Two abdal stars have equal altitudes and equal declinations when located symmetrically about the observer's meridian.
The stars for which they do give information have widely differing declinations; and at equal altitudes they are far from being equidistant from the observer's meridian.
It was also more convenient, as mentioned earlier, to use star pairs as equal altitudes initially with either equal or unequal declinations as in abdal measurements.
The pair Deneb and Capella is almost exact abdal, having nearly equal altitudes and equal declinations at all latitudes when symmetrical about the observer's meridian.
What difference does it make whether the stars have equal altitudes or not at the initial position?
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.
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.
It consists of two stars at equal altitude, one on either side of the observer's meridian, and with the Geographical Positions either on the same parallel of latitude, in which case they are symmetrical about the observer's meridian, or on different parallels, in which case they are unsymmetrical.