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 develops a novel philosophical view on science and scientists. Kuhn introduces in this work the concept of paradigm shift, a sudden change in outlook and theorizing by members of a science community that occurs during a revolutionary change in their field. He describes scientists during periods of "normal science" as solvers of the sorts of puzzles that are not unlike jigsaw or crossword puzzles. A reader who expects to see some of this renown philosophy foreshadowed in Kuhn's older work, The Copernican Revolution, will be disappointed. The terms "paradigm" and "normal science" do not appear in this book, which is more a historical than a philosophical work.

By the "Copernican Revolution" Kuhn means the period and its changes that more commonly are 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 the Copernican Revolution covers this period of one-and-a-half century, while the first half of the book treats over two thousand years of development of pre-Copernican cosmology.

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 starts discussing 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. Kuhn states that the Copernican revolution was not only a revolution in astronomy but also entailed a revolution in science and philosophy and he 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

Chapter 1 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

Chapter 2 deals with 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 model.

Chapter 3

Chapter 3 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 is 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.

(To be continued)