NavList:

Hi Tony,

   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...)