The Greek philosophers eventually ceased to believe their ancient myths about the Milky Way. Instead, they began to speculate about its origins in a more systematic, “scientific” way. They used their limited observations to come to three distinct categories of conclusions.
By the time of Aristotle in the fourth century BCE, the universe had been systematized into a series of nested regions, or “spheres.” Of course, Earth was at the center.
Above the Earth was the changeable region between the Earth and moon — spheres of the air and meta-aer, “beyond the air.” To Aristotle, those zones were corruptible and ever-changing.
Then began a series of spheres starting at the moon and at increasing distance from it: the sun and planets, each in its separate sphere. The outermost region was the celestial sphere in which the stars existed.
Ancient scholars described the spheres as crystals. The stars and planets were embedded in their own unique and distinct spheres. Hence, the planets and stars were unchangeable in their physical qualities or in their motions. With the moon began the regions of incorruptibility and changelessness.
Seekers after the origin of the Milky Way divided into two camps. One group believed its glow was a meteorological phenomenon in the lower spheres of water, air and fire. A second school of thought argued that the Milky Way was a part of the starry outer sphere that set the outer boundary of the cosmos.
Aristotle was firmly in the first camp. He saw the imperfect shape of the Milky Way and concluded that it must be in the “sublunary” region above Earth’s airy sphere but below the moon. He imagined it as a steady “dry exhalation” coming from Earth’s marshy areas.
Other proponents of the sublunary zones attributed the Milky Way to clouds of tiny particles or a stream of heat designed to temper the coldness of the universe.
Such theories must be false, as critics were quick to point out. The Milky Way moved with the stars. Exhalations change their shape. The Milky Way did not change. It must exist in that outer realm among the stars.
The second category of explanation assumed that the sun’s light had something to do with the glow of the Milky Way. Some thought it was the direct reflection of the sun’s rays.
Anaxagoras believed it was the shadow of the Earth cast upon the sky by the sun. Alternatively, some said it was the true light from stars seen when the sun’s rays are obscured as it passes behind the Earth.
Others saw it as a physical characteristic of the starry sphere where somebody, presumably the gods, had soldiered or melted it together. Or perhaps, harkening back to the Phaethon story, it was the burn mark left when the sun had moved along another path.
And then there was Democritus. Out of all the ancients, he was the only one to provide a glimpse of the truth.
None of his writings have survived, but he has been called the father of modern science. We discover what we know of him in the writing of Aristotle, that great cataloger of Greek knowledge.
According to Aristotle, Democritus posited an infinite universe of stars unfettered by crystalline spheres. He believed that the Milky Way consisted of innumerable, “very small, tightly packed stars” that, as the Christian writer Achilles Tatius described it in the second century CE, “seem to us joined together because of the distance of the heavens from the earth, just as if many fine grains of salt had been poured out in one place.”
Democritus was right! Sort of. His description came closest to the truth of the matter, but it didn’t matter. Without direct observational evidence, one speculation was as good as another. It took 2,000 years and the development of a revolutionary technology to transform the musings of Democritus into observations that would fundamentally change our view of the universe — and our place in it.
Tom Burns is the former director of the Perkins Observatory in Delaware.
