Galileo pointed his telescope to the sky in 1609 and applied his experimental technique and careful attention to details to what he saw. Galileo found that many stars too faint to be seen with the naked eye became visible with his telescope. He found that the Milky Way was actually composed of millions of stars. Galileo's observations revealed much about the "heavenly bodies" which did not seem consistent with Church teachings. Particularly, he found that the Moon's surface was "uneven, rough, and full of cavities and prominences, being not unlike the face of the Earth". Galileo guessed that the prominent flat dark areas might be water (the dark maria, or "seas", on the Moon were thought to be water until long after Galileo's time).

These discoveries showed that the heavenly bodies previously regarded as perfect, smooth, and incorruptible, did indeed have irregularities. Further, that fact that the Moon was "not unlike the Earth" suggested that the Earth, too, might belong to the realm of celestial bodies.