System of the Universe

a term used in astronomy to signify a conception of the structure of the system of heavenly bodies—the earth, moon, sun, and planets. Attempts to create a system of the universe were made in ancient Greece as early as the sixth century B.C. by Thales, Anaximander, and Anaximenes. The most historically significant systems were the geocentric system developed by the ancient Greek philosophers Aristotle (fourth century B.C.) and Ptolemy (second century B.C.) and the heliocentric system of the Polish astronomer Copernicus (first half of the 16th century A.D.).

The geocentric system of the universe, which was accepted as valid for approximately 2,000 years, was a striking embodiment of anthropocentrism; in this system the earth occupied the central place in the universe. In Aristotle’s system of the universe a fixed earth was surrounded by seven “heavens” belonging to the “planets”—the moon, Mercury, Venus, the sun, Mars, Jupiter, and Saturn. The stars occupied the eighth “heaven.” In the ninth was a “spirit” or “prime mover,” which in some incomprehensible way imparted movement to all the “heavens.” In order to explain the rather complex apparent motion of the planets across the sky, Aristotle used the idea of Eudoxus of Cnidus (fourth century B.C.), who postulated a system of concentric, rotating, transparent spheres. According to Eudoxus’ theory, there were 56 spheres in all. This complexity of explanation was linked to the fact that Aristotle, following his teacher Plato (fifth-fourth centuries B.C.), proposed that the motion of the planets was caused by the perfectly equal rotation of several concentric spheres. The mutual tilt of the axes and the speeds of rotation of the spheres were worked out individually for each planet.

In the second century B.C., Hipparchus substituted a system of epicycles for the system of spheres, an idea which he borrowed from Apollonius of Perga (circa 200 B.C.). Hipparchus’ system was used and received its final development in Ptolemy’s A Imagest. In place of rotating spheres, the theory of epicycles introduced equal motions of the planets around circles, called epicycles. At the same time, the epicycles themselves were assumed to be shifting in such a way that their centers moved around other circles, called deferents. In most cases, a single epicycle was not sufficient to represent complex observed motion of the planets with satisfactory precision; therefore, a second epicycle was introduced, and then a third, and so forth. It was held that the planet moved along the last epicycle and that the center of each epicycle moved along the periphery of the preceding one. The angles of inclination of the planes of the deferents and epicycles, the relative radii, and the angular velocities of the motions were chosen to give the best description of the apparent motions of the planets across the sky.

The geocentric system of the universe was proclaimed by the Catholic Church throughout the Middle Ages as the only system consistent with the Christian faith. During the Middle Ages, one or two more spheres were added to the original nine. The outermost sphere was called the empyrean and was declared to be the residence of God and the “just.”

The heliocentric system of the universe was created during the Renaissance and played a revolutionary role in the development of natural science. Copernicus’ remarkable work De revolutionibus orbium coelestium (On the Revolutions of Heavenly Spheres), which contains an exposition of the heliocentric system of the universe, was published in 1543 and marks the beginning of the modern era in natural science. Copernicus refuted the doctrine that the earth was stationary. In his system, he demonstrated that the earth and the other planets revolve around the sun, which is the central body of the solar system. The complex, loop-shaped motions of the planets received a natural and simple explanation: their observed shifts in the sky are relative motions observed from a moving earth. Thus, according to Copernicus’ system, the earth is not the center of the universe but merely one of the planets. Copernicus’ doctrine delivered a decisive blow to anthropocentrism.

One of the followers of Copernicus’ doctrine was G. Bruno. Bruno correctly arrived at the materialist conclusion that the universe is infinite and that the sun is only the center of the solar system, one of an infinite number of worlds existing in the universe. Beginning in the late 16th century, progressive science fought fiercely against belief in the geocentric system, which was supported by the Christian Church. Accused of heresy by the Roman Inquisition, Bruno was burned at the stake. The scientific discoveries of Galileo provided important physical and philosophical arguments in favor of the heliocentric system. Galileo’s observations with the telescope confirmed that the sun is merely one of an infinite number of stars. In connection with these discoveries refuting Christian doctrine, the Catholic Church, which had not reacted to Copernicus’ book in the first decades after its appearance, published a decree of the Inquisition in 1616, which stated that defending the Copernican doctrine would be viewed as a manifestation of heretical attitudes. In 1632 legal action was begun against Galileo. The Catholic Church cruelly persecuted scholars and scientists who developed and advocated the heliocentric system and used the terrorism of the Inquisition tribunals as a weapon against the advocates of new ideas concerning the universe.

After the discoveries made in the 16th and 17th centuries, the issue of whether the earth or the sun was at the center of the universe essentially died away. It was clear that the sun was one of the stars and thus, like the earth, could not be the center of a large group of stars. Because of its infiniteness, the universe cannot have any center at all. After detailed clarification of the structure of the solar system, the structure of the Galaxy came under study in the late 18th century. In the 20th century, with development of the means and techniques of astronomical observation, it became possible to investigate the structure of the metagalaxy. In connection with this, the term “system of the universe” came to be used sometimes in Russian in a new, expanded meaning that included notions of the basic features of the structure of the objects under investigation.