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Inventing the Solar System

Early Greek Scientists Struggle to Explain
How the Heavens Move

Ellen N. Brundige
University of CA, Irvine
ebrundig@uci.edu

I. Introduction

Since the first Egyptian farmers discovered the annual reappearance of Sirius [(1)] just before dawn a few days before the yearly rising of the Nile, ancient civilizations around the Mediterranean have sought to explain the movements of the heavens as a sort of calendar to help guide them conduct earthly activities. Counting phases of the moon or observing the annual variations of daylength could, after many years' collection of observations, serve as vital indicators for planting and harvesting times, safe or stormy season for sailing, or time to bring the flocks from winter to summer pastures. With our millenia of such observation behind us, we sometimes forget that seeing and recording anything less obvious than the rough position of sun or nightly change of moonphase requires inventing both accurate observation tools (a stone circle, a gnomon used to indicate the sun's shadow, a means to measure the position of stars in the sky) and a system of recording that could be understood by others (how many fingers' width or degrees is that star from the horizon? Which direction is due north?).

The ancient Greeks struggled with these problems too, using both native technology and inquiry, and drawing upon the large body of observations and theories gradually gleaned from their older neighbors across the sea, Egypt and Babylon. Gradually moving from a system of gods and divine powers ordering the world to a system of elements, mathematics, and physical laws, the Greeks slowly adapted old ideas to fit into a less supernatural, hyper-rational universe. This paper is a short survey of the Greeks' earliest attempts to explain why and how the sky changes.

II. Mythology

As ancient peoples began to realize that sun, moon and stars follow certain rhythms in step with the seasons, they made the leap of thought to postulate that some conscious set of rules must be dictating these movements and seasonal changes which, for agrarian or pastoral societies, were a matter of life or starvation. Who or what could be causing these all-important changes to come about? Certainly nothing on earth, no beast or human, had the power. Thus gods were born.

II.A. Homer and Hesiod: Chariots [(2)] of Fire in the Vault of Heaven

There are hints of the Greek conception of the universe in Homer, who mentions many subjects on his two epics describing war and the perils of trying to come home after long absence. For Homer, heaven is a solid inverted bowl (Od. 15.329 sideron ouranon) straddling the earth, with fiery, gleaming aither above the cloud-bearing air. (Il. 14.288 `fir-tree reached through the aer to aither).[1] Homer mentions the movements of sun (Od. 4.404), moon, and many stars by name. The fact that Hades is on the underside of earth has an important impact on conceptions of heaven: it is unlit by the sun (in Homer and in Hesiod), therefore, the sun--and by extension, other heavenly bodies-- must sink only to the level of Ocean, which is conceived as a river circling earth's edge. From it the Sun must also rise--though how it gets back to the eastern bank of Ocean is never explained.

These popular conceptions of sky are more fully explained in Hesiod, whose works on gods and on agriculture and animal-herding are more closely connected to the practical application of astronomy. He clocks spring, summer, and harvest by solstices and the rising and setting of certain stars, and notices that the sun migrates southwards in winter.[2] Night is a substance welling up from under the earth, as if it were a dark flowing mist (Hes. Th. 726).[3]

II.B. Other mythological schemes of the heaven

One early and popular cult, that of Orpheus, developed its own of gods and universe-creation variant from those in Homer and Hesiod; there, a primeval egg is birthed by the early gods, and the upper half of its broken shell becomes heaven's vault.[4] Various cults, cities and tribes of Greeks (who were unified only by language and common culture, and both of these had regional variants) probably had different versions of cosmogony and slightly different gods in charge of astronomical movement, but the general physical conception of the sky is alluded to by many authors of plays and other popular works (Eur. Melanippe 484), and was probably held by the majority of people.

III. The Ionian Revolution: Fiery Clouds and Wheels of the Sky

Many Greeks settled on the coast of Turkey in the early migrations of the eleventh century BCE, and there enjoyed rich cultural mingling with their neighbors (and sometimes their conquerers) the Lydians and Persians, latest descendents of Mesopotamian civilization. (See Hdt.1.142 for a discussion of Ionia; Strab. 14.1.7 about famous Ionian scholars.) The freshwater conception of the Ocean river is seen by many scholars as a telltale sign of early adoptations from that ancient `between the rivers' people.[5] They kept in touch with their western cousins, who began a second wave of settling across the Aegean in the seventh century, as well as with other rich sea-faring cultures like Egypt. It is not surprising that, by the sixth century, these Ionian navigators of the sea began to develop new ideas about the sky they steered by. The most fundamental of these was that the universe might run, not only by the whim of gods, but by physical, mechanical rules and principles that might, through study, be understood and predicted.

III.A. Thales of Miletus (c. 585 BCE): Eclipses and Stellar Studies

Our sources for all early Greek astronomy are scant, none more so than for Thales, supposedly the first of the philosophers. Various inventions and discoveries are attributed to him, most famous of which is his prediction of an eclipse of 585 (Hdt. 1.74). Modern scholars are fairly sure he was able to do this by consulting known Babylonian eclipse and lunar observations going back about 150 years, long enough to notice that eclipses recur after about 18 years[6]. His activities also seem to have included star-observations and trigonometry, which he is credited with having founded, but the details of his theories are either lost or obscured by later legends about this early thinker who left no written record. He seems to have conceived of earth as flat and water-borne, and to have postulated that there must have been some first substance out of which the world arose, which he guesses is water (Aristot. Met. 983b 6)[7].

III.B. Anaximander of Miletus c. 550 BCE: Celestial Fire and Pipes

The earth for Anaximander is still a cylinder circled by air and then fire "like the bark of a tree" (Ps-Plut. Strom. 2)[8], which separated off at an early stage. We still see echoes of the early cosmologies here, but an attempt is made to explain the scheme in purely physical--in fact, in mathematical--terms. The heavenly bodies are all described as wheels of fire (Homerically described as like chariot wheels) enclosed by are via a further separating-off. Their light which we actually see is only a part of them, described as an axle, pipe, vent, or bellows-nozzle, through which fire jets (Aetius 2.20-21).[9] Eclipses and lunar variations are accordingly caused by these vents opening or partially closing.

All these ideas are attempt to explain the universe in physical terms, though as yet there is only the vaguest theory as to why these things are so. Anaximander seems to suggest a process of separation and "equilibrium", with the earth suspended in the middle and the various heavenly bodies "balanced" all around it by some unseen rule ( Aristot. de caelo. 295b 10). Here, as with all the early thinkers for whom direct quotes are almost non-existent, we must be cautious about later sources who tend to read later theories into earlier philosophers' ideas. Yet with that caution in mind, we can see by Anaximander's "equilibrium" (and the "condensation" of Anaximines and all his successors, next section) that the Greeks were beginning to be aware of gravity but still needed to put two and two together and recognize it explicitly.

III.C Anaximines of Miletus (c. 525 BCE): Condensation of All From Air

The third of the Ionian thinkers refined the flat-earth idea, suggesting that all things are produced through a process of gradual condensation and "rarefication"[(3)]: earth condenses out of air, and fire is "exhaled" from the earth (Ps.-Plut. Strom. 3).[10] The earth and heavenly bodies are flat and loft on infinite air like a leaf (Aristot. de caelo. 294b 13).[11] Celestial bodies do not set beneath the earth, just as in mythology, but instead turn at an angle (the axis of rotation, after all, is visible to us in the northern part of the sky) so that many are obscured by the "higher" parts of earth to the north ( Aristot. Meteor. 354a 28).

IV. Ionic thought's Influence: Wheels and Bowls of Fire

Xenophanes of Colophon (c. 570-490 BCE) migrated from Ionia to Italy fleeing the Medes' takeover, bringing Milesian theory with him. Although largely concerned with deanthrophomorphizing god to make it infinite and all-encompassing, he propagates the view of heavenly bodies condensing into fiery clouds from earth's exhalations (Aetius 2.20.3). His heavenly bodies, like Anaximines', follow circular courses (conceived as bands or zones) and are obscured behind high parts of the earth.

Heraclitus of Ephesus (c. 500 BCE), though criticizing his predecessors' work as data, not understanding, continues the idea of creation through balance of different substances and the process of condensation, this time, from fire. Night is formed of murkier exhalations from earth (as from Tartaros in Hesiod above) and day from exhalations ignited by the sun. Sun, moon, and stars ae fire caught in bowls, which tip away to cause eclipses and lunar phases. The moon travels through the less purified air close to earth, so is dim, and the sun is the closest and thus brightest and hottest of stars (Diog. Laert. 9.8-11).[12]

V. Parmenides, Empedocles, and Pythagoreans: Paradoxes, Spheres, and Cycles

The later of these "Presocratic" philosophers began to specialize, develop, and apply the systems of empirical observation and deduction which their predecessors had invented. Some, like Parmenides and Zeno, concentrated on exposing the fallacies and logical traps to which the first uses of analytical thinking often fell prey. Others, like followers of the semi-legendary Pythagoras, used their theories about how the universe worked to develop new ideas of divinity, astronomy, universal harmony, and mathematics, and extended these ideas to dictate a proper, "harmonious" lifestyle. All these continued to refine and argue over the basic precepts put forth by the Milesian thinkers.

V.A. Parmenides (c 450 BCE): The Sphere of All, Wreaths of Fire

Parmenides of Elea, in Italy, manages to demolish all physics by his proof that neither motion, change, nor differences in matter can exist (quoted by Simplicius in Phys. 146.5).[13] Having done this, he coyly outlines the "beliefs of mortals", which must in fact be his view of the"deceitful" physical world, a sort of Greek version of the Buddhists' maya. His heavenly bodies, separating out with the heaviest matter towards the center, are again concentrations of fire-vapor, here regulated by "Necessity" to move them between an inner "wreath" of fire and an outer solid sphere (Aetius 2.7.1). It sounds like he conceived of the "wreath" as a belt like an asteroid belt, and the outer shell as a true sphere; whether or not his earth was flat is difficult to tell.[14]

V.B. Empedocles and Anaxagoras: Reflections of Light

Empedocles of Acragas (mid 5th cent. BCE) works on a system to reconcile the "unchanging" universe of Parmenides' sphere with chaotic, differentiated matter by having the universe in a state of flux (as in Heraclitus) between harmony and strife. Along the way, he propounds an outer, hard universal sphere upon which the stars are fixed, and an inner sphere of double hemispheres, one of lighter fire for day, one of darker for night. The sun and moon are not physical bodies but concentrated, polished spots on this inner surface which reflect the outer fire (Eusebium P.E. 1.8.10).[15]

Anaxagoras, friend of the Athenian statesmen Perikles and thus slightly younger than Empedocles, follows the usual theory of separation and condensation, but his heavenly bodies are again solid objects (Plat. Apol. 26D). His most important contribution to astronomy was the claim that the moon's light is a reflection of the sun (Plat. Crat. 409b),[16] and that eclipses of the moon were caused by earth's shadow, eclipses of the sun by the moon passing before it (Aetius 2.29.6).

V.C. Pythagoreans: The Hearth of the Universe

The Pythagoreans first proposed a non-geocentric system, perhaps partly on the basis of moral and religious grounds: to them, humanity and earth were imperfect, and only by sacrifice and a strict regimen of personal conduct could one strive to reach the divine. Accordingly, they placed the divine, poetically called the "Hearth of the Universe" or "Throne of Zeus", at the center of a finite, spherical universe (Aristot. de. caelo B13, 293a-b30). The sun is a glass sphere (Aetius 20.12) which catches and reflects this hearth-light. A counter-earth, the "antichthon", had to be invented, supposedly to make the number of planetary spheres ten. These include the five visible planets out through Saturn, earth, the moon, the sun, and the heavenly sphere on which were the stars.(Aetius 2.7.7,Aristot. Met. A5,986a1). Heath, in outlining this system, suggests that the counter-earth was invented to account for the frequency of lunar eclipses [17]. The counter-earth also solves a major problem in this view, serving to eclipse the Hearth-Fire so that we never look God in the face, so to speak. The concepts of number, harmony, and music all influenced the Pythagoreans to invent this fully-realized version of the concentric celestial orbits, which resonate with "the music of the spheres" .[18]

VI. The Socratics and Beyond: A Geocentric System

The atomists Leucippus and Democritus in the generation preceding Socrates refined the various pre-Pythagorean views of space: there is a drum-shaped earth (in Leucippus), condensation is the falling-together of atoms, and centrifugal force helps keep the earth and bodies of fire in place (Diog. Laert. 9.30ff).[19] Leucippus was probably from Miletus, and Democritus from Abdera; their development of atomic theory was a refinement of two centuries of Ionian scholarship.

After them, however, Socrates' pupil Plato and Plato's pupil Aristotle would espouse Pythagorean harmony and spheres and a geocentric system. Their many works analyze, refute, discuss, and expand on their successors; two of the passages representative of their views on astronomy are found in Plato's Timaeus 37dff and Aristotle's De Caelo 2,289a 11-291b 23. In keeping with these theories now becoming prevelant, the early fourth century mathematician

Eudoxis of Knidos mathematically described the idea of concentric spheres (Aristot. Met. 1073b 17-1074 a 15)[20], anticipated by the "wreaths" and "zones" of many earlier scientists and probably assumed by the Pythagoreans.

Having discovered a theory of the solar, or rather, geo- system which accounted for all visible phenomenon (and was, moreover, aesthetically pleasing), subsequent astronomers and philosophers fine-tuned the idea for their particular fields. The philosophers dwelt on harmony, cycle, and a new scheme of the divine; the mathematicians, a description of heaven in the marvelous language of geometry which was nowhere else in the physical world more eloquently expressed. Sophisticated three-dimensional moving systems were worked out by various geometers to account for observed inconsistencies in their basic theory. It would take many centuries before anyone had accurate enough observations to realize that the theory could not account for all data.

By then, people would have even more difficulty letting go of their clockwork, geocentric, "divinely subsidized" [21] universe than the Greeks, who had placed their version of a Bible, the Homeric and Hesiodic myth-cycle, into the realm of metaphor after executing only one gadfly of a philosopher.

Endnotes

  1. Sepdet, or Sothis - A summary of info about the star Sirius and sky-myths

    Sepdet is the Ancient Egyptians' goddess who appeared to them each year as the dog-star, Sirius, brightest star in the sky. She disappeared from the sky for several months of summer, when the Nile was at low ebb, and hopeful Egyptian farmers planted seeds in the fields along the banks of their irrigation ditches. She reappeared just before dawn at the end of our July (Hom. Il. 5.5), and a few days later, the Nile would begin to rise too, so that she was thought to be a herald calling the life-giving waters back to the fields. Naturally, the star came to be revered for this.

    The Egyptian New Year began on the first day whose dawn saw Sirius rise just before the sun, nowadays close to August 1st, a harvest holiday for many cultures coming as it does midway between the summer solstice and autumnal equinox. She was sometimes called Wep-renpet for that reason, which means, "Opener of the Year", and the month that followed her rising was often called this too.

    Sepdet's symbolism and meaning were absorbed into the well-known national goddess, Isis, due to another legend about the star. Sirius happens to travel the sky just ahead of the large constellation of Orion. (His belt of three stars serves as an easy pointer towards Sirius, the unmistakable bright star that is one of the few visible even in city lights' glare). Orion was identified with the dying-and-resurrected god Osiris, in Egyptian mythology, who was one of the most well-known gods of the pantheon. His wife and sister Isis was Lady of Magic, who brought her husband back to life, and the bright star his constellation followed naturally came to be associated with her. The function of Sirius as herald of the life-giving inundation of the Nile added another layer of metaphor to this age-old story.

  2. While popular conception of Greek myths paints a quaint picture of the solar chariot riding across the sky, a search for "sun", "Helios", "Apollo", and his son "Phaethon" yielded only one instance of this motif in Homer or in Hesiod, although all are mentioned many times. Various gods do employ chariots to travel the sky throughout Homer, Poseidon more often than Apollo. Later authors and Greek art begin to use this scene more explicitly and frequently.
  3. Many of the Greek thinkers had earth condensing from water, or earth exhaling watery mist which condensed through "rarefication" into fire. Anaximenes started with earth condensing from air, which seems even more counter-intuitive to us, but we can understand it better if we consider the following.

    Perhaps Anaximenes was observing the way dust precipitates out of the air onto tables, chairs, and any surface. The bright sky was often thought by the ancients to be a bowl or curtain of "shining" air, which they often assumed was smoke or mist refined one step further. Although we find these laughable concepts in the way the Greeks expressed them, we do in fact believe something of the same thing: the gas giants Jupiter and Saturn glow with heat of their own like stars, but are not large enough to collapse, ignite, and form suns. Similarly, we know that compressed gas makes heat, and that the difference between solid, liquid, and gas is that solid matter has the atoms packed closely together so that they cannot move freely, whereas in liquids or gases the atoms are looser.

References

Heath--Heath, Sir Thomas L., Greek Astronomy. New York: Dover Publications, 1991.

KRS--Kirk, G.S., Raven, J.E., and Schofield, M., The Presocratic Philosophers.Cambridge University Press,1957.

[1] KRS. p. 9

[2] Heath. p. xii.

[3] KRS. p. 726.

[4] KRS. p. 25. Aristophanes spoofs this in The Birds.

[5] KRS. p. 11.

[6] Heath. p. xvi.

[7] KRS. p. 88.

[8] KRS. p. 131.

[9] KRS. p. 135.

[10] KRS. p. 151.

[11] KRS. p. 153.

[12] Heath. p. 15.

[13] KRS. p. 252.

[14] KRS. p. 258

[15] Heath. p. xxxvii, KRS. p. 300.

[16] Heath. p. 27.

[17] Heath. p. xxxvi-xxxvii.

[18] Heath. p. 34.

[19] Heath. p. xxxviii.

[20] Heath. p. xliv.

[21] Christopher Fry, The Lady's not for Burning. Second Edition. 1950. A skeptic chastises a fellow rationalist for waxing poetic about the moon: "The moon is nothing but a circumambulating aphrodisiac, divinely subsidized to promote a rising birthrate, a veneer of sheerest Venus upon the planks of time, which may fool the Ocean, but which fools not me."

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Last modified: Sun Dec 27, 1998 / Jeremiah Genest