it, inferred incorrectly that the sun had receded in the mean
time. The modern explanation of this observation is that the
measurement of Albategnius was somewhat in error, since we know
that the sun's motion is steadily progressive. Arzachel, however,
accepting the measurement of his predecessor, drew the false
inference of an oscillatory motion of the stars, the idea of the
motion of the solar system not being permissible. This assumed
phenomenon, which really has no existence in point of fact, was
named the "trepidation of the fixed stars," and was for centuries
accepted as an actual phenomenon. Arzachel explained this
supposed phenomenon by assuming that the equinoctial points, or
the points of intersection of the equator and the ecliptic,
revolve in circles of eight degrees' radius. The first points of
Aries and Libra were supposed to describe the circumference of
these circles in about eight hundred years. 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