I wouldn't call realising there has to be a curve to explain why you can see further in a straight line on earth if looking straight up you can see far further too smart for a kid whose spent most of their life living in an environment best suited to figuring it out, though hey people today with even better tools make that mistake.
Also into 1000CE and someone of a that culture so willing to go to sea thinking its flat, while both Nordic and Abrahamic religions texts (as we have the for the former and for basically all but the most edited too look better for the latter) describe Earth as a circle or some other flat shape they really should know its a Sphere since that's the kind of textual literalism that's gonna be dropped fast and is a few hundred years after it was basically universal for anyone religious who would teach them about the shape of the earth would know its some kind of Sphere, so then they would just need to be a fool who can't do the basic observation or some dick lied to him.
Also Bug Eyes lay off the weed.