A man [Galileo] is here revealed who possesses the passionate will, the intelligence, and the courage to stand up as the representative of rational thinking against the host of those who, relying on the ignorance of the people and the indolence of teachers in priest’s and scholar’s garb, maintain and defend their positions of authority. His unusual literary gift enables him to... overcome the anthropocentric and mythical thinking of his contemporaries and to lead them back to an objective and causal attitude toward the cosmos, an attitude which had become lost to humanity....
— Albert Einstein
Galileo was ordered to stop believing what he could see with his own two eyes.
— Peter Sis
We are all familiar with the story of the Copernican Revolution that gave birth to modern science, and of its great champion Galileo Galilei. In that story, Galileo, the heroic representative of rational thinking, stepped forth to fight for a modern scientific outlook against a host of ignoramuses who used Aristotle, religion, and their positions of authority to uphold old ideas and suppress the truth.
But suppose that the Copernican Revolution had happened in a nobler and more edifying way. Let us use our imaginations to see what that might have looked like and what it would have meant for science.
Imagine, then, a Copernican Revolution that featured scientific debate of the best kind. In this imagined Revolution there would have been rational thinking and brilliant insight on both sides. This Revolution would have showcased the scientific process as it ought to be. In it, clever people on both sides would have honed ideas, pushed instruments to their limits, and engaged in fascinating scientific arguments about whether the Earth orbits a central Sun (heliocentrism) or the Sun orbits a central Earth (geocentrism).
In this imagined Revolution, the supporters of a geocentric universe would not have relied upon lame arguments about how birds would be left behind by a moving Earth --- as though not a one of them had ever poured a drink on a smoothly-moving ship. No, in this Revolution, the brightest geocentrists would have used more sophisticated reasoning. If they were exceptionally clever, they might even have been able to figure out that on a rotating Earth there should be Coriolis forces, despite living two centuries before Gaspard-Gustave de Coriolis discovered this effect. That is, they might have realized that the differing speeds of different points on a spinning globe would cause detectable effects more subtle than anything birds would notice, and that the failure to detect such effects could be used as an argument against the heliocentric theory. And they might also have been able to develop a geocentric model of the universe that conformed to the latest telescopic observations better than the 1500-year-old model of Ptolemy.
In our imagined Revolution, the proponents of heliocentrism would have been equally sophisticated. They would have been defending heliocentric theories that were not quite our modern one, for they would have assumed the Sun to be the unique central body of the entire universe, not just of the solar system. But the most intelligent heliocentrists would have defended these theories with keen observations and impeccable logic. And both sides would have interpreted the available data persuasively to support their views.
Lastly, let us imagine how science might have benefited from having that kind of Copernican Revolution in its history. Modern science would then have been able to point to a brilliant and fascinating scientific debate as its founding story. It would not be weighed down by a depressing tale of broad resistance to scientific truths and widespread refusal to acknowledge what plain observation revealed. We would not be in a position now where crackpots can claim the mantle of Galileo, declare that a benighted Establishment is suppressing The Truth, and launch movements that end up bringing back the measles. Instead, science would be buoyed up by a story that shows how the scientific enterprise has always featured vigorous, intelligent and informed debate in which the truth is not so readily suppressed.
Now, stop imagining. What I have just described is the Copernican Revolution that actually occurred.
Opponents of Copernicus’s theory did envision the Coriolis Effect! Frs. Giovanni Battista Riccioli and Francesco Maria Grimaldi, both Jesuits, described it in Riccioli’s Almagestum Novum of 1651. Fr. Claude Francis Milliet Dechales, also a Jesuit, produced diagrams and discussions of this effect that could serve in a modern textbook. These Jesuit priests were not describing an effect that they had observed, but an effect that they realized must occur on a rotating globe. They said that the fact that no such effect had ever been observed militated against any rotation of the Earth. They were well beyond crude claims that in a heliocentric universe birds would be left behind by a moving Earth.
FIGURE 1. Illustrations of the Coriolis Effect from “Objectiones contra Copernicum” in Dechales’s Cursus seu Mundus Mathematicus (1674). Left: A cannon firing north at a castle. Because the cannon is moving eastward faster than the castle, due to the Earth’s rotation, the cannonball lands East of the castle. Right: The top of a tower moves faster than its bottom, owing to Earth’s rotation. So a ball released from the top (F), strikes the ground at L, rather than at the base of the tower I.
Advocates for Copernicus’s theory did in fact argue for a heliocentric universe that bore little resemblance to any modern conception of it. Johannes Kepler described a heliocentric universe which consisted of one unique, tiny, brilliant, central body, the Sun, orbited by still tinier planets, all surrounded at vast distances by stars that were so immense that each star was larger than the Earth’s orbit, and vastly larger than the Sun. The largest of the stars, Kepler said, was larger than the orbit of Saturn, larger even than an entire geocentric universe! And he said that while the stars were gigantic, they were also dim --- the combined light output of these monsters was as nothing compared to that of the Sun.
These were not arbitrary assumptions on Kepler’s part. They were what heliocentrism logically required, given what 17th century observations seemed to show. Stars, as seen in the telescopes used at that time, did not appear to be mere points, but had discernable widths. It was not realized until centuries later that this was due to distortions produced by the telescopes themselves. (See footnote 1 for a brief explanation.) From these apparent widths (or “angular widths”) and the great distances to the stars required by Copernicus’s theory and other heliocentric theories, it followed that the stars must be of enormous size compared to all the other astronomical bodies, including the Sun. (See footnote 2 for a brief explanation of why stars in heliocentric theories had to be much farther away than in geocentric theories.) The dimness of stars was implied by the fact that they failed to illuminate the night. One could not plead that their distance makes them dim, Kepler noted. This does not help at all, he said, because given their angular sizes, the further away they were assumed to be, the larger they would also have to be --- “the greater their distance, the more does every single one of them outstrip the sun in diameter”.
FIGURE 2. Left: Illustration of a star as seen through the kind of telescope used for stellar observations in the seventeenth century (from John F. W. Herschel’s 1828 Treatises on Physical Astronomy, Light and Sound). Right: Illustration, from Kepler’s 1618 Epitome Astronomiae Copernicanae, of a small sun (arrowed) surrounded by a universe of much larger stars.
Kepler saw this heliocentric universe—with its giant dim stars, tiny brilliant sun, and tinier planets—as elegant, revealing God at work. God’s power was on display in the creation of stars so immense. God’s care was on display in the creation of the tiny Earth and its inhabitants. Thus Kepler wrote:
Where magnitude waxes, there perfection wanes, and nobility follows diminution in bulk. The sphere of the fixed stars according to Copernicus is certainly most large; but it is inert, no motion. The universe of the movables [the planets] is next. Now this—so much smaller, so much more divine—has accepted that so admirable, so well-ordered motion. Nevertheless, that place neither contains animating faculty, nor does it reason, nor does it run about. It goes, provided that it is moved. It has not developed, but it retains that impressed to it from the beginning. What it is not, it will never be. What it is, is not made by it—the same endures, as was built.
Then comes this our little ball, the little cottage of us all, which we call the Earth: the womb of the growing, herself fashioned by a certain internal faculty. The architect of marvelous work, she kindles daily so many little living things from herself—plants, fishes, insects—as she easily may scorn the rest of the bulk in view of this her nobility.
Lastly behold if you will the little bodies which we call the animals. What smaller than these is able to be imagined in comparison to the universe? But there now behold feeling, and voluntary motions—an infinite architecture of bodies.
Behold if you will, among those, these fine bits of dust, which are called Men; to whom the Creator has granted such, that in a certain way they may beget themselves, clothe themselves, arm themselves, teach themselves an infinity of arts, and daily accomplish the good; in whom is the image of God; who are, in a certain way, lords of the whole bulk.
And what is it to us, that the body of the universe has for itself a great breadth, while the soul lacks for one? We may learn well therefore the pleasure of the Creator, who is author both of the roughness of the large masses, and of the perfection of the smalls. Yet he glories not in bulk, but ennobles those that he has wished to be small.
In the end, through these intervals from Earth to the Sun, from Sun to Saturn, from Saturn to the fixed stars, we may learn gradually to ascend toward recognizing the immensity of divine power.
Kepler was hardly alone among heliocentrists in his views. He couldn’t be --- the case for giant stars in a Copernican cosmos was just too compelling. Other heliocentrists shared his view of the sizes of stars, and of their being an illustration of God’s power. Kepler even used the giant stars to argue that the sort of universe advocated by Giordano Bruno --- in which the stars were other suns, orbited by other Earths --- was not scientifically possible. No careful astronomer could subscribe to Bruno’s ideas. Basic observations, measurements, and calculations showed that the stars were not suns, and Bruno’s ideas were absurd.
Needless to say, geocentrists attacked the giant stars required by heliocentrism as themselves absurd. They saw them as inelegant, a great weakness of the Copernican theory. Geocentrists were unimpressed by heliocentrists invoking God’s power concerning the giant stars; that, said Riccioli, cannot satisfy the more prudent minds. No such weaknesses were to be found in the new hybrid geocentric theory (proposed by the great astronomer Tycho Brahe) that they supported. Tycho's theory was fully compatible with telescopic discoveries. In it, the Earth was orbited by the Moon, the Sun (which in turn was orbited by planets), and the stars. These stars were only a little more distant than, and little larger than, Saturn. The geocentric universe was elegantly compact, with all bodies in it commensurate in size. Thus in 1674 Robert Hooke referred to the star size issue as
a grand objection alledged by divers of the great Anti-copernicans with great vehemency and insulting; amongst which we may reckon [Riccioli], who would fain make the apparent Diameters of the Stars so big, as that the body of the Star should contain the great Orb [Earth’s orbit] many times, which would indeed swell the Stars to a magnitude vastly bigger then the Sun, thereby hoping to make it seem so improbable, as to be rejected by all parties.
Is this not all fascinating? Even exciting? Here is a great scientific debate playing out. Even now, you are drawn in, wondering how the story ends, wondering how the arguments get resolved. (Spoiler alert: they did finally get resolved in favor of heliocentrism when Newton came along decades after Galileo.)
This is a great story of science. It happens to be true; and as so often is the case, the truth is more interesting than the myth. This story can be found in the published writings of Riccioli, Dechales, Kepler, and others. One just has to know Latin, be familiar with the science being discussed, and have access to a large library of rare, four-century-old books. Thanks to the digitization of many rare books, and to the internet, access is no longer a hurdle. That is why you are reading this now.
This story also happens to be good for science, at a time when, thanks in large part to that same internet, the sorts of people who claim to know the Truth that the Establishment is suppressing are having an impact, once unimaginable, at science’s expense. Note that, in this story of the Copernican Revolution, machinations of the powerful Establishment play no leading role. This story of the Copernican Revolution does not provide much of a mantle for the crackpot or pseudoscientist to grasp when claiming that “They” are suppressing the truth.
The real story of the Copernican Revolution is worth telling, not only because it is more interesting than the myth and better for science, but simply because it is true. This a story that needs to be told.
[Prof. Graney's most recent book is Mathematical Disquisitions: The Booklet of Theses Immortalized by Galileo (University of Notre Dame Press, 2017), a complete translation of a scientific work by an anti-Copernican astronomer, Fr. Christoph Scheiner, S.J., and his student, Johann Georg Locher. He is also the author of Setting Aside All Authority: Giovanni Battista Riccioli and the Science against Copernicus in the Age of Galileo (University of Notre Dame Press, 2015).]
1… Not until the latter part of the seventeenth century would astronomers start to publish evidence that the appearance of stars, even when seen through a telescope, might be entirely spurious. In fact, the diffraction of light waves through the telescope’s aperture creates the globe-like appearance, greatly inflating the apparent sizes of stars. (This is called the “Airy disk.” See https://en.wikipedia.org/wiki/Airy_disk) A full understanding of diffraction and the wave nature of light was not developed until the early nineteenth century.
2… It was realized in the time of Kepler and Galileo that in heliocentric theories the Earth’s motion around the Sun should cause annual variations in the appearance of the stars. One such effect is called “annual parallax.” Such an effect had not been observed (and would not be, until the early nineteenth century). The answer that Copernicus proposed for this lack of parallax was that the stars were so distant that Earth’s annual motion was insignificant compared to the distance to the stars. In geocentric models, by contrast, the Earth is assumed to be motionless and so no annual parallax is expected, and therefore the stars do not have to be assumed to be so far away. Consequently, in geocentric theories the stars could be assumed to be of a more “reasonable” size.