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Cosmology and Astronomy

Cosmology and Astronomy

Early cosmologies, or worldviews, envisioned a universe subject to the whims of gods and were anthropocentric: focused on the role and fate of human beings. As such, cosmology and religion were often intertwined.

Babylonian Cosmology

Elimination of everything other than empirical observation and mathematical computation from Babylonian astronomical texts of the Seleucid period has been hailed by some as the first appearance of modern science and cosmology. Ancient Babylonians studied how the celestial motions went, but not why. Nor did they seek causes for physical phenomena or develop comprehensive theories about the universe. Consequently, some scholars dismiss Babylonian cosmology and astronomy as merely a set of mechanical procedures with no more theoretical content than recipes in a cookbook. They look instead to the Greeks for the birth of modern science.

Greek Cosmology

For two millennia, the Aristotelian physical cosmology of rotating spheres carrying the sun, moon, planets, and stars around the central earth permeated Western thought. The natural place for earthy material was at the center of the universe, and earthy material tended to move to its natural place. Planetary spheres rotated because that was their natural motion. Aristotle's teleological explanations, with the world fulfilling a purpose formed by a superhuman mind, would not necessarily be incompatible with Christian philosophy, which also envisioned the world as inherently meaningful and purposive. Concurrent with Aristotle's physical cosmology was Plato's geometrical cosmology, in which astronomers strove "to save the appearances": to explain apparently irregular planetary motions with combinations of circular motions at constant speeds.

In the context of modern science, saving the appearances with uniform circular motions is an arbitrary and absurd task. The task was, however, widely accepted and pursued for around two thousand years, from the Greeks in the fourth century b.c.e. through Copernicus and the European Renaissance in the sixteenth and seventeenth centuries c.e. The historical importance of a cosmology is not necessarily negated by its absurdity, especially when that label is applied in hindsight by different people in a different age with different standards and values.

Saving the appearances is plausible in the context of Plato's philosophy, which in turn can be understood as his reaction to the moral and political chaos of his age, which left him highly dissatisfied with the physical world. His concept of an ideal reality is simply illustrated. A circle drawn on paper is an imperfect representation in the visible world of experience of a perfect circle, which exists only in the world of thought. In his Republic, Plato wrote that the sky was part of the visible world, and the true revolutions of the planets, sun, and moon were to be discerned by reason and thought, not by sight.


Greek geometrical cosmology was systematized and advanced, with rigorous geometrical demonstrations and proofs, by Claudius Ptolemy (second century c.e.). He did for astronomy what Euclid had done for geometry and earned a reputation as the greatest astronomer of the ancient world. Indeed, his Almagest, of about 140 c.e., was so comprehensive that its predecessors ceased to be copied and failed to survive.

The match between Ptolemy's reported observations and his theory is too good to be true, and the Almagest has been called the greatest fraud in the history of science. It is not a modern scientific research paper, however, but a textbook, and it is in this context that Ptolemy should be judged. He made many observations; errors largely canceled each other out; and he thus obtained an accurate planetary theory. Next he would have selected a few observations in best agreement with the theory to illustrate it, and even fabricated examples to neaten up his pedagogy. The Greek astronomical tradition was far more concerned with general geometrical procedures than with specific numerical results.

Islamic scientists rescued the Aristotelian and Platonic cosmologies when Western civilization crumbled; they transmitted Greek learning back to the West when learning revived there. Initially, Aristotelian cosmology, including the eternity of the world and denial that God could make other worlds, seemingly contradicted dogmas of the Christian faith, and in the thirteenth century the bishop of Paris condemned hundreds of Aristotelian propositions. The condemnations helped free cosmology from Aristotelian dogma and also led to the nominalist, instrumentalist, or positivist thesis. Cosmology was understood as a working hypothesis, the truth of which could not be insisted upon because God could have made the world in a different manner but with the same observational consequences. Imaginative and ingenious discussions, including possible rotation of the earth, subsequently flourished.

Hypothetical cosmologies, however, are not the stuff of revolution. Confidence that the essential structure and operation of the cosmos is knowable would be essential to the achievements of Nicolaus Copernicus (14731543), Galileo Galilei (known as Galileo; 15641642), Johannes Kepler (15711630), and Isaac Newton (16421727).

The Copernican Revolution

Ptolemy's planetary theory matched observations, but in Copernicus's opinion, it violated the standard of uniform circular motion. Also, Ptolemy's scheme did not automatically produce phenomena that followed naturally in Copernicus's heliocentric model. An unauthorized foreword to Copernicus's 1543 De revolutionibus orbium coelestium presented the heliocentric theory as a convenient mathematical fiction. Copernicus, however, believed that he was describing the real world.

Revolutions in science, as in politics, often exceed the limited changes envisioned by their creators. The sphere carrying the stars was now obsolete, and soon human imagination distributed the stars throughout an infinite space. Furthermore, the earth was no longer unique. Galileo's telescopic observations emphasized that the moon was uneven and rough, like the earth. Also, Jupiter had four satellites similar to the earth's satellite.

Logical consequences of the Copernican system, Galileo's telescopic discoveries, and the principle of plenitude, which interpreted any unrealized potential in nature as a restriction of the Creator's power, all encouraged belief in a plurality of worlds. Also, political and societal critics used the moon as a literary convention, its inhabitants' arrangements being either a model of perfection or a mirror of the earth's vices. Faith in an anthropocentric universe lay shattered, leaving people's relationship with God uncertain. John Donne's 1611 poem An Anatomy of the World, its line the "new Philosophy calls all in doubt," and later "all Relation: / Prince, Subject, Father Son, are things forgot," refers to Christian morality as much as to the relative positions of the sun and earth.

The Newtonian Revolution

Tycho Brahe's (15461601) observations of comets coursing through the solar system shattered the Aristotelian crystalline spheres, and his observation of a nova, a star flaring up in brightness, pierced Aristotelian belief in an unchangeable heaven. Belief in uniform circular motion died next when Kepler used Brahe's observations to show that planets travel around the sun in elliptical orbits. A new explanation of how the planets retrace the same paths forever around the sun became a central problem of cosmology. Newton showed mathematically how a force of attraction or gravitation toward the sun continually draws planets away from straight-line motion and holds them in Kepler's elliptical orbits.

The Newtonian example of general laws in natural philosophy excited searches for general laws in other realms, including economics. In this branch of philosophy, Adam Smith (17231790) played Newton's role. Also inspired by Newton's achievement, Voltaire (16941778) and Montesquieu (16891755) searched for natural laws of politics. The French Revolution owed much to Newton.

Newton was convinced that his discoveries demonstrated God's wonders. Yet the revolution in thought following from his new cosmology, particularly the concept of a mechanical, clocklike universe, threatened the historic link between cosmology and religion in Western thought. Writers of the Romantic period sought to breathe divine life back into an overly mechanized and increasingly godless universe. Under the sway of the French Enlightenment's atheistic approach to nature, however, Pierre-Simon Laplace (17491827) happily replaced the hypothesis of God's rule with a purely physical theory. According to legend, when Napoléon Bonaparte (17691821) asked Laplace whether he had left any place for the Creator, Laplace replied that he had no need of such a hypothesis.

The Newtonian solar system provided Immanuel Kant (17241804) a model for the larger stellar system. The same cause that gave the planets their centrifugal force, keeping them in orbits around the sun, could also have given the stars the power of revolving. And whatever made all the planets orbit in roughly the same plane could have done the same to the stars. Nebulous-appearing objects in the heavens became, in Kant's mind, island universes, like colossal solar systems. Late in the eighteenth century, William Herschel's (17381822) large telescopes expanded the heavens from a starry sphere to a three-dimensional firmament. He observed that most stars seemed to lie between two parallel planes. This stratum, seen from earth, is the Milky Way.

Twentieth-Century Cosmology

At the beginning of the twentieth century, it was generally thought that the Earth's galaxy was some ten thousand light-years across and that the solar system was near the center of the galaxy. This vision of the universe was soon replaced with a revolutionary new conception. Harlow Shapley at the Mount Wilson Observatory showed that the galaxy is hundreds of thousands of light-years in diameter and the solar system is far from its center. The significance of humans and their particular planet had dwindled still further. Shapley noted a historical progression from belief in a small universe with humankind at its center to a larger universe with the earth farther from the center. The geometry had been transformed from geocentric to heliocentric to acentric. The psychological change was no less, Shapley insisted, from homocentric to acentric. Furthermore, the galaxy containing the Earth is but one of many "island universes," as Edwin Hubble soon proved. Next Hubble found that the universe is expanding. Georges Lemaître, a Belgian priest and astrophysicist, explained the expansion of the universe from an initial "cosmic egg." Einstein confirmed that Lemaître's theoretical investigations fit well into the general theory of relativity.

Advances in nuclear physics fed speculations about an expanding universe resulting from thermonuclear reactions in an early, hot, dense phase. Fred Hoyle at mid-century derisively called it the "big bang," and the term stuck. Hoyle championed instead steady-state creation, in which the universe expands but does not change in density because new matter continuously appears. Pope Pius XII announced in 1951 that big-bang cosmology affirmed the notion of a transcendental creator and was in harmony with Christian dogma. Steady-state theory, denying any beginning or end to time, was associated with atheism, though Hoyle associated it with personal freedom and anticommunism. The debate was resolved observationally with detection of a faint cosmic background radiation, a remnant of the big bang.

In 1979 an American particle physicist, Alan Guth, proposed that important cosmological features can be explained as natural and inevitable consequences of new theories of particle physics. Guth's theory of inflation states that in the first minuscule fraction of a second of the universe's evolution, a huge inflation occurred. After that, the inflationary universe theory merges with the standard big-bang theory.

See also Astrology ; Cosmology: Asia .


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Norriss Hetherington

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