Parmenides may well, as most scholars think, have developed his alternative cosmology mainly as an object lesson on the futility of cosmologies. After all, he announced at the outset of his cosmological treatise that positing two “forms” or elements, namely Light and Night, was a mistake. Yet he put a great deal more effort into his alternative cosmology than was really necessary if it was, in the end, only a throw-away theory. Although we do not know how many lines his cosmological treatise took up in the poem, by all accounts, it was much longer than the Truth part of the poem. The new cosmology was, it seems, a labor of love, for the critic of cosmologies.
And it was amazing.
After a century of Greek philosophers theorizing about the cosmos and how it arose and how it functions, Parmenides, who had a deep distrust about the validity of theories about nature, offered an alternative cosmology that made three scientific breakthroughs.
As I have mentioned, his theory that the Earth was spherical was not an immediate success. For about a century afterwards, (most) natural philosophers continued to portray the Earth as a flat disk. But the identity of the Morning Star and the Evening Star, and the theory of the moon’s phases as caused by the reflection of the sun’s light, were taken over immediately. In the next generation, both Anaxagoras and Empedocles accepted the view, followed by almost every other natural philosopher from then on.
Up until Parmenides discovered the source of the moon’s light, and with it, the reason for the phases of the moon, virtually every philosopher had proposed a different (and often very elegant) theory of the moon. Anaximander saw it as the light emanating from a circular tube like a bicycle tire with a valve that let the light inside shine out. Anaximenes saw it as a thin circular disk that flew around something like a kite sometimes seen from the side. Xenophanes saw it as a cloud. Heraclitus saw it as a bowl of fire, with the opaque bowl sometimes covering the spherical fire partially or fully, producing the phases. After Parmenides, there were no new theories. Everyone repeated Parmenides’ account. Sometimes with minor modifications, but always with his central insight. Philosophers would continue to develop new theories of other general natural phenomena such as earthquakes and localized phenomena such as the Nile floods, but there would be no more theories of the moon’s light.
Indeed, Parmenides’ account is the one we still use today. To be sure, we know that the Earth revolves around the sun and not vice versa, that the sun is a star like the pinpoints of light we see in the sky, that the moon is composed of material from the Earth resulting from an ancient cosmic collision, and so on. We know a lot more, then, about the history of the solar system and its motions. But we still hold to Parmenides’ account of lunar light. That is what a scientific revolution looks like: it changes our understanding of the world, making it impossible to accept earlier accounts without serious modification.[1] Many scientific revolutions do not last forever. For instance, Newton’s account of gravity gave way to Einstein’s General Relativity. But without Newton there would have been no Einstein. And some of the discoveries that emerge from revolutions do last, including Parmenides’ account of the moon’s light.
And often, great discoveries open the door to new discoveries. We shall see this in the case of Parmenides’ discovery.
Parmenides’ discovery allows us to turn the sky into a laboratory and watch as the moon follows its path across the sky. At the beginning of a lunar month, we, as the sun sets, a thin crescent moon just above and to the left (east) of the setting sun. The next evening at sunset, the moon is a little higher and farther left (about 12⁰ away), and a little thicker. The next evening the pattern repeats, until, after about a week, the moon is high up in the south, appearing as a half-moon. About a week later, the full moon is seen just rising in the east as the sun sets in the west. After that, the moon rises after sunset, and reverses in its phases, now with the left (east) side facing the sun that will rise at dawn. Parmenides’ theory explains the sequence of phases, and indeed, predicts them, like no other theory ever proposed.
Parmenides’ discovery makes it clear that the neither the sun nor the moon is “new every day,” like some theories, including that of Xenophanes, claimed. It allows us to plot the course of the sun even when it is invisible during the night hours, by its angular distance from the moon. Hence it proves that the sun is shining all night, that it moves under the earth, not around it, as some theories, such as that of Anaximenes, claimed. It allows us to affirm that the moon is in existence even when it disappears at the time of the new moon. It also shows that the sun is not only higher or more distant than the moon (contra Anaximander), but also very much more distant.
In short, Parmenides’ theory renders obsolete not some, but all of the preceding theories of lunar light. It replaces them. It provides the first major clue to the architecture of the solar system.
So what was the effect of Parmenides “refutation” of philosophical cosmologies? It did not doom the project of rationally explaining the cosmos. It gave it new life. It transcended all previous attempts, revealed a secret about the structure and dimensions of the cosmos—one that had been staring humans in the face since time immemorial—and proved: the world is accessible to rational explanation.
However revolutionary Parmenides was as a philosopher, he was, incredibly, even more revolutionary as a scientist.[2]
[1] See Thomas S. Kuhn, The Structure of Scientific Revolutions ([1970] 1996).
[2] See D. W. Graham, Science Before Socrates: Parmenides, Anaxagoras, and the New Astronomy (Oxford, 2013).