For two thousand years, the pale band of light arching across the night sky was explained as a celestial river or an atmospheric glow. It was only when Galileo turned his telescope toward the heavens that this continuous stream dissolved into a multitude of individual stars. Yet identifying what the Milky Way was made of did not explain why it formed a band, where we were located within it, or how it was shaped.

This episode traces the slow accumulation of better instruments and reasoning that revealed our galaxy's true structure. We explore how Thomas Wright and Immanuel Kant conceptualized a flat disk of stars, how William Herschel attempted to map it by counting stars, and how Harlow Shapley used globular clusters to prove the Sun occupies the galactic suburbs. We also examine the twentieth-century discovery that the outer edges of the g ...