Nicolaus Copernicus Begins a Revolution in Astronomy with His Heliocentric Model of the Solar System

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Nicolaus Copernicus Begins a Revolution in Astronomy with His Heliocentric Model of the Solar System

Overview

The publication of Nicolaus Copernicus's (1473-1543) De Revolutionibus Orbium Celestium in 1543 was attended by no official opposition. The heliocentric system Copernicus presented was initially viewed as a hypothetical model devised merely to facilitate computation. For many, the most attractive feature of the new system was Copernicus's abolition of the equant, which restored uniform circular motion as the basic axiom of astronomy. Most early supporters passed over in silence the question of the system's physical reality. Theoretical improvements made possible by Copernican theory and new observations helped undermine Aristotelian physics and with it geocentrism—the idea that the Sun and all other planets in the Solar System revolved around Earth. By the mid-seventeenth century the heliocentric view reigned supreme, though Copernicus's circular orbits had by then been replaced by Johannes Kepler's (1571-1630) elliptical orbits.

Background

The theoretical framework of pre-Copernican astronomy was established in the Almagest of Ptolemy (c. 100-c. 170). Drawing heavily on the work of previous Greek astronomers, especially Hipparchus (c. 170-c. 120 b.c.), this work developed a theory of the universe employing geocentric models to predict planetary motions. Ptolemy appealed to Aristotelian physics to show Earth was at rest. He then geometrically demonstrated Earth is the center of the universe with the fixed stars moving together as a sphere. He further assumed, in accordance with Aristotelian teaching, that all celestial bodies move about Earth in perfect circles. To obtain agreement between observations and predictions based on circular motions, Ptolemy used epicycles, deferents, eccentrics, and other noncircular motions. He also found it necessary to introduce the equant—an imaginary point around which celestial objects moved—to approximate uniform motion about an off-center point.

Aristotle (384-322 b.c.) also regarded the planetary orbs as solid celestial spheres that formed a unified mechanism explaining the movements of celestial bodies. Though relying on Aristotelian physics, the Almagest makes no attempt to interpret epicycles and deferents as physically real, nested spheres. Regardless of whether or not Ptolemy actually held this view, it was adopted by Medieval astronomers who considered it an essential component of the Ptolemaic system.

Models resembling those of the Almagest were used in the Middle Ages to calculate tables of planetary motions, an example being the Alfonsine Tables (1252). However, most medieval European astronomical work amounted to little more than the collection and reorganization of Arabic and ancient Greek material. No new observations of importance were undertaken, and by the mid-fifteenth century the Alfonsine Tables(1252) were sorely in need of revision.

Georg von Purbach (1423-1461) pointed out to Johannes Regiomontanus (1436-1476) the inaccuracies of existing tables as well as the need for better translations of Greek texts. Purbach attempted to produce a revised and corrected version of the Almagest but died before finishing it. Regiomontanus completed Purbach's Epitome of Astronomy. This work contained, in addition to the Purbach-Regiomontanus translation of the Almagest, critical commentary and revised computations. Published in 1496, the work was a great success and attracted the attention of Copernicus, who was particularly struck by the errors inherent in Ptolemaic lunar theory.

Copernicus was not so much exercised by inaccurate predictions as he was by the lack of "perfection" exhibited by the Ptolemaic system, especially with respect to uniform circular motion. His solution was to give Earth a simple circular orbit about a static, off-center Sun. Furthermore, in this heliocentric model (more accurately heliostatic, since the Sun is not really the center of the system) the diurnal rotation of the heavens is accounted for by the rotation of Earth. A canonical wobble about Earth's axis of rotation was also proposed to explain precession.

As radical as Copernicus's system was, his firm commitment to maintaining uniform circular motion precluded any real simplification over the Ptolemaic system. He continued using epicycles, deferents, and eccentrics. By replacing Earth with the Sun, Copernicus immediately eliminated five large planetary epicycles. However, his rejection of the equant, because it failed to preserve uniform circular motion, required the introduction of secondary epicycles. The real power of Copernicus's system was to be found in the trigonometric methods he used and his improved lunar theory. It should also be mentioned that Copernicus believed his heliocentric model was physically true.

Copernicus first described his heliocentric system in the brief essay "Commentariolus." Composed sometime before 1514, it was privately circulated. De Revolutionibus was completed in the early 1530s but Copernicus delayed publication. Hearing of this work, Georg Joachim von Lauchen, self-named Rheticus (1514-1574), traveled to Frauenburg in 1539 to examine the manuscript. Rheticus realized the full revolutionary impact of the work and urged Copernicus to publish immediately. Copernicus refused but did allow Rheticus to publish a summary account that appeared under the title Narratio prima (1540). Copernicus finally relented and agreed to let Rheticus prepare De Revolutionibus for publication. Andreas Osiander (1498-1552), a Lutheran minister who saw the book through press, penned an anonymous and unauthorized preface stating the heliocentric hypothesis was merely a mathematical model and not intended as a true description of the universe.

Impact

De Revolutionibus appeared in March 1543. Though Osiander's preface may have forestalled official opposition to the work, many theologians denounced Copernican heliocentrism because of its conflict with the Bible, and Aristotelians everywhere objected to the very idea that Earth could be in motion. Despite such objections, De Revolutionibus acquired a small following.

Erasmus Reinhold (1511-1553) was one of the earliest supporters of the Copernican system. He made extensive corrections to De Revolutionibus and calculated the Tabulae Prutenicae (1551). This was the first set of practical planetary tables based on Copernicus's theory. More accurate than the Alfonsine Tables, Reinhold's tables were widely adopted and provided a strong argument in favor of Copernicanism. However, Reinhold's focus on Copernicus's mathematical modeling and silence regarding the physical reality of heliocentrism encouraged a similar attitude in German astronomers that persisted well into the 1570s.

Observations of the nova of 1572 by Tycho Brahe (1546-1601) and Michael Mästlin (1550-1631) indicated this phenomenon was located among the fixed stars. This undermined the Aristotelian notion that the heavens were perfect and unchanging. Brahe and Mästlin's observations of the comet of 1577 dealt another blow to Aristotelian cosmology. Because the erratic behavior of comets was incompatible with the immutability of the heavens, Aristotle maintained they were atmospheric exhalations. Brahe and Mästlin's parallax measurements for the 1577 comet indicated it was more distant than the Moon. Furthermore, Brahe concluded that its orbit was elongated, suggesting it had passed through several planetary spheres—impossible if planetary spheres were solid.

These celestial events led Mästlin to completely reject Aristotelian cosmology and adopt Copernican heliocentrism. After attending Mästlin's lectures on the superiority of Copernicus's cosmology, Kepler embraced Copernicanism as well. Meanwhile, Thomas Digges (c. 1546-1595) had already taken up the Copernican cause, becoming its foremost exponent in England. Digges translated portions of Copernicus's De Revolutionibus and appended his own views on an infinite universe with fixed stars at varying distances from Earth (1576).

Brahe also rejected the Ptolemaic system as incompatible with his observations, but belief in the Bible and a lack of stellar parallax prevented him from accepting Copernican heliocentrism. As a compromise he advanced his Tychonic theory, in which all of the planets but Earth orbit the Sun, with the Sun and its train of planets revolving about a stationary Earth (1588). This system avoided many of the pitfalls of the Ptolemaic system while preserving a stationary Earth. Brahe's theory gained acceptance in many quarters over the next 50 years.

Galileo (1564-1642) announced his support of Copernicanism with the publication of his telescopic discoveries in Sidereus nuncius in 1610. Galileo's discovery of four satellites orbiting Jupiter contradicted the widely held belief that Earth was the center of rotation for all celestial bodies, and his observation of mountains and depressions on the lunar surface refuted the Aristotelian notion that the Moon was a perfect sphere. Galileo also discovered sunspots and the phases of Venus. The latter discovery removed a serious objection to Copernican heliocentrism. The primary effect of Galileo's findings was to further undermine Aristotelian cosmology, which served as the foundation of the Ptolemaic system. Galileo furnished further support for Copernicanism in his Dialogue Concerning the Two Chief World Systems (1632). Having introduced the principle of inertia, he demonstrated that objects on a rotating Earth would behave no differently than on a stationary Earth.

Johannes Kepler provided the crucial advance in the ascendancy of heliocentrism. He joined Brahe in Prague in 1600. After Brahe died the next year, Kepler secured control of his incomparable data set and spent the next eight years devising various geometrical schemes to account for the observations of Mars. Kepler finally determined that the orbit of Mars, as well as those of the other planets, describes an ellipse with the Sun occupying one foci. This is known as Kepler's first law. At once, the Ptolemaic and Copernican epicycles and eccentrics were completely eliminated. Furthermore, since one of the foci of each planetary ellipse was anchored on the Sun, the Sun truly occupied a central position within the solar system. Kepler also showed that planets sweep out equal areas in equal times as they move about in their orbits. Known as Kepler's second law, this implies that planets move faster when closer to the Sun. Published in Astronomia nova (1610), Kepler's first two laws directly challenged the traditional canon of uniform circular motion. Kepler's third law appeared in Harmonices mundi (1619).

Kepler completed the Tabulae Rudolphinae (Rudolphine Tables) in 1627, having developed their theory in accordance with his new planetary laws. The predictive accuracy of these tables was two orders of magnitude better than anything previously achieved and did much to recommend Kepler's heliocentric theory to right-thinking minds. The success of heliocentrism was sealed when Pierre Gassendi (1592-1655) in 1631 observed the transit of Mercury across the disk of the Sun—just as Kepler predicted.

STEPHEN D. NORTON