Copernican heliocentrism

• 1. Introduction

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  In 1543, a book appeared that asked its readers to believe the Earth was moving. Its author, Nicolaus Copernicus, placed the Sun near the center of the Universe and set Earth and the other planets circling it. For centuries, astronomers had worked with the opposite picture, in which Earth stood fixed at the center. Oddly, Copernicus kept much of the old machinery. His planets still traveled in circles, carried on smaller circles called epicycles, at perfectly uniform speeds. Those inherited assumptions were false, yet they sat beside ideas that proved sound.

  This documentary asks where such a mixed model came from, and who had imagined a moving Earth before him. It asks why most astronomers who read the book declined to accept it, and what finally turned the argument. And it asks a quieter question that historians still dispute, which is whether any of this deserves to be called a revolution at all.

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• 2. Ancient Precedents

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  Philolaus, writing in the 4th century BC, is among the first thinkers known to have suggested that the Earth moves. He was probably inspired by Pythagoras, who described the Earth as a spherical, moving globe. In the 3rd century BCE, Aristarchus of Samos went further and proposed what appears to be the first serious heliocentric model of the Solar System. He built on Heraclides Ponticus, who had spoken of the Earth turning on its axis once every 24 hours.

  Aristarchus's own text is lost. We know of it because Archimedes described it in The Sand Reckoner, addressing King Gelon directly. In that account, the fixed stars and the Sun stay unmoved, while the Earth travels around the Sun on a circle. The sphere of stars is so vast that Earth's orbit stands to it as a center stands to a surface. Archimedes noted that this made the universe many times larger than astronomers usually claimed.

  A common belief holds that Aristarchus's contemporaries rejected his idea. That belief rests on a tampered translation. Plutarch reported that Cleanthes, head of the Stoics and a worshiper of the Sun, opposed the heliocentric view. Aristarchus joked that Cleanthes should be charged with impiety. Gilles Ménage, shortly after the trials of Galileo and Giordano Bruno, swapped the grammatical roles in the passage. The accusation then fell on Aristarchus, and the image of an isolated, persecuted astronomer survives to this day.

  In 499 CE, the Indian astronomer and mathematician Aryabhata, influenced by Greek astronomy, described a planetary model in which the Earth rotates on its axis. He used that rotation to explain the apparent westward motion of the stars. His followers were especially strong in South India, where they taught the daily rotation of the Earth and wrote secondary works built on his principles. The model that ruled Europe, however, came from a treatise dating to about 150 AD.

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• 3. Ptolemy's Machine

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  Around 150 AD, Claudius Ptolemy produced the Almagest, and for roughly 1,400 years afterward it shaped European astronomy. Throughout the Middle Ages it was treated as the authoritative text, though its author remained poorly understood. People often mistook him for one of the Ptolemaic rulers of Egypt. The historian Thomas Kuhn gave the Almagest a striking verdict in The Copernican Revolution. He called it the "first systematic mathematical treatise to give a complete, detailed, and quantitative account of all the celestial motions."

  In this geocentric picture, the Earth stood still at the center. The stars were embedded in a large outer sphere that rotated relatively fast. The planets lived in smaller spheres between, with a separate one for each. Planets sometimes appear to run backward against the stars, and the system handled this with deferents and epicycles. A planet revolved in a small circle, the epicycle, about a point. That point itself revolved in a larger circle, the deferent, centered on or near the Earth.

  A rival toolkit ran alongside it. Homocentric spheres, first devised by Eudoxus of Cnidus, let the spheres that carried planets rotate somewhat themselves. By the time of Copernicus, this approach was associated with Averroes. Astronomers also liked eccentrics, in which the rotational axis was offset and not quite at the center.

  Ptolemy's own unique contribution was the equant. This was a point about which the center of a planet's epicycle moved at a steady angular speed, even though the point sat off-center from the deferent. That broke a fundamental principle of Aristotelian cosmology, which demanded that planetary motions be explained by uniform circular motion. Many medieval astronomers saw the equant as a serious defect. Removing it would later become the stated aim of Copernicus.

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• 4. Borrowed Mathematics

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  Al-Sijzi, an Islamic astronomer, invented an astrolabe based on a belief held by some of his contemporaries. In his words, "the motion we see is due to the Earth's movement and not to that of the sky." A 13th-century Arabic work shows he was not alone. It reports that, according to the geometers, the earth is in constant circular motion.

  In the 12th century, Nur ad-Din al-Bitruji proposed a complete alternative to Ptolemy, though not a heliocentric one. He called the Ptolemaic system an imaginary model, successful at predicting planetary positions but neither real nor physical. His alternative spread through most of Europe during the 13th century.

  Then came the mathematics. Mu'ayyad al-Din al-Urdi, Nasir al-Din al-Tusi and Ibn al-Shatir, working in the 13th and 14th centuries, devised techniques for geocentric models that closely resemble those Copernicus later used. Copernicus's planetary models contain the Urdi lemma and the Tusi couple, as in the Arabic sources. In his Commentariolus, the way he swapped the equant for two epicycles appears earlier in al-Shatir's work. Al-Shatir's lunar and Mercury models match his exactly.

  How these ideas reached Copernicus is unsettled. Some scholars argue he must have seen a work not yet identified, though no likely candidate has turned up. Others say he could have developed the ideas on his own. One possible route ran through Byzantine science. Gregory Chioniades translated some of al-Tusi's works into Byzantine Greek, and several Byzantine Greek manuscripts containing the Tusi couple still survive in Italy. Other proposed sources are Proclus's Commentary on the First Book of Euclid, which Copernicus cited, and Nicole Oresme's Questiones de Spera.

  Europeans had their own hints of a Sun-centered order. Martianus Capella, in the 5th century AD, held that Venus and Mercury circled the Sun and not the Earth. Copernicus mentions him as an influence. Macrobius described a heliocentric model in 420 CE. John Scotus Eriugena, who lived from 815 to 877 CE, proposed a model reminiscent of the one Tycho Brahe would later devise.

  The closest teachers of Copernicus were Georg von Peuerbach, who lived from 1423 to 1461, and his student Regiomontanus, who lived from 1436 to 1476. Their Theoricae novae planetarum, compiled from Regiomontanus's lecture notes in 1454, was printed in 1472. It presented Ptolemy with more mathematical elegance but did not reach heliocentrism. By 1470, the observations of their Vienna school were accurate enough to make heliocentrism look almost inevitable. Regiomontanus may even have reached it some 30 years before Copernicus, since a late work attends to Aristarchus and a letter mentions the "motion of the Earth." Regiomontanus taught Domenico Maria Novara da Ferrara, and Novara taught Copernicus.

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• 5. The Book

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  De revolutionibus orbium coelestium, or On the Revolutions of the Heavenly Spheres, was printed in Nuremberg in 1543, the year Copernicus died. A second edition followed in Basel in 1566. Copernicus had circulated an outline of his theory to colleagues sometime before 1514. He decided to publish only after his pupil Rheticus urged him to.

  Inspiration came not from watching the planets but from reading. Cicero gave Copernicus an account of the theory of Hicetas. Plutarch described the Pythagoreans Heraclides Ponticus, Philolaus and Ecphantes. All of them proposed a moving Earth, though none put it in orbit around a central Sun. An early manuscript that survives cites two of them: "Philolaus believed in the mobility of the earth, and some even say that Aristarchus of Samos was of that opinion." The passage was dropped from the printed book. The reason is unknown, though reluctance to quote pre-Christian sources is one guess.

  The universe he described had the Sun fixed near its center. Around it, in order, came Mercury, Venus, the Earth with the Moon, Mars, Jupiter, Saturn, and the fixed stars. The Earth carried out three motions: a daily rotation, an annual revolution, and an annual tilting of its axis. The distance from Earth to the Sun was small compared with the distance to the stars. Strictly, Copernicus put the Sun near the center of the universe and not exactly at it. That makes his model technically heliostatic rather than heliocentric.

  Most sixteenth-century readers valued something else in it. By replacing the equant with additional epicycles, Copernicus delivered what they considered his major achievement.

  The opening pages carried an unauthorized, anonymous preface by Andreas Osiander, a Lutheran theologian and friend of the author. It declared that the Earth's movement was a mathematical hypothesis, not a claim of truth or even probability. The idea seemed to contradict the account of the Sun's movement in Joshua 10:12-13, so the preface was apparently meant to soften religious backlash. No evidence suggests Copernicus himself saw his model as merely convenient mathematics.

  The author's own front matter began with a letter from his friend Nikolaus von Schönberg, Cardinal Archbishop of Capua, who had died by then. The letter urged Copernicus to publish. A lengthy introduction followed, dedicating the book to Pope Paul III. Copernicus named his ostensible motive as the failure of earlier astronomers to agree on a theory of the planets. He added that better predictions would let the Church build a more accurate calendar. Reform of the Julian Calendar was considered necessary then, and it was a major reason for the Church's interest in astronomy.

  The work ran to six books. The first gave a general view of the heliocentric idea. The second set out spherical astronomy and a list of stars. The third dealt with the apparent motions of the Sun, and the fourth with the Moon. Only the last two books, covering planetary longitude and then latitude, laid out the new system in concrete detail.

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• 6. Doubting Contemporaries

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  Around 500 copies of the first and second editions of the book have survived, a large number by the scientific standards of the time. Readers took it seriously, yet few conceded that the Earth actually moves. The model was not markedly easier to use than the geocentric theory, and it produced no more accurate predictions of planetary positions. Copernicus knew this. He offered no observational proof, relying instead on arguments for a more complete and elegant system. To many readers it also seemed to defy common sense and the Bible.

  Tycho Brahe, arguably the most accomplished astronomer of his time, admired the elegance but objected on grounds of physics, astronomy and religion. Aristotelian physics could not explain how a massive body like Earth could move. It could easily explain the heavens, which were made of a substance called aether that moved naturally. Tycho wrote that the system "expertly and completely circumvents all that is superfluous or discordant in the system of Ptolemy." Then came his complaint: it gave the Earth, "that hulking, lazy body, unfit for motion, a motion as quick as that of the aethereal torches, and a triple motion at that." Forty-five years after publication, Tycho built a cosmology precisely equivalent to that of Copernicus, but with the Earth held fixed. Practicing astronomers who accepted heliocentrism appeared only after Galileo.

  His sharpest astronomical objection concerned the sizes of the stars. To the naked eye, stars appear to have measurable sizes. Unknown to sixteenth-century astronomers, this came from diffraction, an optical effect of the wave-like nature of light. Geocentric astronomers could set the stars just beyond Saturn's orbit, and even the largest would be comparable to the Sun. A moving Earth, however, should produce stellar parallax, a shift in the positions of stars. Nobody could detect it, so the stars had to lie much farther away.

  Tycho reasoned that parallax could not be significantly above a minute of arc, or his measurements would have caught it. That meant the stars lay over 700 times the Sun-to-Saturn distance away. A typical third magnitude star measured about one arc minute across. With an annual parallax of one arc minute, its size would be comparable to the orbit of Earth. First magnitude stars would be larger still. Brahe declared such immense stars absurd.

  Christoph Rothmann and other Copernicans answered by appealing to God. They said the stars were enormous as symbols of divine greatness, and likened them to the fore-court in front of God's palace. Their opponents replied that God could indeed make such stars. But they regarded everything else in creation as proportionate and harmonious, so gigantic stars seemed out of character.

  The telescope at first made things worse. Small-aperture instruments of the 17th century produced Airy disks, artefacts of diffraction, which astronomers mistook for the physical bodies of stars. Simon Marius even argued that the disks undermined Copernicus and supported Tycho. In the late seventeenth century, astronomers found that the apparent sizes were spurious. Stars passing behind the Moon vanished at once, not gradually as a large body would. By the early 18th century, Edmund Halley was calling the bigness of stars an "Optick Fallacy."

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• 7. Vindication Through Others

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  Johannes Kepler and Galileo adopted the Copernican model, championed it, and in Kepler's case sought to improve it. In 1609, Kepler published Astronomia nova. Using Tycho Brahe's detailed observations, he found that the orbit of Mars is an ellipse with the Sun at one focus. Its speed varied with its distance from the Sun. He claimed that all planets had elliptical orbits and non-uniform motion, adding that the Sun "will melt all this Ptolemaic apparatus like butter." The irony is that Kepler's laws of planetary motion later replaced the Copernican model itself.

  In 1610, Galileo pointed the newly invented telescope at the sky. He saw four large moons of Jupiter, proof that the Solar System held bodies that did not orbit Earth. He saw the phases of Venus, which the Ptolemaic theory could not properly explain. And he followed sunspots, whose apparent annual variation indicated that the Sun rotates about a fixed axis. Giovanni Zupi used a telescope to see the phases of Mercury in 1639.

  In 1687, Isaac Newton proposed universal gravity and the inverse-square law of gravitational attraction. These explained Kepler's elliptical planetary orbits.

  From a modern point of view, the Copernican model has real advantages. Copernicus gave a clear account of the seasons, tracing them to the fact that Earth's axis is not perpendicular to the plane of its orbit. He also offered a simpler explanation of retrograde motion, treating it as a parallactic displacement caused by the Earth's own travel around the Sun. This weighed heavily in Kepler's conviction that the theory was substantially correct. In the geocentric model, by contrast, epicycles handled the same apparent backward motion in an ad hoc way, and their revolutions were mysteriously tied to that of the Sun.

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• 8. Revolution or Patch

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  In the 20th century, Thomas Kuhn popularized the phrase "Copernican Revolution." He also presented the model as the first example of a paradigm shift in human knowledge. Historians of science still argue over whether Copernicus's propositions were revolutionary or conservative.

  Herbert Butterfield, in The Origins of Modern Science, calls the model "irrelevant to the present day." Arthur Koestler, in The Sleepwalkers, published in 1959, denies that the system was a discovery. He calls it "a last attempt to patch up an out-dated machinery." Even Kuhn conceded that Copernicus only transferred "some properties to the Sun's many astronomical functions previously attributed to the earth."

  Otto Neugebauer goes further, writing that "Copernican solar theory is definitely a step in the wrong direction." The science historian David Wootton denies that Copernicus was a catalyst for the later intellectual revolution that turned astronomy into the first modern science.

  Koestler's book carries the subtitle A History of Man's Changing Vision of the Universe, which shows how large a claim these critics were weighing against one astronomer's modest, circular, epicycle-filled model.

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