Recall from last time that the ancient Greeks organized the cosmos into a set of nested spheres centered on Earth.

Above the Earth was the changeable region between the Earth and moon — spheres of the air and meta-aer, “beyond the air.”

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.

Astronomers had to “break” the solid spheres to understand the Milky Way properly. Around 150 CE, Greek astronomer Ptolemy broke at least the inner planetary spheres.

To keep Earth at the center of things and to explain the resultant unusual orbits of the planets around Earth, he had to create a set of orbits that might be described as wheels within wheels. You can see a rendition of Ptolemy’s “epicycles” here:

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 of Democritus maintained a few 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.”

They attributed their inability to see it as individual stars 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, with the stars embedded in it, still remained.

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 that the universe consisted of an infinitude of stars extending in all directions into space.

Why was the sky not filled with stars? Their great distances, he said, would make most stars invisible. He attributed their spacing in the sky 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 number of stars might vary in different directions. As we shall see, that idea 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 a vital piece of the Milky Way puzzle into place.

Galileo had heard of a marvelous invention by spectacle makers in Holland. He used a self-designed telescope to discover the phases of Venus, the craters on the moon, and the four brightest satellites of Jupiter.

Almost in passing, he solved 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.”

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.

Galileo gave us a three-dimensional cosmos, and that eventually helped. However, that pesky outside sphere where the stars resided still bounded the universe.

However, Galileo’s failure was mainly a failure of methodology. 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.

More on that next week.

Tom Burns is the former director of the Perkins Observatory in Delaware.