A weather balloon ascending
The following illustration depicts a weather balloon on the ground, the objects A,B,C in the surrounding area and a nearby pylon. In the picture above the weather balloon does not see the pylon and the three nearby objects as their light bodies are not intersecting.
The weather balloon has been launched. As it gains altitude, its circle of sight expands. It now sees the object A, and the upper parts of the pylon. The objects B and C remain below his horizon line.
As the weather balloon continues its ascent, its circle of sight expands and covers the objects A, B, and the major part of the pylon, while the bottom of the pylon still remains below his horizon line as it is not covered by the circle of sight of the balloon.
The weather balloon has reached its highest altitude. It now has a full view on the entire pylon, on the objects A, B, while the object C remains below his horizon line.