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The Galilean moons are actually bright enough to be visible to the unaided eye if it weren't for the glare of Jupiter.
Now presume a very clever observer invented a device to get rid of that glare.
Frex a tube with a tiny occultation disk at the end kept in place with hairs.
The relevance is that prior to 1610 all celestial objects were thought to revolve around the Earth.
All the existing cosmologies were based on this notion.