What fundamental measurement did the astrolabe primarily utilize the alidade to determine relative to the observer?

Altitude

The Alidade is specifically identified as the rotating sighting rule mounted on the back of the instrument. Its function is to allow the observer to align vanes or pinholes directly toward a celestial object, such as a star or a planet. Once aligned, the instrument allows the user to read the angle corresponding to that object's height above the horizon directly from the degree markings inscribed around the rim of the Mater. This angle is defined as the object's altitude. Determining accurate altitude measurements was critical because, especially for mariners, this data was the necessary input for calculating latitude, which was vital for navigation and accurate mapping.