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It's a circle. It can't be an ellipse.DaveC426913 said:This arc would be of a portion circle/ellipse that would subtend a measurable amount of his view (at that specific distance from the window).
You can't see a defined radius, you can only measure a radius. A ball held at a certain distance will appear to have the same radius as the Moon. You need to calculate the distance before you can say how big something is.DaveC426913 said:I doubt the radius of that curve would be the same as the actual horizon if he were looking straight down on the Earth, because, looking out the porthole like this, the rest of the horizon wouldn't be below his feet, it would be literally behind him, almost viewable out the opposite porthole.
It's no longer points on a horizon; It's a curve on a plane perpendicular to the observer. That curve is what the observer sees, absent any 3-dimensional/foreshortening clues.
Maybe I've got it wrong. If the OP can't seem to nail down a description of what he's doing, I'm not going to do much better.