"The first thing Galileo discovered was that the moon was not smooth and homogeneous, as everyone believed. Instead, it was covered with craters and mountains whose peaks became awash with light when the “terminator” — the line that separates the illuminated and dark parts of the moon — inched forward through the night. Art historians Samuel Edgerton and Horst Bredekamp have written insightfully about how his skills as a draftsman were key to this discovery. Young artists in training during this period were drilled on treatises designed to, in effect, reshape their perception, so that they unthinkingly interpreted certain configurations of two-dimensional light and dark shapes as the surfaces of three-dimensional figures hit by a light source. Galileo’s draftsman eye thus gave him a crucial advantage over other observers, such as Englishman Thomas Harriot, who, a couple of months earlier, had carried out the first recorded telescopic observation of the moon. To Harriot the moon remained smooth and the terminator a fairly clean line. He only saw mountains and craters after he learned of Galileo’s novel description.

The implications of Galileo’s discovery were mindboggling. Aristotelian physics had been based on a fundamental distinction between Earth and the Heavens. Everything on Earth was subject to processes of corruption and change. The Heavens were incorruptible, made of perfectly smooth material, and moved only along circular paths. A pockmarked moon made no sense.

Galileo’s telescope was about to deliver even more shocking news. In the clear sky of January 1610, he pointed it toward Jupiter, and noticed three small stars peculiarly aligned next to it. He recorded their position on a now-famous piece of paper. The following night, he could scarcely believe his eyes: they had moved. And now there were four. A few nights later, Galileo realized that they were not stars but planets orbiting Jupiter as it moved westward against the backdrop of the fixed stars. For the first time ever, someone had observed a celestial body that orbited around something that was not Earth. This was a formidable blow to both the Ptolemaic system and Aristotelian physics, which did not allow for multiple centers of gravity. Galileo’s discoveries spelled the end of conceptions of Earth, and hence of man, as the center of everything."

https://lareviewofbooks.org/article/faking-galileo/