In speculating about the nature of the cosmos and particularly the heavens, Parmenides had done something that no one on earth had ever done before. He had made a major scientific breakthrough. And, as we shall see, soon everybody—well, at least everybody who studied the cosmos—knew it.
1.5.3.1 Anaxagoras the Cosmologist
Writing in the early fifth century BCE, Anaxagoras probably led the way in appreciating Parmenides’ breakthrough. Born in Clazomenae, a Greek colony in Ionia on the Aegean coast of Anatolia, he was the first Ionian philosopher to set up shop in Athens, where he joined the circle of the great political leader Pericles. Athens was now emerging as a leading power in the Greek world, and also, with the help of enlightened leaders like Pericles, a cultural center. Previously, Athens had been a scientific backwater, having provided none of the luminaries of the natural philosophy movement. Anaxagoras would change all that.
Today, unfortunately, he is often known as a caricature of the movement, the man who said the sun was a stone and the moon was made of earth. At his trial, Socrates was accused of holding that view, but he pointed out that Anaxagoras was the author of the theory, which was treated by the speakers as bizarre and impious.[1] But that is to miss the brilliance of a great thinker.
To begin with, Anaxagoras accepted Parmenides’ great insight that the moon’s phases resulted from its reflection of the sun’s light off its spherical surface. An ancient source reports some of his theories:
The sun and the moon and all the heavenly bodies are fiery stones carried around by the revolution of the aether. And there are below the stars certain bodies invisible to us that are carried around with the sun and the moon. … The moon is below the sun and nearer to us. The sun exceeds the Peloponnesus in size. The moon does not have its own light but gets it from the sun. The revolution of the heavenly bodies carries them under the earth.[2]
He recognized the existence of the sun, the moon, the stars and (perhaps for the first time) what we may call asteroids (second sentence).[3] The moon must be “below” or nearer the earth than the sun (third sentence) because at the time of the new moon its surface is all in shadow and hence invisible to us. Anaxagoras could infer this from the fact that the moon gets its light from the sun (fifth sentence). He could also infer that the heavenly bodies pass under the earth because, for instance, at the time of the full moon, the moon is at its zenith around midnight, while the sun, which is illuminating it at about 180⁰, must be directly below the earth. (This fact seems obvious now, but some thinkers, including Anaximenes and Xenophanes, thought the heavenly bodies all traveled above the surface of a flat earth.) Anaxagoras also knew from this that the sun shines all night, despite what some earlier philosophers (and mythographers) had thought.
So we have, from Anaxagoras, a cosmology and astronomy that integrates the fact that the moon shines with borrowed light, with the positions and motions of the heavenly bodies. He grasped the implications of Parmenides’ insight and built a cosmos incorporating that insight.
But he went beyond Parmenides with his next account:
The moon is eclipsed when the earth blocks it, or sometimes one of the bodies below the moon; the sun is eclipsed when the moon blocks it at the time of the new moon.[4]
Consider what Anaxagoras revealed: if you think about the model of the heavens Parmenides presented, you can account for eclipses, both solar and lunar. The sun and the moon appear to be the same size (½⁰ of the heavenly sphere by later calculations). During the time of the new moon, on this model, the moon is close to the sun, because the shadowed side of the moon is turned toward the earth, while the bright side is facing the sun, at roughly 0⁰ as seen from earth. The moon must be opaque, since its shadow is dark; so it is earthy (as Socrates reports, not fiery, as Hippolytus reports). If it should line up perfectly with the sun, it would inevitably block the sun’s light to us observers on earth and voilà, a solar eclipse. If, on the other hand, the full moon, the earth, and the sun should line up perfectly, with the sun behind us at 180⁰, the moon would fall into the shadow of the earth and suffer an eclipse.
Very simple, very elegant. And true. Notice that the predictions made by this theory are very risky: they tell us that a solar eclipse can happen only at the time of a new moon, and a lunar eclipse can happen only at the time of a full moon. This is a sign of a theory that is subject to confirmation or disconfirmation by empirical evidence. If an eclipse happens at any other time, the event will refute the theory. But in the almost two and half millennia since Anaxagoras published his theory, it has never been refuted. Anaxagoras got it right.
What about the odd claim Anaxagoras made that the sun exceeds the Peloponnesus in size? (The Peloponnesus is the large peninsula at the bottom of Greece.) Why measure the sun in terms of this peninsula? (Anaxagoras also said the moon is about the size of the Peloponnesus.)[5]
Well, it turns out that in 478 BCE there was an annular solar eclipse that passed over southern Greece, which happened to pretty much exactly cover the Peloponnesus with its umbra, its shadow. By one account, Anaxagoras was living in Athens at the time, at the ripe old age of 22, perhaps a refugee from the Persian War. The eclipse happened on February 17 by the Julian calendar, at a time when Greek ships stayed in port because of bad winter weather. But within a month or so, ships would be sailing again, and all it would take for an enterprising scientist to plot the width of the eclipse would be to visit a waterfront tavern in Piraeus and buy a few drinks for thirsty sailors. Where were you last month? Did you see the eclipse? What did you see? Collecting observations was a good piece of old-fashioned Ionian historia.
There was a flaw in Anaxagoras’ reasoning. He was assuming the size of the moon’s shadow would be roughly the diameter of the moon.[6] But in fact the moon’s shadow forms a diminishing cone toward the earth, and in fact, with an annular eclipse, the shadow diminishes to a focal point and then expands again to form an antumbra. Anaxagoras could measure the width of the eclipse, but not soundly deduce the size of the moon from the result.
Still, think about what Anaxagoras did. He took a correct theory about the moon’s light, that explained the phases of the moon and their correlation with the angular distance of the moon from the sun, and applied that knowledge to solve another, seemingly unrelated, problem of astronomy: how do eclipses of the sun and moon occur? Without a telescope or sextant, without tables plotting the paths of sun and moon (such as the Babylonians had), without accurate clocks or calendars, he explained correctly, and for the very first time we know of in the history of the world, how eclipses work, predicted at what phases they could occur, and showed that they were natural and normal phenomena, not prodigies or portents. Presumably at the age of 22. But, sadly, to this day, hardly anybody gives him credit for making the second major historical breakthrough in astronomy.
He said the sun what a fiery stone and the moon was made of earth, LOL. Incidentally, however, the moon is made of earth. Samples of moon rocks brought back by Apollo astronauts show that the moon is composed of minerals that have the same isotopes as matter from the earth’s mantle and crust. The earth collided with a Mars-sized planet in the distant past and sent debris from planet earth into orbit, where it eventually coalesced through gravity to form a satellite.[7] The biggest news of all was the Parmenides’ scientific breakthrough discovery that the moon shines by reflected sunlight, led to Anaxagoras’ scientific breakthrough, that eclipses are caused by the blocking of the sun’s light—to the earth by the moon (solar eclipse), or to the moon by the earth (lunar eclipse).
[1] Plato Apology 26d-e.
[2] Hippolytus Refutation of All Heresies 1.8.6-8.
[3] Hippolytus Refutation 1.7.5 attributes this view to Anaximenes; but this account seems to conflict with other aspects of Anaximenes’ theory.
[4] Hippolytus Refutation 1.8.9.
[5] Plutarch The Face on the Moon 932a.
[6] Anaxagoras does not explicitly state this assumption, but his contemporary Empedocles does in his fr. 42.
[7] See https://en.wikipedia.org/wiki/Giant-impact_hypothesis .