Airplane approaching and receding
Airplanes is another proof for a straight surface and a curved light. According to the round Earth model, airplanes are constantly arching around a spherical Earth, yet we observe them shooting across the sky in a straight line. An approaching airplane appears to be ascending in a straight line, all the way to the point when it is closest; from there, it appears to be descending in a straight line. Even if the Earth were a sphere, the relatively smooth curvature would not account for the drastic ascents and descents which we observe in the sky. Reminder - in the case of a spherical Earth, the size of the sphere has to be only be a few hundred kilometers in diameter to make the observed steep ascents and descent possible. In this example we will examine approaching and receding airplanes.
The following is an illustration of an airplane at an average altitude of ten kilometers and an observer standing on the ground. The airplane has a much bigger circle of sight due to its altitude. At this point, the light body of the airplane has not yet entered the circle of sight of the observer and is therefore below his horizon line.
The light body of the airplane enters the circle of sight of the observer and the airplane appears on the horizon. The airplane appears on the horizon and begins its apparent vertical ascent.
The airplane now appears to be ascending in a steep, straight line.
The airplane is at its closest point, right above the observer. From there, it commences its apparent descent towards the horizon line.
The airplane is receding in an apparent vertical ascent.
As the light body of the airplane withdraws from the circle of sight of the observer, the airplane eventually disappears below his horizon line.
...as seen by the observer
Commercial airlines have an average altitude of 10 kilometers. The circle of sight at those altitudes is somewhere between 100 and 200 kilometers in diameter. The reason why airplanes appear to be descending and ascending in a steep, straight line is because it's the moving light body of the airplane which determines the apparent motion of the airplane. As the light body of the airplane enters the circle of sight of observer, the airplane enters the field of view and remains visible as long as it takes for the light body to pass by. The airplane appears the moment its light body intersects the light body of the observer, and disappears the moment their light bodies disconnect. Considering a light body with the circumference of 100-200 kilometers, the airplane remains within the range of vision for about 10-15 minutes - the same amount of time that it takes for the airplane to cover the distance corresponding to the diameter of its circle of sight.