A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Position-Finding
From: Bill Gray
Date: 2018 May 13, 19:06 -0700
Yes, you could have seen Ganymede. You could, I think, spot all four moons on a good night. Galileo's first scope was roughly the size you're talking about (I think maybe even smaller), and not of as good quality.
I vaguely remember reading an article by an amateur astronomer running a public viewing night. A kid looked through the scope at Jupiter, looked up at Jupiter in the sky, and asked why the moons were backward in the scope. The guy running the scope stuttered a bit, and tried to explain that telescopes involve mirrors and can reverse images. The kid left convinced that the telescope was a fake.
One's night vision definitely deteriorates with age, and I could believe that a young kid might conceivably be able to spot the moons with an unaided Mk-1 eyeball. (I asked my daughter if she could see them a few years back, when she was eight. She couldn't. I should try again...)