Here's a breakdown:

* Angular Distance: This refers to the angle between two points, as measured from a specific location (in this case, the observer on Earth).

* Horizon: The line where the sky appears to meet the Earth.

* Altitude: The angle measured vertically upwards from the horizon to the object in the sky.

So, if you see a plane high in the sky, its altitude is the angle between the horizon and the plane, as seen from your location.