Ancient Greek Astronomy
Since the first Egyptian farmers discovered the annual reappearance of Sirius 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 day length 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 millennia 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 moon phase 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. 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 Babylonia. 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.
As ancient peoples began to realize that sun, moon and stars follow certain rhythms in step with the seasons, they began to hypothesize that some conscious set of rules must be dictating these movements and seasonal changes that, for agrarian or pastoral societies, were a matter of life or starvation. So, to explain the natural phenomena a complicated system of gods’ interactions with men was born.
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 straddling the earth, with fiery, gleaming “aither” above the cloud-bearing air. Homer mentions the movements of sun, 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, 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, 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
As the Greeks began to travel and explore, their ideas of the order of the universe began to change. 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 the Lydians and Persians, latest descendents of Mesopotamian civilization. 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. The 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 BCE. 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. 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. The next ancient Greek philosopher was Anaximander of Miletus who lived c. 550 BCE. The earth for Anaximander was still a cylinder circled by air and then fire “like the bark of a tree,” which separated off at an early stage. Although his theory still echoes the early cosmologies, it is an attempt to explain the scheme in purely physical–in fact, in mathematical–terms. The heavenly bodies are all described as wheels; their visible light that we actually see is only a part of them, described as an axle, pipe, or vent. These vents opening or partially closing cause lunar variations. All these ideas did attempt to explain the universe in physical terms, though as yet there was only a vague theory as to why these things are so. Yet it is possible to see by Anaximander’s “equilibrium” that the Greeks were beginning to be aware of gravity but still needed to put two and two together and recognize it explicitly.
Through further study and thought the Greeks progressed towards a more concrete and accurate image of the earth and the heavens around it. Anaximines of Miletus (c. 525 BCE) refined the flat-earth idea, suggesting that all things are produced through a process of gradual condensation and “rarification”: earth condenses out of air, and fire is “exhaled” from the earth. The earth and heavenly bodies are flat and loft on infinite air like a leaf and do not set beneath the earth, just as in mythology, but instead turn at an angle so that many are obscured by the “higher” parts of earth.
Ionic thought brought new influences to Greece. Xenophanes of Colophon (c. 570-490 BCE) migrated from Ionia to Italy. He propagated the view of heavenly bodies condensing into fiery clouds from earth’s exhalation. His heavenly bodies, like Anaximines’, followed circular courses and were obscured behind high parts of the earth. Heraclitus of Ephesus (c. 500 BCE), though criticizing his predecessors’ work as data, not understanding, continued 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 and day from exhalations ignited by the sun. Sun, moon, and stars are fire caught in bowls, which tip away to cause eclipses and lunar phases. The moon travels through the less purified air close to earth, and the sun is the closest and thus brightest and hottest of stars.
The later of these “Pre-Socratic” philosophers began to specialize, develop, and apply the systems of empirical observation and deduction that 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. Parmenides of Elea, in Italy, managed to demolish all physics by his proof that neither motion change, nor differences in matter can exist. 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. Empedocles of Acragas (mid 5th cent. BCE) worked on a system to reconcile the “unchanging” universe of Parmenides’ sphere with chaotic, differentiated matter by having the universe in a state of flux between harmony and strife. He also advocated 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
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. His most important contribution to astronomy was the claim that the moon’s light is a reflection of the and that eclipses of the moon were caused by earth’s shadow, eclipses of the sun by the moon passing before.
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. The sun is a glass sphere 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. Also, the counter-earth was invented to account for the frequency of lunar eclipses and to solve 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.”
Thus, by a roundabout way the early Greek philosophers were reaching the Aristotelian view of the universe. The atomists Leucippus and Democritus in the generation preceding Socrates refined the various pre-Pythagorean views of space: there is a drum-shaped earth, condensation is the falling-together of atoms, and centrifugal force helps keep the earth and bodies of fire in place. After them, however, Socrates’ pupil Plato and Plato’s pupil Aristotle reflected upon Pythagorean harmony and spheres and a geocentric system. Their many works analyze, refute, discuss, and expand on their predecessors; two of the passages representative of their views on astronomy are found in Plato’s Timaeus and Aristotle’s De Caelo.
Having discovered a theory of the solar, or rather, geo- system that 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” universe than the Greeks, who had placed their version of a Bible, the Homeric and Hesiodic myth-cycle, into the realm of metaphor.
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