In which Socrates studies the cosmological theories of the first philosophers and goes on a trip to meet a critic of such theories.
Eclipse
On February 17th, 478 BC, at Athens, a little before noon, the sky grew unusually dark as though it was twilight. The bright disk of the sun narrowed to a crescent, and then to a thin ring. People ran outside to see the ominous sight. A few experts tracked the reflected images in basins of water to protect their eyes and see more clearly what was happening. As the earth turned slowly the sun and moon danced across the sky together, the moon screening the sun’s light and casting a small circular shadow on the earth, traveling rapidly west to east as the moon orbited eastward in front of the sun while the earth revolved more slowly eastward. The people of Athens had seen a rare solar eclipse, this one an annular eclipse in which the sun is not completely blocked by the moon, as it is when the moon is orbiting closer to the earth.
To the Greeks, eclipses were terrifying signs from the gods that were carefully recorded and whispered about in hushed tones. But on that day in Athens, a young foreigner named Anaxagoras saw not a supernatural failure or abandonment (ekleipsis) of the sky by the god Helios with his shining chariot, but a natural event. The moon was a solid, spherical body orbiting below the sun. When its path intersected with that of the sun, it would, quite naturally, block the sun’s light to earth. There was no lapse in the divine order of things, only a rare intersection of two heavenly bodies, a coincidence of celestial geometry. That was his hypothesis. But his hypothesis needed to be proved. If an eclipse were a shadow, it would fall on a limited area of the earth. To verify the hypothesis, one would need to determine that the shadow was not seen everywhere alike, but only over a limited area.
In February the boats were still laid up for the stormy winter season. In a few weeks they would be launched again and seaborne commerce would resume. Anaxagoras then went down to the ports of Athens, Piraeus and Phaleron, perhaps sent letters to learned philosophers in distant cities, and collected the results. He found that the full annular eclipse was visible only in the Peloponnesus, not to the north or south. Since he presumed the moon was relatively close to the earth, the sun relatively far away, the shadow would approximate the diameter of the moon. He drew a conclusion: the moon was the size of the Peloponnesus, the sun larger (for it was farther away and yet its outline was visible outside the darkened disk of the moon).[1]
Anaxagoras was a refugee from his city of Clazomenae in Ionia. Athens was still a pile of rubble after the Persian invasion, but her people were triumphant and her navy had recently liberated Ionia from Persian control. Anaxagoras was one of a new breed of thinkers, later called philosophers, who were then still called sophoi, or wise men, like the fabled Seven Sages of Greece from the previous century. The new thinkers, however, did not compose proverbs and aphorisms about how to get along in life, as had their predecessors. They developed theories about how the universe began and how the universe was now. They employed the word kosmos to designate the orderly natural world we live in, and they sought to explain its workings as the product of natural regularities rather than of divine interventions. Lightning was the product of wind bursting out of a cloud, not of Zeus throwing a missile. Earthquakes were the result of a drying out of the earth, or movements of air in underground passages, not the act of the Earthshaker, Poseidon. Each philosopher developed his own version of how the world had come to be what it was then, but they agreed that whatever the story was, the world was a collection of natural bodies interacting in natural ways. The task was to devise a cosmology, a reasoned description of the world and its major components.[2]
The Philosophers of Miletus
Greek Philosophy had originated a century earlier, with a man named Thales from Miletus, a flourishing city on a peninsula jutting into the Aegean Sea from Ionia on the sea’s eastern shore. Miletus, the “jewel of Ionia,” had colonies and trading partners all over the Mediterranean, from the Black Sea in the north to the Nile delta in the south to southern Italy in the west, from which she imported and exported merchandise. Thales, who likely traveled to Egypt on a merchant vessel and returned with amazing new ideas, theorized that the earth was like a raft floating on a vast sea; when there were ripples on the sea, they caused earthquakes. He is credited by early sources with predicting a solar eclipse in 585 BC—although there was no scientifically reliable way for him to do this. He may have used some method of counting months from one eclipse to another, which can work for certain periods of time. We shall never know, as he seems not to have left a written record of his views.[3] Thales speculated that everything came from water, and thus began the tradition of explaining cosmic phenomena by deriving them from everyday natural processes rather from supernatural interventions.
His student Anaximander did write a book, presumably a short papyrus roll summarizing his views; but his book was one of the first, if not the first, prose treatises in Greek. In his view the earth was a disk like a column drum (large columns were made by stacking disk-shaped stones on top of each other) hanging in space, surrounded by rings of fire. These rings were enclosed by air, which rendered them invisible except at an opening, where the fire shone out. Thus each ring was something like a bicycle tire; only at the valve stem would the fiery content be visible. The outermost ring was that of the sun, the second that of the moon, and inside that a ring or plural rings for the stars. Anaximander drew a map of the earth that divided the land into roughly equal continents of Europe, Asia, and Africa, with the Mediterranean Sea in the middle and “Ocean” flowing around the rim of the disk, as it did in the mythical geography of the poets Homer and Hesiod. He saw the world as composed of opposites, which were balanced in a cosmic process of justice, so that an excess of one element, say heat in the summer, was repaid by an excess of its opposite, cold in the winter.[4]
Anaximander’s student Anaximenes proposed yet another cosmology, in which a flat, thin earth floated on a cushion of air. The sun and moon circled above the earth, being hidden at times by high mountains in the north.[5] Anaximenes made an important step in explaining the structure of matter, or the “chemistry” of the world. He said that everything came from air. He was specific for the first time about how that happened. Air is characterized by a certain density of matter. When air gets more rarified, it becomes fire; when it gets more condensed, it turns into wind; when it is still more condensed, it becomes cloud; when still more condensed, it becomes water; when still more condensed, earth; when as condensed as possible, it becomes stones. Thus there is a kind of gradation of matter according to its density. Air successively becomes all things, and since the processes are reversible, other kinds of matter turn into air. Thus when water is evaporated from the sea it becomes cloud, then wind, then air; but in the reverse step air becomes wind, then cloud, then water—as we observe when clouds form and rain falls from the sky.[6]
[1].Graham & Hintz 2007; Graham 2013 b.
[2].Graham 2006; Gregory 2007.
[3].See O’Grady 2002, which tends to be too credulous of some claims. See Wöhrle 2009 for sources.
[4].See Kahn 1960; Rovelli 2011; Couprie 2011.
[5].See Wöhrle 1993.
[6]. For sources of Anaximander and Anaximenes, see Wöhrle 2012.