The original astronomic concepts were that planets, stars, and the sun were small, close light sources. Being in heaven, they were perfect (aside from the moon, which was smudged due to closeness to this imperfect sphere). They were embedded in clear solid domes at varying distances. But in general, the idea that the heavens were made for us on Earth to look at persisted a long time.
The overturning of these ideas, showing that some planets revolved around the sun or that moons existed around Jupiter upset this apple cart when the first telescopes were turned upward. So the crystal spheres had to go.
But still, you could see these objects as smooth lights or globes to reflect light, thus 'perfect'. The discovery of spots on the sun hurt this idea of perfection, as did the discovery of mountains, plains, and geographic features like earth on the moon. So a mountain on the moon means the moon is another world like earth. Another blow for the idea that there is an essential division between Earth and "The Heavens".
From Wiki, Planetary Science:
In more modern times, planetary science began in astronomy, from
studies of the unresolved planets. In this sense, the original
planetary astronomer would be Galileo, who discovered the four largest
moons of Jupiter, the mountains on the Moon, and first observed the
rings of Saturn, all objects of intense later study. Galileo's study
of the lunar mountains in 1609 also began the study of
extraterrestrial landscapes: his observation "that the Moon certainly
does not possess a smooth and polished surface" suggested that it and
other worlds might appear "just like the face of the Earth itself".
Advances in telescope construction and instrumental resolution
gradually allowed increased identification of the atmospheric and
surface details of the planets. The Moon was initially the most
heavily studied, as it always exhibited details on its surface, due to
its proximity to the Earth, and the technological improvements
gradually produced more detailed lunar geological knowledge. In this
scientific process, the main instruments were astronomical optical
telescopes (and later radio telescopes) and finally robotic
See Also: Celestial Spheres