Anybody with a $200 telescope can look at the surface of the Moon in the light-to-shadow transition zone and observe how the sunlight interacts with the Moon's mountains and craters.

If the Moon was further away than Sun, it's side visible from Earth would always be fully (or almost fully) illuminated by the Sun, i.e the Moon would always be full. You can make a very simple observation using a lightbulb (Sun) and a ball (Moon). This exercise should be mandatory for all Iskcon devotees.

The object of this discussion is not to destroy anybody's faith in their guru, but to put that faith into a proper context.

Srila Prabhupada was a perfect guru because he gave his disciples Krsna, and because he was fully surrendered to Krsna's service - not because everything he said was infallible. He was infallible in his devotion to Krsna.