How Far the Stars?

Setting Aside All Authorityby Christopher M. Graney

University of Notre Dame Press. 280p $29

Is there something to be gained by assessing the merits of the losing side of a long-settled scientific argument? Christopher Graney’s answer in Setting Aside All Authority is an unequivocal yes, and he takes one of the most celebrated cases of scientific advance—the victory of heliocentrism over geocentrism—to show us. The work contributes to an effort among historians of science to demonstrate how much more genuinely scientific the entire dispute was than is suggested in the conventional story of it as a titanic struggle between the scientifically rational and the religiously superstitious.

Setting Aside All Authority focuses on the 17th-century status of a modified form of geocentrism developed by the Danish astronomer Tycho Brahe (1546–1601). Renowned to this day for his studies of the heavens with the unaided eye, Brahe was troubled by several problems in Copernicus’s heliocentric model: the mathematics was sometimes inaccurate in predicting the movement and placement of celestial bodies, and there was no physics that could explain, for example, why the Earth did not fall into the fiery sun at the center of a heliocentric cosmos. So he wove together his own abundant observational data, the strengths of the Copernican astronomical mathematics and time-tested principles of Aristotelian physics to develop a third model of the cosmos, a helio-geocentric hybrid: The planets revolve around the sun; and that “solar system,” plus the Moon and the stars, rotates around the Earth.

Graney has long had a scholarly interest in a 17th-century supporter of Brahe’s model, the Jesuit astronomer Giovanni Battista Riccioli. Setting Aside All Authority elaborates on the succinct praise of the eminent historian of science Edward Grant that Riccioli was the intellectually most formidable 17th-century opponent of Copernicanism. Riccioli’s monumental Almagestum Novum (1651) was in its day the most up-to-date compendium of astronomical knowledge and the one most widely distributed across Europe. On the question of the new cosmos, its dramatic frontispiece paints an answer worth a thousand words. The original Ptolemaic model of geocentrism lies at the feet of Urania, the muse of astronomy, who holds in a balance hanging from her hand Copernicus’s heliocentric model and Brahe’s helio-geocentric model. The lower, because weightier, model is Brahe’s.

The next 1,500 folio pages argue why. Riccioli included an outline of 126 nontheological arguments for and against heliocentrism. He explained, one by one, why nearly all of them were scientifically unconvincing. Two objections, however, struck him as significant: First, a rotating and revolving Earth should produce specific observable phenomena, such as stellar parallax and indicators of the Coriolis effect. The absence of such observations were indeed serious concerns, not put to rest until the 19th century: Stellar parallax was not observed until 1838, and Foucault’s pendulum demonstrated the Coriolis effect only in 1851.

The second serious challenge had to do with the size of stars and the size of the universe. Measurable stellar parallax and a better understanding of optical problems with telescopic observations of stars as points of light would have made the problem moot. But without those, this second challenge was the more serious, as the heliocentrists acknowledged. The width of stars was measured first in relation to the width of planets. Relative widths suggested the greater distance from the Earth of the stars than the planets. Geocentrists situated the stars on the outermost celestial sphere. By having to take into account a stellar parallax too small to measure and, on account of optical diffraction, the size of stars relative to planets, the stars were computed to have enormous size and to be put at enormous distance. The telescope exacerbated the problem on account of optical problems in focusing and diffraction patterns that would not be understood adequately until the 19th century. The magnitudes of size and distance were large enough to shock everyone.

Riccioli pursued the problem by checking and cross-checking the telescopically magnified width of stars relative to the diameter of Jupiter. How could the existence of stars of such size be explained? The Copernicans developed an answer, but not exactly the sort that scholars today would guess as possible for them. The heliocentrists used theology to explain what their science could not: God creates the way he wants. Riccioli’s cool response was, “That cannot satisfy more prudent men.” Graney effectively turns the conventional story on its head: Geocentrism, though hardly without its mathematical and physical problems, had been convincing for reasons that had much to do with observation and mathematics, and nothing to do with new 16th-century biblical sensibilities. Aristotle and Ptolemy had, after all, no truck with Christianity.

Graney underscores Riccioli’s scientific integrity by looking at his testing of gravity. An explanation for gravity and momentum adequate for the newest cosmology needed to wait for Newton. In the meantime, Riccioli and others worked on the hypothesis that falling bodies accelerated. Riccioli was initially skeptical. Graney isolates a set of experiments involving the count of pendulum swings as weights fell from towers and the measurement of the sound at impact. The noise at impact increased with the height, Riccioli noted, thus suggesting acceleration. Riccioli changed his mind accordingly.

Graney’s book is a fascinating read. By outlining the strength of the opposing arguments at the time of gravity problem’s emergence and the scientific limitations of all sides, Graney makes the victory of heliocentrism far more scientifically interesting than the conventional history suggests. The Almagestum Novum, printed almost a century and a half after the circulation of Copernicus’s first, celebrated outline of heliocentrism in the Commentariolus (ca. 1514) and 15 years after Galileo’s second set of hearings before the Inquisition, was not one more contribution to a shouting match between the ill-informed and the hardheaded. It was a scrupulous assessment of the status questionis orbium coelestium and the fruit of careful and ingenious experimentation.

Graney’s book reminds us that science is a messy venture. Tidying it up into neat stories with serious scientists always winning, even if only by a hair’s breadth, and the bad scientists ultimately collapsing under the weight of their own stubbornness, distorts the real history and leaves us with no helpful lesson for negotiating the true and false scientific hypotheses of our own day. In science, no less than in politics, after all, few things are more dangerous than creating a caricature of one’s opponent and refusing to admit his strengths.