The late summer sky is crowned by the most beautiful — and mysterious — sight the universe has to offer us. The glorious Milky Way, a great river of light, dominates the night. During September’s early evenings, the silvery band is high in the sky as it extends from the southeast to the northwest. It flows across the sky like a stream of glowing milk. It stretches across the vault of heaven like, as some would have it, the backbone of the night.
Up to about 400 years ago, not a single human could determine Milky Way’s composition. It was certainly not made up of the goddess Hera’s spilled milk or a celestial spine.
You’ll need a dark, rural sky to see it in all its glory. The growth of city lighting has seen to that. Our ancient forebears had one thing going for them that all our technological advances have sadly come to erase. Their skies were unmarked by the stain of light pollution of city lights. They saw the Milky Way in all of its complex, mind-altering glory.
However, the question of its composition and the mystery of its origin lasted thousands of years. Over the next couple of weeks, I intend to trace the history of our slowly rising knowledge of its true nature. I hope you will come along for the ride.
The story starts with the myths and legends that people told long ago. Many natural mysteries were explained — or explained away — by the capricious nature of our gods or by a culture’s concerns about their survival or success. The Milky Way is no exception.
Practically every culture on Earth tells such stories. The ancient Egyptian culture saw it as the work of the god Isis, who had spread an abundance of life-giving wheat across the sky. Across the world, the Incas, who were lovers of the beauty and wealth that gold produces, saw it as a steam of gold dust. The Inuit people of the Arctic believed they saw a snowy band. The inhabitants of Australia saw the ash of the campfires they had set against the cold and dark of the night.
Often, ancient people depicted rivers and great waters and the creatures that lived in them. The Arabs who lived in the parched deserts saw an unattainable river of water. Native Americans of the Great Lakes region saw a muddy creek stirred up by a giant mud turtle crawling across the sky. Polynesians called the Milky Way a cloud-eating shark. Fishers in the Orient saw a school of fish frightened away by a fishhook, which they fancied to be the thin, crescent moon.
Most often, ancient cultures saw it as a road or path, quite literally a “way.” To some early Hindus of India, it was the path of their god Aryaman upward to his heavenly throne. To those who lived on the banks of the Yellow River in China, it was a yellow road, their life-sustaining river extended into the heavens. The Celtic denizens of Wales saw it as the path that the sly trickster and warrior Gwydion left as he pursued his fleeing wife. In Norse mythology, the Milky Way is the road to Valhalla, the final home of brave warriors who died in battle. The Iroquois described it as the path to eternal life after death, the “Road of Souls.” To travelers seeking the center of civilization of its time, it was the “Road to Rome.”
Ovid, perhaps the greatest of all Roman poets, describes it as the shining road to Olympus and from there to the palace of Zeus, the king of all the gods: “By this way, he wrote, “the gods fare to the halls and royal dwelling of the mighty Thunderer.”
The ancient Greeks saw the Milky Way as the emblem of a tragic tale of betrayal and vengeance. As told by the poet Eratosthenes, the story reflects their almost-human failings raised to superhuman proportions.
Zeus, or Jupiter to the Romans, was the king of all the gods, and he was used to getting his own way. He betrayed his wife, Hera, with a kind of offhanded but merciless zeal.
Hera put up with his philandering. She was more or less forced to do so. She couldn’t express her anger directly. Zeus was far more powerful than she.
At long last, she had a mortal baby to torment. Alcides, as he was called at birth, was the illegitimate offspring of Zeus, king of the gods, and Alcmene, a mortal woman. His half mortal, half immortal parentage caused no end of trouble for the babe throughout his exceedingly difficult life.
For starters, Hera sent two monstrous serpents to kill the baby in its crib. But the babe was exceedingly strong. He reached out with his chubby, little hands and squeezed the life out of the slithering beasts.
Zeus knew that the baby was in mortal danger. Heracles, as he came to be called, must drink the mother’s milk of a goddess to achieve the immortality of a god. Hera, as you might imagine, was unwilling to provide such a service to Zeus’ lust child. Zeus thus sent his messenger, Hermes, to lay Alcides beside her as she slept. The infant did what came naturally and suckled the milk of immortality. Did I mention that the babe was strong? Hera awoke in pain and pushed the babe away. Her milk streamed across the sky as a glowing band of light we call the Milky Way to this very day.
None of those tales sound particularly “scientific” by our standards. However, in another Greek myth, we find the hint of a more satisfactory explanation that would come far down the path of centuries.
The ancient Greek story of Helios and Phaethon is the case in point. According to the myth, the universal predominance of the gods began with an apocalyptic war between the gods and the Titans, the race of supernatural creatures that preceded them.
The gods were triumphant. They cast many of their titanic enemies into the outer darkness. Others were given onerous jobs. Helios was burdened with the sweaty task of transporting the sun across the sky in a horse-drawn chariot.
Presumably, he took a long break at night. To him and the sea nymph Cymene was born Phaethon, a son. Phaethon’s playmates teased him relentlessly. They did not believe that his father was the solar charioteer. He begged his father for some proof, and his father foolishly promised him anything he wanted.
The boy insisted on driving the chariot for a day, but he was not able to control the solar steeds. As he careened around the sky, he dislodged some of the stars. The Milky Way is the burn mark left by one of the dislodged stars or even the sun itself.
The Greek philosophers eventually ceased to believe the ancient myths. Instead, they began to speculate about the origins of the Milky Way in a more systematic, “scientific” way. They used what limited observations they could make to come to four categories of conclusions.
By the time of Aristotle in the fourth century BCE, the universe had been systematized into a series of 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 their own sphere. The outermost region was the celestial sphere in which the stars existed. Those spheres were often described as crystals in which the stars and planets were imbedded and hence unchangeable in their physical qualities or in their motions. With the moon began the regions of incorruptibility and changelessness.
Where does the Milky Way fit in? Aristotle 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. It must be some sort of steady “dry exhalation” coming from the marshy regions of the earth.
Other proponents of the sublunary zones attributed the Milky Way to clouds of very small 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.
A 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 the sphere had been soldiered or melted together. Or perhaps, harkening back to Phaethon, it was the burn mark left when the sun had moved along another path.
And then there was Democritus, who out of all the ancients was the only one to provide a glimpse of the truth. More on him next week.
Tom Burns is the former director of the Perkins Observatory in Delaware.