The Inquisition followed sound science
Two brilliant astronomers, one powerful church, and a handful of planets: It's easy to mock geocentrism hundreds of years later, but evidence at the time spoke otherwise.
toni demuro for the boston globe
By Jacob Haqq-Misra

When Pope John Paul II announced in 1992 that Galileo was correct, more than 350 years after his condemnation by the Inquisition, the world reacted with apathy, relief, and amusement. No one doubts any more that Earth revolves around the sun, and even private Catholic schools had been teaching heliocentricity to their students prior to the official apology.

Our historical understanding of the Galileo affair tends to implicate the church as clinging unnecessarily to a literal interpretation of the Bible, which required the faithful to accept the untenable theory of geocentrism. From elementary school onward, we’re taught that the church stood firmly athwart scientific progress, bellowing “Stop!’’ Indeed, the clash has gone down through the ages as a sort of morality play of science versus religion, pitting the proponents of progress against religious reactionaries. But what if that morality play itself is nothing more than dogma?

We know that Galileo’s ideas were correct, yet the best science of the 17th century tended to favor a universe with Earth at its center. Did the Inquisition rely too heavily on theology, or did its perpetrators instead judge Galileo according to the scientific standards of their day? In other words: Did the Inquisition get it right?

Like all good science, the answer to that question starts with the evidence. And in the case of Galileo and the mystery of the design of the cosmos, evidence includes two specks: one a mark of punctuation, the other the width of a distant star as seen from the ground with the naked eye.

The supernova of 1572 perplexed kings, citizens, and astronomers alike. Up until then, the distant stars were assumed to be eternal and immutable, so the world took notice when an aging star imploded on itself in a fiery conflagration in the middle of the night sky, visible to the naked eye. Some astronomers claimed that this strange phenomenon occurred lower in the celestial sphere, below the moon, in order to preserve the prevailing Aristotelian worldview. Others were not convinced.

Danish astronomer Tycho Brahe ranked among the most meticulous astronomers of his time. He also is remembered for the consequences of his hot temper, which lost him the bridge of his nose during a duel. Although legends report he wore a prosthetic nose of gold or silver, subsequent exhumations of his grave revealed his nose was actually made of brass.

Tycho, as he is known, set his sights on the study of the supernova, and his state-of-the-art measurements showed that this explosion was held fixed in the sky like the distant stars, rather than wandering like the planets. The heavens, he found, were not immutable but capable of creating new stars.

A year later, he published “De nova stella,’’ which so impressed the king of Denmark that Tycho was given the island of Hven to lord over. The island provided Tycho with an isolated location for his observing program of carefully cataloging the nightly positions of the stars and planets.

He worked without a telescope, and Tycho’s naked-eye measurements were assisted by his immense and accurate sextants and quadrants, all held in a dark underground facility to optimize observing conditions. The number and precision of celestial objects studied by Tycho and his assistants were unmatched by any of his contemporaries.

Tycho would further solidify standing with his patron due to his combined astronomical observation, and favorable astrological interpretation, of a great comet that appeared in 1577. Although he was obliged to continue occasional service as the king’s astrological adviser, Tycho’s long-lasting contribution to his scientific contemporaries was his cosmological model that combined the best aspects of the prevailing geocentrism with Nicolaus Copernicus’s new theory of heliocentrism.

Tycho realized that strict geocentrism, where all celestial objects orbit Earth, was difficult to justify from observations. The planet Mars, for example, sometimes appears to move backward across the sky before again moving forward — a phenomenon known as retrograde motion. This illusory motion makes the most sense under a model where the planets revolve around the sun; geocentrists must resort to complex orbital tricks (known as epicycles) to justify their worldview. Likewise, astronomers observed phases on the planet Venus, just like on the moon, which could only occur if Venus orbits the sun.

Tycho resolved these problems by constructing a model where the planets orbit the sun, while the sun and moon both orbit Earth. Earth itself remains unmoving at the center of the universe, while the distant stars occupy the farthest sphere that rotates uniformly around Earth. This hybridization in Tycho’s cosmology provided a mathematically consistent framework for interpreting contemporary observations. In fact, observations available at the time tended to favor the Tychonic system over the heliocentric theory of Copernicus.

The primary criticism of a sun-centered cosmology was the size of the distant stars. To Tycho’s naked eye, the stars looked like small circles in the sky. Galileo and other astronomers who pointed telescopes to the heavens confirmed these observations and even measured the relative size of the brightest stars. For Tycho, this simply meant that the stars were objects similar in quality to the sun but residing along a sphere at a much greater distance. But for heliocentrism, this posed a problem. Copernicus’s theory claimed that the stars were very far away. Given their measured size, this implied that the stars themselves must be colossal in size, many times the size of the sun, and some nearly as big as the universe itself. This nonsensical claim was hard to defend.

Today we recognize that the round shape of stars is due to interactions of point-like sources of starlight with Earth’s atmosphere (known as an airy disk). Tycho, Galileo, and all the others were basing their arguments on faulty observations, but the rudimentary technology of their day wasn’t able to resolve the issue.

Furthermore, defenders of Copernicus’s theory of heliocentrism often resorted to religion as a justification for the large size of stars. When questioned by Tycho about the star size problem, German mathematician Christoph Rothmann replied, “Grant the vastness of the Universe and the sizes of the stars to be as great as you like — these will still bear no proportion to the infinite Creator.’’ Religion, not science, was the only way out for the Copernicans.

Tycho was also a religious man, and he extorted the harmony that his cosmology displayed with theology. But he was also wary of basing scientific arguments solely on religious arguments, preferring instead to resolve scientific disputes by observation when possible. The star size problem remained a fundamental objection to heliocentrism for Tycho and many others throughout the 18th century.

The literal interpretation of scripture favored by the church remained in harmony with Tycho’s system, while heliocentrism roused both scientific and theological suspicions. Historical accounts of Galileo’s indictment often focus on the claim that heliocentrism is heretical, but Galileo could have been fairly criticized based on scientific arguments alone.

On Feb. 24, 1616, a team of 11 consultants to the Inquisition of Rome, hired to investigate a complaint filed against Galileo, issued a statement condemning the Copernican system that Galileo supported. The idea of the sun at the center was said to be “. . . foolish and absurd in philosophy; and formally heretical, since it explicitly contradicts in many places the sense of Holy Scripture.’’

The words of this statement (written in Latin) have been accurately preserved in historical accounts, but a peculiar issue with punctuation suggests both scientific and religious reasons for implicating Galileo. A semicolon separates the clause about philosophy from the next about heresy; sometimes this appears as a comma, and other times the punctuation mark is omitted entirely. The difference is critical (in the original Latin as well): Did the inquisitor’s consultants charge Galileo for separate scientific and theological objections, or is the objection regarding philosophy simply a parallel statement about heresy?

Physicist Christopher Graney recently generated new high-resolution images of the original verdict, showing that the full semicolon rightfully belongs in history. The consultants, and the inquisitors that followed, all believed that scientific arguments provided sufficient grounds for objecting to Galileo’s ideas. The charge of heresy provided further reason to act, but the scientific case against Galileo remained strong. Similar statements from Galileo’s indictment and trial further suggest that the Inquisition was well aware of the scientific objections to Galileo’s cosmology.

The church had obvious theological reasons to prefer Tycho’s ideas over Galileo’s, but it also had contemporary astronomy on its side. It was not until the 19th century, when the phenomenon of atmospheric diffraction was understood, that the star size problem was resolved by realizing that the airy disk of a star’s appearance is deceptive. Pioneering measurements by German astronomer Friedrich Bessel in 1838 also showed that the distant stars really do show small changes in position across the sky due to Earth’s movement in orbit (known as parallax), which provided conclusive evidence that Copernicus — and not Tycho — was correct.

Galileo had stumbled upon the right idea, but the tools and theory were lacking to persuade his skeptical contemporaries. If Popes Paul V (who censured Galileo) and Urban VIII (who placed Galileo under house arrest) had reason to believe Tycho’s model for its science, then can we blame them for being wary of an idea that also seemed theologically dangerous?

The first opportunity to exonerate Galileo would have probably been around the mid-19th century as sensitive measurements of parallax were first becoming possible. However, this took place during the 31-year papacy of Pius IX, who issued condemnations against Darwin’s theory of evolution and any “conclusions of science those opinions which are known to be contrary to the doctrine of faith, particularly if they have been condemned by the Church.’’ Evolution subsequently took the stage as a primary issue of tension between church and science, while heliocentrism quietly gained popular acceptance.

The pronouncement by John Paul II came only about 150 years after conclusive evidence for heliocentrism was possible. While this still betrays a stoic resistance of the church, we cannot retroactively convict the church of engaging in scientific slander against Galileo. When the brightest minds in Tycho and others could answer all questions scientific and theological better than Galileo, who were they to believe?

Galileo was clearly ahead of his time, even while observations favored a cosmology that differed from his own. But there was no evidence of a grand conspiracy against Galileo and no falsified data or pseudoscience was invoked as an alternative. The church certainly had other motives than the pursuit of pure science, but it also chose to accept the science of the day rather than remain wholly ignorant. The science of the day was extolled during Galileo’s trial not covered up.

Scientific ideas are difficult to suppress. We should always be mindful of science as a tool to expose the weaknesses of inference, legend, and superstition when unmatched by careful observation. At the same time, we must also be credulous of allegations of institutional conspiracy. Unscrupulous organizations sometimes do hire unethical scientists to perform contrived experiments and reach predetermined conclusions, and plenty of practitioners of pseudoscience abound. Such bogus experiments are usually easy to refute. When institutions claim to be acting based on principles of science, we must be careful to rigorously analyze and replicate their conclusions in light of current evidence before passing judgment. A scientist like Galileo would expect nothing less.

Jacob Haqq-Misra is a research scientist with the Blue Marble Space Institute of Science. Follow him on Twitter @haqqmisra.