The human eyes and brain can easily and quickly detect very small changes in angle.
Parallax is a difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle between the sight lines.
Parallax, and our ability to sense it, provides our brains with the information we need to control the descent of a glider.
|Flight Training Manual for Gliders – Holtz|
|Lesson||5.3||Glide Slope Control Using the Airbrakes|