Copernican revolution (book)

The Copernican Revolution: Planetary Astronomy in the Development of Western Thought is a book written by Thomas S. Kuhn and published in 1957 by Harvard University Press.

Thomas S. Kuhn is the author of the epoch-making Structure of Scientific Revolutions (1962), a book that pioneers a novel philosophical/sociological view on science and its practitioners. Kuhn introduces in this work the concept of paradigm shift, a sudden change in outlook of members of a science community that occurs during a revolutionary change in their field. He describes scientists working during periods of non-revolutionary ("normal") science as solvers of a kind of puzzles that are not unlike jigsaw or crossword puzzles. A reader who expects to see foreshadowed some of Kuhn's renown philosophy in the five years older Copernican Revolution will be disappointed. The terms "paradigm" and "normal science" do not appear in it; the book is more a historical than a philosophical work.

The Copernican Revolution, Kuhn’s first book, is one of the best selling books ever written on the history of science. Around 2000 the Harvard University Press edition was in its 19th printing, and this excludes the Vintage Book edition. It was one of Kuhn’s first publications in the history of science; previously he had published six papers in this field, on seventeenth-century chemistry and on the Carnot cycle. The book had its origin in notes for a science course at Harvard based on a historical approach. The course was not so much about science itself, but more on an understanding of science aimed at students outside the sciences. This origin of the book is important to understanding the character of the book.

By the "Copernican Revolution" Kuhn means the period in the history of science that is more commonly referred to as "the Scientific Revolution". The period is sharply defined: it begins with the publication of Copernicus' work De Revolutionibus Orbium Coelestium in 1543 and closes with the appearance of Newton's Philosophiae Naturalis Principia Mathematica in 1687. The second half of Kuhn's The Copernican Revolution covers the period of one-and-a-half century after Copernicus' death, while the first half of the book treats over two thousand years of development of pre-Copernican cosmology. The present article is a review of the Copernican Revolution, and, more importantly, describes a relevant part of science history as seen through the eyes of Thomas S. Kuhn.

Contents

As stated, Kuhn spends the first half of his book on the pre-Copernican view of mankind on the universe. His exposition starts with the Egyptians, goes from Antiquity through the Dark Ages and the later Middle Ages up to Copernicus. Kuhn describes Western Civilization's slowly awakening recognition of a cosmos that seemed to consist of the Sun, the Moon, the planets, the stars on a surrounding sphere, and, of course, the Earth at the center of it all.

When Kuhn discusses Copernicus' work halfway the book, it is remarkable that he refers to the latter's discovery (the Sun, not the Earth, is the geometric center of the Universe) as Copernicus' "innovation" not as his "revolution". One may argue that this underplays the importance of Copernicus' historic contribution to astronomy, but it is consistent, as Kuhn prefers to call the whole 145 year period starting in 1543 as "Copernicus' revolution". Notwithstanding, the book treats Copernicus' innovation—the shift from a geocentric to a heliocentric universe—as a crucial and pivotal point in the development of cosmology and astronomy. According to Kuhn, the Copernican revolution was not only a revolution in astronomy but also entailed a revolution in science and philosophy and Kuhn relates how an astronomer's solution to an apparently technical problem fundamentally altered men's attitude to the basic problem's of everyday life.

Chapter 1: The Ancient Two-Sphere Universe

The first chapter explains the primitive cosmologies of the Egyptians and Babylonians. It treats a good deal of astronomical theory, such as the apparent motion of the Sun as seen from Earth; it introduces concepts as ecliptic, winter/summer solstice and vernal/autumnal equinox. When the ancient Greek culture comes in the picture, the oldest cosmological model, the Two-Sphere Universe (a term coined by Kuhn), is introduced. It consists of a tiny spherical and stationary Earth at the geometric center of the large rotating (with 24 hour frequency) sphere of the stars. Kuhn argues that the idea that astronomy may supply a cosmological model is one of the most significant and characteristic novelties that we inherited from ancient Greek civilization.

Chapter 2: The Problem of the Planets

For the Greek and their successors the Sun and the Moon were two of the seven planets. Kuhn describes a rudimentary image of the universe that remained current in elementary books on astronomy and cosmology until the early 17th century, long after Copernicus' death. The Earth is at the center of the stellar sphere that bounds the universe. From the outside in are the orbits of Saturn, Jupiter, Mars, Sun, Venus, Mercury, and the Moon. Chapter 2 details how in a more refined model the retrograde motion of the planets is explained by epicycles, small circles that rotate uniformly about a point on the circumference of a second, uniformly rotating, circle, the deferent. This Hellenistic cosmology culminated in the Almagest of Ptolemy (ca 150 AD), a book that treats a complicated theory designed to predict the times of the occurrences of the planets in the sky. Generally, the planetary motions in the Almagest are composed of epicycles with centers on deferents, but Ptolemy also introduced equants. An equant is a point with respect to which the rotation of the deferent is uniform but the equant is shifted off the center of the deferent. Copernicus' dislike of equants was one of his main reasons to search for a better planetary model.

Chapter 3: The Two-Sphere Universe in Aristotelian Thought

This chapter gives an account of Aristotelian cosmology and world view. According to Aristotle (384–322 BC) and his successors, the universe is finite and bounded by the sphere of the stars and its interior is mainly filled with aether. Aristotle believed that the very notion of vacuum is absurd, space and matter are inextricably bound together and therefore the universe must be filled with matter. The planets are moved by homocentric spherical shells consisting of aether. (Later the shell were thought to be thick enough to contain the deferent of the planet and its epicycles). The underside of the innermost shell—that of the moon—divides the universe into two totally disparate regions, filled with different sorts of matter and subject to different natural laws. The terrestrial, sublunar, region in which man lives is filled with the elements: fire, air, water, and earth. It is the region of variety, change, birth, death, generation and corruption. The motion of the lunar shell continuously pushes the four elements and therefore they can never be observed in their pure form. The celestial region, the moon and beyond, in contrast, is eternal and changeless; it consists solely of the pure, transparent, weightless, and incorruptible element aether.

Chapter 4: Recasting the Tradition: Aristotle to the Copernicans

Chapter 4 describes the period between Ptolemy and Copernicus. Soon after the beginning of this period, with the fall of the Western Roman Empire, most of ancient knowledge was lost in Europe. The Islamic Chaliphates and, to a lesser extent, the Byzantine empire became the guardians and conservators of this knowledge. During the Dark Ages (that lasted until ca 1000 AD) even the fact that the earth is spherical was forgotten. At the beginning of the 4th century Lactantius ridiculed the concept of the spherical earth. In the middle of the 6th century, Kosmas, an Alexandrian monk, derived a Christian cosmology from the Bible. His universe was shaped like the tabernacle that the Lord instructed Moses to build. However, as stressed by Kuhn, these cosmologies never became official Church doctrine.

In the 11th and 12th century some of the ancient knowledge was rediscovered, at first via the Córdoba Caliphate in Spain. Astronomical tables were imported from Toledo (the center of learning in the Córdoba Caliphate) in the 11th century. Ptolemy's Almagest and most of Aristotle's astronomical and physical writings were translated from Arabic into Latin from the 11th century onward. This was the time that the European awe for "ancient wisdom" and "The Philosopher" (Aristotle) was born. Initially, the Catholic Church considered the rediscovered antique science pagan, but scholastics as St. Thomas Aquinas (1225-1274) managed to reconcile the Aristotelian knowledge with Christian doctrine and its combination became the all embracing Christian world view.

In this intellectual climate there wouldn't have been room for Copernicus to posit his heliocentric model. However, as discussed in the second half of the chapter, slowly but surely some criticisms against Aristotle's world view were germinating. In the Parisian nominalist school Nicole Oresme (d. 1382) tore some rents in the fabric of Aristotelian thought. Renaissance explorations and voyages (Columbus' first landfall in America was made when Copernicus was 19 years old) raised new questions and set the example for more innovations. Ancient astronomical computational techniques turned out to be fallible, as clearly brought to light by the cumulative errors of the Julian Calendar. Compared to the political uproar associated with the religious reforms of Luther and Calvin an innovation in astronomy seemed a completely negligible event. Also more intellectual aspects of the Renaissance played a role. Humanism, the dominant learned movement of the age, was dogmatic anti-Aristotelian, and its criticism facilitated scientists to break away from Aristotle's roots. In addition, the Neoplatonic outlook of the Humanists, with its aesthetic taste for pure mathematics, created the atmosphere that instilled into Copernicus its dislike of the non-uniform motion of the planets that Ptolemy had introduced by using equants.

Chapter 5: Copernicus' Innovation

As is well-known, Copernicus' innovation, described in detail in chapter 5, consists of two steps. First, the Earth, still at the center of the stellar sphere, is assumed to make a diurnal (24 hour) rotation around its axis. This explains the diurnal rotation of the Sun and the stars, which now are assumed to be at rest. Once the step of a moving Earth is made, the next step, orbiting of the Earth around the Sun, is conceptually easier. Kuhn explains that these two steps (see the article ecliptic for diagrams) are not very consequential for the understanding of the apparent daily and yearly motion of the Sun. The second step, however, the replacement of a geocentric by a heliocentric system, has far reaching consequences for the understanding of the motion of the planets. Especially the retrograde motion of the planets becomes a more elegantly explained and therefore much easier understood phenomenon. Interestingly, Kuhn points out that Copernicus was aware of the model of Aristarchus (ca. 310–230 BC) who also assumed that the Earth orbits the sun.

Copernicus adhered as closely as he could to the classic Ptolemaic ideas. He based his theory on a finite universe bounded by the sphere of the stars and also believed that the motion of the planets must be composed of perfect circles and that motions are uniform. He considered his elimination of equants (inducing non-uniform motions) as one of his most important contributions to mathematical astronomy. For these reasons Kuhn states that the De Revolutionibus is not a revolutionary but rather a revolution-making text. Copernicus' purpose was not to give the world a new cosmology, but to resolve the technical flaws he perceived in Ptolemaic astronomy. According to Kuhn, Copernicus' work consists of fairly narrow technical planetary astronomy, not of cosmology or philosophy.

From Kepler's work (around 1610) it is known that the planetary orbits are elliptic rather than circular and hence it is not surprising that Copernicus' simple model based on circular orbits is only qualitatively correct. In order to obtain quantitative results Copernicus was forced to introduce epicycles, although fewer than most medieval astronomers had applied. Even so, Copernicus' predictions of planetary positions were as accurate as Ptolemy's, not better.

Chapter 6: The Assimilation of Copernican Astronomy

(To be continued)