As we saw last week, that silvery streak of light in the sky, which has been for a long time called the Milky Way, confounded astronomers from ancient times.
We saw that the seekers after its origin divided into two camps. One group believed that its glow was some sort of meteorological phenomenon in the lower spheres of water, air and fire. The solid, upper spheres, which were crystalline in nature, contained the moon, sun and planets.
To both groups, the universe was bounded by a fixed sphere in which the stars were embedded. The second group believed that the Milky Way was a part of that starry, outer sphere.
And then there was Democritus, none of whose writings have survived, but who might be called the father of modern science. What we know of him we discover 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 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.
The solid spheres had to be broken to come to a true understanding of the Milky Way. Around 500 CE, Greek astronomer Ptolemy broke at least the inner, planetary spheres. In order to keep Earth at the center of things and to explain the resultant unusual orbits of the planets around earth, Ptolemy had to create a set of orbits that can only be described as wheels within wheels. The planets would thus have to wobble around earth and not orbit in a perfect, single circle. Therefore, the planets could not be in crystalline spheres. However, Ptolemy left intact the outer, crystalline sphere of stars.
Below the moon, the lower spheres remained regions of air, fire and water. Around 1200 CE, Roger Bacon, for example, suggested that the Milky Way might be an optical phenomenon. It was, he said, the reflection of the light from stars and sun in the outermost layer of fire below the moon.
The unresolved-star theory still had its proponents during the medieval period and beyond, especially among Persian and Arabic astronomers. As early as 1000 CE, the Persian astronomer Abu Rayhan al-Biruni suggested that the Milky Way was “a collection of countless fragments of the nature of nebulous stars.” Three hundred years later, Arabic astronomer Ibn Qayyim Al-Jawaziyya described it as “a myriad of tiny stars packed together in the sphere of the fixed stars.” The inability to see it as individual stars was attributed to their closeness to each other or turbulence in earth’s airy sphere.
Copernicus’s revolutionary notion that Earth revolved around a central sun did little to resolve the Milky Way mystery. The starry sphere still remained, even if some believed that the stars were embedded in it.
The key was to get some depth into the starry realm, and the first to do so was Giordano Bruno around 1600 CE. He believed with religious fervor that the universe consisted of an infinitude of stars extended in all directions into space. Why then was the sky not filled with stars? Their great distances, he said, would make most stars invisible. The spacing of them in the sky he attributed to their varying distances from Earth.
Bruno never applied his infinite universe of stars to the Milky Way. That omission is not surprising. Bruno’s infinity of stars was the same in all directions. He rejected the notion that the stars might be denser in one direction than another. That idea, as we shall see, is the key to unlocking the mystery of the Milky Way.
One of Copernicus’s proponents got it partly right in a world-changing way. In 1610, he published his groundbreaking work, The Starry Messenger. In a scant 12 lines, he finally pushed into place a key piece of the Milky Way puzzle.
Galileo had heard of a marvelous invention by spectacle makers in Holland. The telescope was at the time mostly a toy, but Galileo used it to discover the phases of Venus, the craters on the moon, and the four brightest satellites of Jupiter. Almost in passing, he managed to solve the Milky Way problem that had “vexed philosophers through so many ages.” He did so, he wrote, with the “ocular certainty” of his telescope. It was “nothing but a congeries of innumerable stars.” Democritus was right all along.
Still, Galileo was wrong about the extent and structure of the Milky Way. He did not see any reason to reject the unchanging crystalline sphere in which the stars of the Milky Way dwelled and that represented the outer limits of the cosmos.
In part, Galileo’s failure of vision was simply a failure of imagination. His primary concern was proving Copernicus’s sun-centered model and disproving the old Earth-centered point of view. Thus, discoveries like the moons of Jupiter were important because they “broke” the spheres once and for all. Jupiter could not be plastered on a sphere if it had moons orbiting around us.
Galileo gave us a three-dimensional cosmos, and that eventually helped. However, the universe was still bounded by that pesky outside sphere where the stars resided.
However, Galileo’s failure was also a failure of technology. Thanks to Johannes Kepler, astronomers had a way of measuring the distance to the planets but no good way of measuring the distance to the stars. For that, another set of technologies had to develop.
The common assumption is that people are smarter now than they were back in ancient times, but that assumption is far from the case. Some ancient thinkers were actually quite clever. They were skilled in using the data they had available to draw the correct conclusions.
For example, Aristotle used the curve of Earth’s shadow during lunar eclipses to correctly conclude that Earth was a sphere, and he did it almost 2,500 years ago.
What Aristotle lacked was information, and new information often involves the development of radically new technologies.
Thus, for new revelations to develop, we often need new technologies and a few minds clever enough to exploit them. Such confluences of tools and intelligence are difficult to come by, but their frequency has accelerated markedly as human civilization developed.
As a result, for a hint of new revelations about the Milky Way, we must wait a scant 150 years and the mind of one of the greatest thinkers who ever walked the planet. More on that next week.
Tom Burns is the former director of the Perkins Observatory in Delaware.