The AI Journal

The familiar narrative of the Copernican revolution ends, for most readers, with Galileo’s trial. After 1633 the remaining resistance is usually presented as institutional stubbornness: the Church clinging to a geocentric cosmos long after the observational case had been settled. That account is convenient and incomplete. By the middle of the seventeenth century the strongest published arguments against the motion of the Earth were not drawn from Scripture but from telescopic measurements of stellar diameters and from the physics of falling bodies. Those arguments retained genuine empirical weight well into the 1650s, and the principal statement of them was made by a Jesuit astronomer who treated the question as a matter of natural philosophy rather than of revealed truth.

Giovanni Battista Riccioli’s Almagestum Novum of 1651 contains a systematic review of 126 arguments concerning the mobility of the Earth—forty-nine in favour of the Copernican hypothesis and seventy-seven against. Riccioli states at the outset that he will set aside all authority, sacred or secular, and weigh the arguments by their intrinsic force. The theological objections appear, but they form a small minority. The decisive cluster is observational. Building directly on Tycho Brahe’s pre-telescopic measurements, Riccioli and his collaborators measured the apparent diameters of stars with telescopes and found them still to possess measurable disks. Under the Copernican hypothesis the absence of detectable annual parallax already required the stars to lie at enormous distances; if those same stars also subtended a finite angle, their physical diameters would exceed the diameter of the Earth’s orbit. Stars the size of planetary systems struck Riccioli, as they had struck Brahe, as physically absurd. The telescope had not dissolved the problem; it had refined the measurements that generated it. The same logic applied, with only minor numerical adjustment, once the telescope entered use. Galileo himself had reported that stars appeared smaller through the telescope than to the naked eye, yet they retained disks rather than becoming pure points. Riccioli’s more systematic campaign of measurement, carried out with colleagues at Bologna, produced a table of diameters that still implied, under Copernican distances, stars of planetary-system scale. The inference was not an artefact of hostile interpretation; it followed directly from the combination of two data sets both sides accepted: the lower bound on distance and the measured angular size.

Brahe himself had already formulated the core of this objection in the 1580s. His instruments gave first-magnitude stars an apparent diameter of roughly two minutes of arc. Combined with the lower bound on stellar distance imposed by the failure to detect parallax, the calculation produced stars whose volumes dwarfed the Sun. Albert Van Helden’s history of cosmic dimensions shows that this inference was treated as a serious quantitative constraint by successive generations of observers. The introduction of the telescope reduced the measured angular sizes, yet the residual disks remained large enough, once multiplied by the still greater distances now required, to keep the absurdity intact. Riccioli’s tables of telescopic star diameters therefore functioned not as a retreat from observation but as an updated version of Brahe’s original case.

The same empirical temper appears in Francesco Ingoli’s 1616 essay to Galileo, written on the eve of the first formal condemnation of Copernicanism. Of the twenty-two arguments Ingoli listed, only four were theological. The remainder were mathematical and physical, most of them drawn from Brahe. Ingoli explicitly invited Galileo to answer the scientific ones and did not press the scriptural points. The essay supplied the chief direct basis for the consultants’ recommendation that led to the 1616 decree. The decree itself therefore rested, at least in its immediate documentary foundation, on a body of arguments that its author regarded as empirical rather than confessional.

A second family of arguments concerned terrestrial physics. Riccioli devoted careful attention to the behaviour of falling bodies and to the absence of any detectable deflection that would betray the Earth’s rotation. He described experiments that anticipate what later physicists would call the Coriolis effect and noted that no such effect had been observed. Projectiles fired vertically, or dropped from towers, failed to show the eastward displacement that a rotating Earth should produce. Galileo and later Newton would attempt to answer the point by refining the analysis of relative motion; in 1651 the observational record still favoured the stationary Earth. Riccioli did not claim that the geoheliocentric system of Brahe was free of difficulty—he called it the least absurd of the available models—but he did claim that the scientific balance of the evidence still tilted against a moving Earth. The geoheliocentric arrangement conserved the relative planetary positions that Copernicus had rendered so successfully, while keeping the Earth immobile and therefore free of the unobserved rotational effects. In that limited but important sense it was the conservative empirical choice.

The cumulative force of these arguments is easy to underestimate from a later vantage point. Historians of science long treated the post-Galilean period as one in which the observational case had already been won and only institutional inertia remained. Riccioli’s careful enumeration of the remaining difficulties shows that the observational case was still contested on its own terms. The star-size problem and the missing rotational effects were not residual theological scruples dressed in scientific language; they were quantitative claims that required quantitative answers. Once stellar aberration had been measured, once reliable annual parallax had been obtained, and once the wave nature of light explained the apparent disks as diffraction artefacts, the older objections collapsed. But those later results were not available to Riccioli. On the data of 1651 an astronomer who genuinely set authority aside could still conclude that the Copernican hypothesis required an implausible physical universe. The geoheliocentric alternative conserved the observed planetary positions while avoiding both the enormous stellar diameters and the missing terrestrial deflections. It was not an elegant system, but elegance is not evidence, and the evidence then in hand did not compel the more elegant solution.

The strongest objection to this reconstruction is that institutional power, not the quality of the arguments, decided the outcome. The 1616 decree and the 1633 condemnation were acts of ecclesiastical authority; even if the arguments placed before the consultants were largely scientific, the decision to enforce them was not. One may grant the point without surrendering the claim. The existence of an institutional decision does not erase the content of the case that decision claimed to rest upon. Riccioli’s 1651 review was published long after the condemnations, under no immediate pressure of trial, and still judged the empirical arguments against a moving Earth to outweigh those in its favour. That judgement was later shown to be mistaken, but it was not irrational on the evidence then available. The conventional story therefore inverts the chronology: the scientific case against Copernicus remained competitive for decades after the institutional case against him had already been made. What finally shifted the balance was not the silencing of theological opposition but the accumulation of further measurements—stellar aberration, better parallax determinations, and the recognition that telescopic star disks were optical illusions—that removed the quantitative force of the older objections. Until those measurements arrived, the rational course for an astronomer who set authority aside was still, on the published evidence, to withhold assent from a moving Earth.