Thanks all, after discussion with the sprog, I found that he had been looking at a picture of the world from space. When the whole disc was visible, he worked out that he couldn’t see the whole surface of the hemisphere due to the angles. So the surface disappears round the corner, as he put it, long before the edge. That is if looking down on the pole, we couldn’t see the surface at the equator. So his question is, how far from earth does an astronaut stop seeng more of the earth.