Born: 19 February 1473, Torun, Poland, Died: 24 May 1543, Frombork, Poland
Polish astronomerNicolaus Copernicus (Polish: Mikołaj Kopernik) proposed that the planets have the Sun as the fixed point to which their motions are to be referred; that the Earth is a planet which, besides orbiting the Sun annually, also turns once daily on its own axis; and that very slow, long-term changes in the direction of this axis account for the precession of the equinoxes. This representation of the heavens is usually called the heliocentric, or “Sun-centred,” system—derived from the Greek helios, meaning “Sun.”
Copernicus’s theory had important consequences for later thinkers of the scientific revolution, including such major figures as Galileo, Kepler, Descartes, and Newton. Copernicus probably hit upon his main idea sometime between 1508 and 1514, and during those years he wrote a manuscript usually called the Commentariolus (“Little Commentary”). However, the book that contains the final version of his theory, De revolutionibus orbium coelestium libri vi (“Six Books Concerning the Revolutions of the Heavenly Orbs”), did not appear in print until 1543, the year of his death.
Science of the Stars
In Copernicus’s period, astrology and astronomy were considered subdivisions of a common subject called the “science of the stars,” whose main aim was to provide a description of the arrangement of the heavens as well as the theoretical tools and tables of motions that would permit accurate construction of horoscopes and annual prognostications. At this time the terms astrologer, astronomer, and mathematician were virtually interchangeable; they generally denoted anyone who studied the heavens using mathematical techniques. Furthermore, practitioners of astrology were in disagreement about everything, from the divisions of the zodiac to the minutest observations to the order of the planets; there was also a long-standing disagreement concerning the status of the planetary models.
From antiquity, astronomical modeling was governed by the premise that the planets move with uniform angular motion on fixed radii at a constant distance from their centres of motion. Two types of models derived from this premise. The first, represented by that of Aristotle, held that the planets are carried around the centre of the universe embedded in unchangeable, material, invisible spheres at fixed distances. Since all planets have the same centre of motion, the universe is made of nested, concentric spheres with no gaps between them. As a predictive model, this account was of limited value. Among other things, it had the distinct disadvantage that it could not account for variations in the apparent brightness of the planets since the distances from the centre were always the same.
A second tradition, deriving from Claudius Ptolemy, solved this problem by postulating three mechanisms: uniformly revolving, off-centre circles called eccentrics; epicycles, little circles whose centres moved uniformly on the circumference of circles of larger radius (deferents); and equants.
The equant, however, broke with the main assumption of ancient astronomy because it separated the condition of uniform motion from that of constant distance from the centre. A planet viewed from a specific point at the centre of its orbit would appear to move sometimes faster, sometimes slower. As seen from the Earth and removed a certain distance from the specific centre point, the planet would also appear to move nonuniformly. Only from the equant, an imaginary point at a calculated distance from the Earth, would the planet appear to move uniformly. A planet-bearing sphere revolving around an equant point will wobble; situate one sphere within another, and the two will collide, disrupting the heavenly order.
In the 13th century a group of Persian astronomers at Marāgheh discovered that, by combining two uniformly revolving epicycles to generate an oscillating point that would account for variations in distance, they could devise a model that produced the equalized motion without referring to an equant point. This insight was the starting point for Copernicus’s attempt to resolve the conflict raised by wobbling physical spheres.
An Orderly Universe
In the Commentariolus, Copernicus postulated that, if the Sun is assumed to be at rest and if the Earth is assumed to be in motion, then the remaining planets fall into an orderly relationship whereby their sidereal periods increase from the Sun as follows: Mercury (88 days), Venus (225 days), Earth (1 year), Mars (1.9 years), Jupiter (12 years), and Saturn (30 years). This theory did resolve the disagreement about the ordering of the planets but, in turn, raised new problems. To accept the theory’s premises, one had to abandon much of Aristotelian natural philosophy and develop a new explanation for why heavy bodies fall to a moving Earth.
It was also necessary to explain how a transient body like the Earth, filled with meteorological phenomena, pestilence, and wars, could be part of a perfect and imperishable heaven. In addition, Copernicus was working with many observations that he had inherited from antiquity and whose trustworthiness he could not verify. In constructing a theory for the precession of the equinoxes, for example, he was trying to build a model based upon very small, long-term effects. Also, his theory for Mercury was left with serious incoherencies.
Any of these considerations alone could account for Copernicus’s delay in publishing his work. (He remarked in the preface to De revolutionibus that he had chosen to withhold publication not for merely the nine years recommended by the Roman poet Horace but for 36 years, four times that period.) When a description of the main elements of the heliocentric hypothesis was first published in 1540 and 1541 in the Narratio Prima (“First Narration”), it was not under Copernicus’s own name but under that of the 25-year-old Georg Rheticus, a Lutheran from the University of Wittenberg, Germany, who stayed with Copernicus at Frauenburg for about two and a half years, between 1539 and 1542.
The Narratio prima was, in effect, a joint production of Copernicus and Rheticus, something of a “trial balloon” for the main work. It provided a summary of the theoretical principles contained in the manuscript of De revolutionibus, emphasized their value for computing new planetary tables, and presented Copernicus as following admiringly in the footsteps of Ptolemy even as he broke fundamentally with his ancient predecessor. It also provided what was missing from the Commentariolus: a basis for accepting the claims of the new theory.
Both Rheticus and Copernicus knew that they could not definitively rule out all possible alternatives to the heliocentric theory. But they could underline what Copernicus’s theory provided that others could not: a singular method for ordering the planets and for calculating the relative distances of the planets from the Sun. Rheticus compared this new universe to a well-tuned musical instrument and to the interlocking wheel-mechanisms of a clock. In the preface to De revolutionibus, Copernicus used an image from Horace’s Ars poetica (“Art of Poetry”). The theories of his predecessors, he wrote, were like a human figure in which the arms, legs, and head were put together in the form of a disorderly monster. His own representation of the universe, in contrast, was an orderly whole in which a displacement of any part would result in a disruption of the whole. In effect, a new criterion of scientific adequacy was advanced together with the new theory of the universe.