Galileo Galilei published his first observations of the night sky in his first book, Starry Messenger. I use Galileo in Chapter 21 of 30 Philosophers to illustrate how people thought at the end of the Middle Ages and how we arrived at our modern cosmological model. In many ways, his story is the story of observation—the struggle between wanting to believe something and reality. It is hard to ignore these starry messages when they come directly from the universe and can be seen by anyone. Four of his observations with his newly built telescope highlight what people believed at the time. He observed the Moon, Venus, the Sun, and Jupiter.

Among his most pivotal were the Moon’s craters and mountains—a transformative moment in our understanding of the cosmos. The celestial realm, including the Moon, planets, and stars, was imagined as the epitome of perfection—a stark contrast to our own rugged, flawed Earth. The realization that the Moon had mountains shattered this long-held belief.

While ancient astronomers had theorized that Venus had phases just like our Moon, Galileo’s observations provided direct evidence. At the time, many believed the phases were caused by intrinsic changes within. Galileo’s discovery showed instead that Venus orbits the Sun—a direct challenge to Church doctrine.

The Sun, the largest object in the sky and the reason we have so many Sun gods, was believed to be flawless too. So when Galileo observed sunspots, it was a shock. The idea that the Sun itself was imperfect contradicted deeply ingrained beliefs.

Perhaps most significant, Galileo observed four moons orbiting Jupiter. This revolutionary sighting meant that not everything revolved around the Earth. Earth’s central position in the universe was no longer a certainty.