—operates the Vatican Advanced Technology Telescope, or VATT, to watch the desert skies. Yet another unexpected juxtaposition of empiricism and faith will take place next spring, in May 2017, when the Observatory hosts an international workshop on “spacetime singularities” and gravitational waves at Castel Gandolfo, the Vatican compound on Lake Albano, the flooded crater of an extinct volcano southeast of Rome.

There is much less of a conflict than it might seem between advanced astronomical observation and Christian faith, Mueller explains to me, let alone between a basic measuring tool like a meridian line and Christian doctrine. In conversation, Mueller is precise and patient—but also slightly prickly. He does, after all, occupy an unenviable position of being attacked from both sides: by believers unsure of why he and his fellow brother-astronomers feel compelled to pry at the edges of the known universe, as if kicking the tires of Creation, and by secular physicists who seem all to eager to dismiss Vatican astronomy as hopelessly clouded by medieval superstition.

But, Mueller emphasizes, reason is crucial to religious practice. “If we are doing math, if we are doing science—for that matter, if we are doing art—any human activity done with generosity and reverence is acting in God’s image. Science is included in that.”

The meridian line at Santa Maria degli Angeli e dei Martiri, Rome
The meridian line at Santa Maria degli Angeli e dei Martiri, Rome Amphipolis/CC BY-SA 2.0

Contemplating the origins of the universe or measuring the obliquity of the Earth’s angular relationship to the sun is not in opposition to Christian faith at all, he stresses, no matter what those observations might reveal. Faith, in fact, he suggests, is precisely pinned on the confidence that religious doctrine and apparently contradictory scientific data will eventually be reconciled. There is no reason to dismiss or otherwise shy away from theologically uncomfortable scientific results, including previously controversial ideas, such as evolution and climate change, both of which Pope Francis has now urged Catholics to accept.

Whether the Church in Galileo’s time would have agreed with Mueller’s generous assessment is perhaps made clear by the astronomer’s cruel fate. Galileo, of course, was accused of heresy for promoting a heliocentric solar system (not to mention, as Heilbron writes in his biography of Galileo, for his arrogance). It was not until 1992, 359 years after his death, that Galileo was finally cleared of these charges—although the Church had quietly lifted its ban on Galileo’s heliocentric astronomy in 1757.

Mueller, however, also cautions me against over-emphasizing the role of meridian lines in Church affairs, in particular with determining the date of Easter. Of course, those lines would have been used to confirm the date of this major holiday, he says; after all, they were ingenious instruments of solar measurement. But remember, Mueller adds, the major Gregorian calendar reform took place in 1582—more than half a century before the era of meridian lines truly began and nearly 75 years before Cassini began his work in Bologna.

“My point is, sure, you can use a meridian line to help you with Easter,” Mueller tells me, “but that really happened after the fact—the calendar was already reformed by that time.” For Mueller, the meridian lines were simply testaments to the success of Gregorian calendar reform, given pride of place inside grand cathedrals “almost as a celebration of the fact that astronomy was used to fix the date of Easter, and in support of the Church’s new engagement with astronomy.” They were evidence of worshipful exactitude in celebrating the anniversary of Christ’s resurrection.


The interior of the Basilica di San Petronio, Bologna.
The interior of the Basilica di San Petronio, Bologna. Giorgia/CC BY-ND 2.0

Alas, by the middle of the 18th century, at the very height of the meridians’ instrumental power—after lines throughout Europe had been reworked, renovated, and fine-tuned over many generations to be more accurate instruments than ever before—they were superseded for the complex work of solar observation. Meridian lines had been made obsolete by ever larger and more powerful telescopes, and by the use of precision-ground glass lenses.

The meridians’ usefulness did not disappear entirely, on the other hand; the lines were simply demoted. Formerly relied upon for measuring solstices, revealing unexpected angular details about the Earth’s relationship with the sun, and marking the date of Christ’s Resurrection, they were now used to determine local noontime.

If you return to the church of San Petronio just before solar noon—which is different than the abstract noon indicated by your wristwatch or smartphone—you will see that the disc of pale light on the floor is shifting horizontally toward the meridian line. At the actual moment of solar noon, when the sun is highest in the sky, the beam of light will touch the line itself, passing across it as morning becomes afternoon.

The meridian line cuts diagonally through the church interior, narrowly missing  this support column. Although cathedral interiors were nearly ideal spaces for the  installation of meridian lines, they also posed many unique challenges.
The meridian line cuts diagonally through the church interior, narrowly missing this support column. Although cathedral interiors were nearly ideal spaces for the installation of meridian lines, they also posed many unique challenges. Geoff Manaugh

The meridian thus also acts like a daily clock—and, following their redundancy at the hands of powerful telescopes, meridian lines did indeed find a new role as a tool used to synchronize church bells. Heilbron refers to this as battling “the inconvenience of multiple noons,” an embarrassing problem where churches might ring out competing middays, sometimes several minutes apart, betraying the fact that the seemingly well-organized temporal structure of Christian civilization was in a state of minor disarray.

To a certain extent, this quotidian fate was built into the meridians’ very workings. As Heilbron points out, many of the craftsmen hired to work on the lines had been trained as clockmakers. Their skills in metalwork and precision instrumentation had prepared them well for work at an architectural scale.

Today, in an era of atomic clocks, smartphones, and digital timekeeping, however, meridian lines are not even useful for that. Now reduced to mere tourist attractions—and often overlooked entirely by visitors paying more attention to the frescoes, arches, and stained-glass windows—meridian lines nonetheless still function. They still silently mark the passage of time and the Earth’s movement around the sun, highlighting solstices and equinoxes as the rhythmic clockwork of the universe continues its restless motion.

Stranger yet, however, is the fact that these instruments are not, in fact, stationary. The churches, bulwarks of eternity, move. Little by little, decade after decade, these mountains of masonry are sinking, unsteadily settling into the soil, throwing off the angle of the sun and introducing a new source of inaccuracy into the resulting measurements.

The hole in the roof of San Petronio is decorated with elaborate solar motifs, both for symbolic purposes and to make it  visible to worshippers down below.
The hole in the roof of San Petronio is decorated with elaborate solar motifs, both for symbolic purposes and to make it visible to worshippers down below. Geoff Manaugh

This is true to the extent that the meridian line installed inside architect Filippo Brunelleschi’s Cathedral of Santa Maria del Fiore in Florence eventually became used as nothing more than a diagnostic tool for determining how much the cathedral itself had shifted. No longer useful for astronomical observation at all, the meridian became something more like an emergency light on the building’s creaking dashboard.

Even in Bologna, however, the church of San Petronio was moving downward. Heilbron explains that the hole punched through the cathedral ceiling, through which daily beams of sunlight blazed, “had fallen over 4 percent of its original height”—and this was in the late 1600s. It was only by comparing solstice measurements taken from San Petronio with other church meridians that the deformation was detected. Arguments over exactly how far the hole had shifted—with some people claiming only 1 percent, not 4—meant that “a battle ensued among the mathematicians of Bologna,” Heilbron writes. They soon descended into arguments over how their models of the cosmos had been distorted not by the Church but by the actual church: the building itself, that is, not the religious institution that funded their astral research.

Later, solutions to the problem of mobile architecture were proposed. These included attaching plaques to internal pillars that were structurally unconnected to the walls of the church. This meant that the plaques would not settle with the rest of the building and could thus be used as what Heilbron calls “a fiducial mark” for checking future measurements. Other approaches suggested using independently suspended floors so that, as the church changed position over time, the scientific instrument at its heart would remain untouched.

In a sense, Mueller’s point that the Church is now fully supportive of contemporary astronomical research is strangely borne out by this architectural detail. As these older buildings unevenly sink into the ground or become treated merely as sites for curious tourists, the real work of engaging with the cosmos has simply changed venue. The lines, however, remain, serving both as scars left by those unfortunate wars within the Church to separate faith and physics, and as seams that were able to bridge that gap.

Update 11/15: An earlier version of this story incorrectly stated that Galileo was executed for his beliefs. He was persecuted by the Catholic church, and forced to recant his views, but died naturally during his eight-year-long house arrest. 

Paul Mueller is the administrative vice director of the Vatican Observatory, not an astronomer there.