The horizon isn't a flat line, it's a circle. That circle is the edge of a very, very small spherical cap. It should look flat, because even at 30,000 feet it is still just a small spherical cap relative to the scale of the planet.
The horizon isn't a flat line, it's a circle. That circle is the edge of a very, very small spherical cap. It should look flat, because even at 30,000 feet it is still just a small spherical cap relative to the scale of the planet.