The shape of the other planets appear to be a flat disc to me through a telescope or otherwise.
A hose pipe appears to be flat to me when I look at it far away enough...
See, now that's just ignorance at its most blissful.
Well, you can't tell that something is round until you get near enough to it.
I see a hose, I look at it, and it looks flat. Therefore, I conclude that it's flat, just like how we conclude that other planets are flat. Now, when someone brings the hose over and shows it to me, I realise my error and correct myself; it's not flat. But the problem is, you RE'rs have yet to bring us over and show us conclusive evidence we can verify about how the planets are round.