I can't tell you off the top of my head just how far it'd have to be to make a visible difference.

On the topic of Newton, though, it was a theory in his time, but after years upon years of his math checking out, one can start taking it as fact. Gravity has been proven to affect both light, time and even space, which we previously thought were immutable constants.

Also, a scientific 'theory' and a hypothesis are often confused. Theories are (as far as I'm aware) accepted as fact, whereas hypotheses are not.