All of which illustrates how a difficult and false explanation may take the place of a simple and correct one.

The observations of later generations have shown conclusively that the sun's shift of position is regularly progressive, hence that there is no "trepidation" of the stars and no revolution of the equinoctial points.
If the Arabs were wrong as regards this supposed motion of the fixed stars, they made at least one correct observation as to the inequality of motion of the moon.

Two inequalities of the motion of this body were already known.

A third, called the moon's variation, was discovered by an Arabian astronomer who lived at Cairo and observed at Bagdad in 975, and who bore the formidable name of Mohammed Aboul Wefaal-Bouzdjani.
The inequality of motion in question, in virtue of which the moon moves quickest when she is at new or full, and slowest at the first and third quarter, was rediscovered by Tycho Brahe six centuries later; a fact which in itself evidences the neglect of the Arabian astronomer's discovery by his immediate successors.
In the ninth and tenth centuries the Arabian city of Cordova, in Spain, was another important centre of scientific influence.

There was a library of several hundred thousand volumes here, and a college where mathematics and astronomy were taught.