1. Explain how the same object can have different weights in different places.
Variations in atmospheric pressure upon the object.
2. Considering the fact that the planets have been known to exist before space travel was even around I would assume that they are visible with a telescope. Also, you can check this yourself.
Go and look through a telescope. Pick out somewhere on land or out at sea and tell me how far you can see before the furthest object becomes indescribable.
Ok, you're looking through a thicker atmosphere, but you'll find that you can't see much after tens of miles ,clearly.
Equate that to the sky and add a larger helping of sight and you'll realise that telescopes aren't what you think they are, in terms of this distant planet carry on. Thy're fine for viewing big things in the atmosphere, like, say reflections off of a dome.
Saying planets existed before space travel is pointless because space travel is a figment of over active imaginations and skulduggery.