The Rigid Sky in Greek Philosophy

The worship of Zeus in the ancient world involved a cosmology that was built on the assumption of a stationary earth. Many arguments were available that appeared to support this idea; clouds would be left behind, it was reasoned, if the earth rotated. Observations showed that a stone or an arrow shot straight up into the air fell back down to the same place, and was not deflected towards the west.

The ancients noted that after sunset, the stars appeared in the formerly bright blue sky, and they observed the regular daily movement of the stars, which seem to rotate about a point in the sky above the north pole each night. To keep the stars in their relative positions, they reasoned a rigid spherical shell was required, centered on the earth's center, in which all the fixed stars were embedded. The rigidity of the heavens was regarded as an amazing discovery, which seemed to account for many observations. The concept was the basis for the worship of the Olympian Zeus in the ancient world.

Zeus was the rigid heaven of the ancient world, which shone bright blue in the day, and held up all the stars, which were thought to be embedded like nails on its inside surface. The sky was the focus of Greek religion. Zeus was chief of the Olympic deities, and was called "the father of gods and men" by Homer. Herodotus says Homer gave the gods their names, or defined them. We might say today that he invented them. There were twelve main Olympian gods. Zeus, youngest son of Cronus, was the supreme ruler, identified with the sky. Hera, his wife, was the protector of women and marriage. Apollo, god of light, was identified with Helios (the sun) in Hellenistic times. Artemis, the twin sister of Apollo, and goddess of hunting and wildlife, was the moon goddess, identified with Diana by the Romans. Athena, born from the head of Zeus in an ancient myth, seems to have originated as a deity of the Athenians, and was a protector of cities. She is identified with wisdom, or craftiness. Poseidon, the sea, was identified with Neptune by the Romans. Earthquakes and volcanic eruptions were attributed to him, reflecting the ancient belief that water underlies the earth's crust. Aphrodite, goddess of love and beauty, was associated with the Roman Venus, Ishtar of Assyria, and Astarte of Phoenicia. Hades, whose name means "the unseen," was also called Pluto. His name referred to the underworld, the place of the dead.

The Greek philosophers taught that the sky was a solid sphere, which rotated, and held all the stars in place. It was necessary to postulate such a sphere since they denied the rotation of the earth. The larger the sphere of heaven was thought to be, the stronger it would need to be, to keep from flying apart. That is why their "wisdom" maintained the sky was strong. In the Iliad 8:5-27, Homer has Zeus boast he is the strongest of all the gods, saying that if a golden chain were fastened to the sky, he could hold up all the other gods, sun and moon and earth and sea, so that they would dangle in mid-air, but all of them combined could not drag him down from heaven.

The strength of the sky suggested by Homer's story implied it was rigid. The works of Homer were the basis for a Greek education down through the Hellenistic period. The plays, poetry, and philosophy of the Greeks all depended a great deal on Homer. What Homer wrote about Zeus defined the conception of him down through many centuries. In some ancient cultures, the sky was thought to be made of stone or iron, and this view was supported by the occasional fall of meteorites that were interpreted as objects of divinity since they came from the sky. The development of Greek thinking about the nature of the universe is revealed in the following brief review of the ideas of Greek philosophers. [For another review, see Inventing the Solar System: Early Greek Scientists Struggle to Explain How the Heavens Move.]

Thales of Miletus (c. 624-548 BC) [See biography] was one of the seven sages of antiquity. He taught the earth was made from water, and rested on water. Herodotus reported that Thales had predicted an eclipse, which occurred on 28 May, 585 BC. The sudden change from daylight to darkness so startled the warring armies of Medes and Lydians, they gave up the battle and made peace. Thales learned geometry from the Egyptians. A story about Thales says he was once was so intent in his observations of the heavens, he fell into a well.

Anaximander (c. 611-547 BC) thought the cosmos was spherical, and rotated. For him, air was the substance of all things in the heavens. The earth was a disk floating on waters which filled the bottom half of the spherical universe. He guessed the thickness of the earth's disk was equal to one-third of its diameter. The sun, he said, was a circle 28 times larger than the earth. It resembled a chariot wheel with a hollow rim full of fire. The sun was an opening in the rim through which the fire shone out. An eclipse results when the opening is closed.

Pythagoras (c. 582-507 BC) was born at Samos, and studied under Thales. He visited Egypt, Babylon and India, and about 530 established a school at Crotona, a Greek colony on the southern coast of Italy. He believed, with the Babylonians, that the earth was a sphere. He identified the evening and morning star as the same planet. He taught the earth is suspended in the midst of the universe, which rotates around it. There were three parts to the universe: the zone of air and clouds above the earth, in which exists all that is subject to change and corruptible, was called Ouranos; the region above the moon, with the sun and planets was called Cosmos, and the sphere of the fixed stars was called Olympus. [Tauber, p. 40]

In Pythagorean cosmology, there were spheres for the sun and moon, one for each of the five known planets, one for the stars, the earth itself, and a "counter-earth" was postulated to bring the number of heavenly spheres up to ten, since ten was considered a perfect number. For Pythagoras and his school, number was reality, and principles of mathematics and number formed the basis of all things. The Pythagoreans discovered irrational numbers, but cult members were sworn to secrecy concerning the discovery, as this was regarded as a threat to their system.

Anaximenes (c. 550-480 BC) said the stars were fixed on the surface of a crystal hemisphere which made a daily rotation around the flat earth.

Philolaus (fl. 470 BC) a pupil of Pythagoras, taught the earth floated in space and revolved in a circle once each day around a central fire, called "the hearth of Zeus," or the hearth of the universe. The side of the earth on which the Mediterranean region is located always faced away from this fire. The heaven appeared to move because of this circular orbiting motion of the earth.

Empedocles (c. 495-435 BC) estimated the moon's distance was one third the distance to the stellar sphere.

Anaxagoras of Clazomenae (499 - 428 BC) went to Athens about 480 BC. He claimed "the sun and stars are flaming stones which are carried round by the revolution of the ether." He claimed the sun is larger than the Peloponnesus, the moon shines with the light of the sun, and he explained a large meteorite which fell on Aegospotamoi as a result of a landslide on one of the heavenly bodies. He was charged with impiety and narrowly escaped being exiled from Athens. Euripides said he had reduced "the all seeing Helios, who traversed the sky every day in his flashing chariot and was the awful witness of men's most sacred oaths, to the status of a lifeless lump of glowing stone." [Olson, p. 79]

Protagoras in 411 BC was less fortunate. He was accused of denying the gods and illegally teaching about the heavens. He was brought to trial for impiety and exiled from Athens. Copies of his books were collected and burned in the marketplace. He died in the following year when the ship was wrecked during departure.

Plato (c. 427-347 BC) derived his cosmology from the Pythagoreans. The heavens were divine, eternal, and characterized by circular motion, while the material world (the earth) was decrepit, and characterized by rectilinear motion. Later, this became a dogma which hindered the discovery of the true nature of the heavens. Plato considered the theory of the invisible celestial spheres to be on a higher level than visible things. He has Socrates saying in a passage about astronomical observations in his Republic, book 7, 529a:

These intricate traceries in the sky are, no doubt, the loveliest and most perfect of material things, but still part of the visible world, and therefore they fall far short of the true realities - the real relative velocities, in the world of pure number and all perfect geometrical figures, of the movements which carry round the bodies involved in them. These, you will agree, can be conceived by reason and thought, not seen by the eye... If we mean, then, to turn the soul's native intelligence to its proper use by a genuine study of astronomy, we shall proceed, as we do in geometry, by means of problems, and leave the starry heavens alone...

Eudoxus of Cnidus (c. 406-355 BC) [See biography] proposed an "onion-like" system of 27 "spheres within spheres," to explain all the motions observed in the heavens. Neugebauer wrote: [Neugebauer 1953, p. 225]

Few astronomical theories have exercised so deep and lasting an influence on human thought as the discovery of Eudoxus that the motion of the planets can be explained, at least qualitatively, as the combination of uniform rotations of concentric spheres about inclined axes. The sphericity of the universe, the fundamental importance of uniform circular motion, must have appeared from then on as an established fact. Combined with Aristotle's idea of the "prime mover" the universe could be understood as one great system, truly geocentric. No wonder that this theory held its fascination for almost two thousand years over the minds of philosophers and even astronomers, in spite of the fact that serious difficulties were apparent almost from the start.

Eudoxus found that the motion of a planet could be represented as the result of four concentric rotating spheres, each with distinct axes. When the axes were suitably oriented, the planet, located on the innermost sphere, moved in a figure-eight loop called the "hippopede" or horse-fetter. When combined with the diurnal rotation, the movement resembled that of a planet in its course among the stars. In this arrangement, each planet required four spheres, while three were necessary for sun, and three for the moon, and one for the stars. The system was developed further and made even more complicated by Callippus (c. 370-300 BC) and Aristotle.

Aristotle (384-322 BC) applied the teachings of Plato and elaborated on the theory of concentric spheres of Eudoxus, modified by Callipus. Aristotle explained the movements of the heavenly bodies in terms of the basic assumption that circular motion characterized the movements of heavenly things. His system contained 55 nested spheres, each with a mind of its own. The outermost sphere in Aristotle's system was kept in motion by the deity, which he called the "prime mover". Summarizing Aristotle's thoughts on the movement of this, the greatest sphere, Dicks wrote:

The first heaven exhibits everlasting circular movement; there must therefore be an entity that moves it, and this entity must be itself unmoved and everlasting. It operates by acting as the goal for desire and thought. Being itself utterly and unchangeably good, the first and best entity in the universe, it causes the motion of the primary form of movement, that of revolution in a circle, which strives after it as a lover strives after a loved object; and on such a principle the whole physical universe depends. The prime mover's activity is the highest form of joy which is pure contemplation with itself as object, such as we mortals very rarely attain to, but which is the natural state for the divine; the prime mover is a living entity, the best possible and eternal, and since these are attributes of the god, it is itself divine; it is immaterial and has no parts and no magnitude.

Aristotle distinguished between heavenly things, that he considered to be incorruptible and eternal, and things of this material world, that were corruptible, and made up of the four basic elements, fire, air, earth, and water.

[See Aristotle's On the Heavens.]

[See Aristotle's Astronomy.]

Heraclides of Pontus (c. 390-322 BC) explained the apparent rotation of the heavens by the rotation of the earth. He proposed that the sun was the center of the orbits of Mercury and Venus, and that it also orbited the earth. The "central fire" was abandoned.

Aristarchus of Samos (320-250 BC) proposed a heliocentric cosmology. [See biography]. This demanded a universe of immense size, which men in those days found too difficult to comprehend. He estimated the diameter of the moon to be one third that of the earth, and the distance to the moon to be 40 earth diameters. The diameter of the sun, he gave as 19 earth diameters, and its distance 764 earth diameters. Aristarchus was accused of impiety, because his theory clearly implied some of the gods did not exist. Plutarch mentions this accusation against Aristarchus in the dialogue of On the face of the moon [923a]:

Do not bring against me a charge of impiety such as Cleanthes used to say that it behoved the Greeks to bring against Aristarchus of Samos for moving the Hearth of the Universe, because he tried to save the phenomena by the assumption that the heaven is at rest, but that the earth revolves in an oblique orbit, while also rotating about its own axis.

Seleucus (2nd century BC) taught at Seleucia on the Tigris in Mesopotamia. Plutarch reports that Seleucus also upheld the heliocentric concept, which he regarded not as a hypothesis, but as a fact.

Hipparchus of Nicaea (fl. 146-127 BC) [See biography] followed the methods of Aristarchus, and estimated the diameter of the moon as 0.29 of the earth's diameter, and its distance 30.25 earth diameters. The sun was 12.33 times as great as the earth and its distance 2550 earth diameters, about 10% of today's values. He discovered the precession of the equinoxes, compiled a star catalogue, and invented trigonometry. He added eccentrics to the arrangement of the spheres of Aristotle, further complicating the system.

Claudius Ptolemy (c. 90-168 AD) [See biography] working at Alexandria, summed up the Greek science based on a stationary earth and geocentricity in his Almagest, the only comprehensive work on Greek astronomy which has come down to us intact. Ptolemy reviewed arguments for the earth's rotation, concluding it did not. As he considered the spheres of heaven to be divine, the assumption that the earth remained stationary seemed necessary and fundamental. He further elaborated on the geocentric cosmology by adding a refinement called "equants."

Plato and Aristotle reasoned that the sphere was the most perfect figure, and that the greatest sphere, which imparted motion to the sphere which held the stars, and all the other spheres, was god. This harmonized quite well with the Homeric tradition, in which Zeus was the identified with the solid sky, Olympus.

The academy founded by Plato in Athens, and Greek education generally, aimed at the training of "philosopher-kings" capable of promoting the Greek ideals in the world. Aristotle was a tutor of Alexander the Great. It is said that Alexander carried a copy of Homer with him, which was kept beneath his pillow when he slept. The theories of Greek philosophers tended to reinforce popular religious beliefs derived from Homer. The Hellenistic Greek rulers promoted the Greek culture and the cosmology underlying it. Neugebauer wrote [Neugebauer 1963, p.532]:

It is useful occasionally to remember that the so-called Greek mind not only produced works of the highest artistic and intellectual level but also could indulge in the development of the most absurd doctrines of a pseudo-rational superstition which contributed heavily to the "darkness" of later ages.

In another paper, Neugebauer commented on evidence that ancient astronomical knowledge and techniques were suppressed by the Greeks [Neugebauer 1946, p 120]:

The unique role of the Hellenistic period in the field of sciences, as in other fields, can be described as the destruction of a cultural tradition which dominated the Near East and the Mediterranean countries for many centuries, but also the founding of a new tradition which held following generations in its spell.

References

Dicks, D.R. 1970. Early Greek Astronomy to Aristotle. Thames and Hudson, London. p. 217-218.

Herodotus, The Histories

Neugebauer, O. 1946. The history of ancient astronomy: problems and methods. Publications of the Astronomical Society of the Pacific 58(340):17-142. (See p. 120.) Reprinted in: Neugebauer, O. 1983. Op. Cit.

Neugebauer, O. 1949. The astronomy of Maimonides and its sources. Hebrew Union College Annual 22:322-363. (See p. 336.) Reprinted in: Neugebauer, O. 1983. Astronomy and History: Selected Essays. Springer-Verlag p. 382-423.

Neugebauer, O. 1953. On the "Hippopede" of Eudoxus. Scripta Mathematica 19(4):225-229. (See p. 225.) Reprinted in: Neugebauer, O. 1983. Astronomy and History: Selected Essays. Springer-Verlag, 305-309.

Neugebauer, O. 1963. The survival of Babylonian methods in the exact sciences of antiquity and middle ages. Proceedings of the American Philosophical Society 107(6):528-535. (See p. 532.) Reprinted in: Neugebauer, O. 1983. Astronomy and History: Selected Essays. Springer-Verlag, p. 157-164.

Olson, Richard. 1982. Science deified & science defiled. University of California Press. Berkeley. p. 79.

Plutarch, Platonic Questions 1006c.

Tauber, Gerald E. 1979. Man's View of the Universe. Crown Publishers Inc. NY.