To remain at fixed altitude with respect to Star B, the observer is always located on a circle of equal altitude whose center on earth is the Geographical Position, directly below Star B at its zenith.
A ship's course would not follow the periphery of a single circle of equal altitude as modern calculations do; rather, it would follow the route dictated by the destinations of its cargo.
So long as the ship sailed within a latitude range whose width never exceeded the diameter selected for the circle of equal altitude of the fettered star, as shown in figure 4, the navigator could use the al-qaid method.
This results from measuring altitudes in the alqaid case from star to periphery of the circle of equal altitude which continuously bends away from or toward the unfettered star, thus giving greater incremental extensions of horizontal distance per 1 |degree~ of latitude change compared to the abdal case, where measurements are made from star to meridian.
The fettered altitude, 10 |degrees~, is low, so the diameter of the circle of equal altitude is very large.
For each successive position of observer, it is necessary to determine a new longitude position of the observer as he moves 1 |degree~ latitude on the circle of equal altitude.
10 Al-qaid, the second method employed by medieval Arab navigators in determining latitude, quite ingeniously anticipates the modern circle of equal altitude used in conjunction with the navigational triangle to duplicate the medieval empirical process.
11 To visualize a circle of equal altitude, picture a maypole with children encircling the pole, each holding a streamer attached to the top of the pole.