A History of Science, vol 2 | Page 8

of the motion of the solar
apogee. That is to say, he found that the position of the sun among the
stars, at the time of its greatest distance from the earth, was not what it
had been in the time of Ptolemy. The Greek astronomer placed the sun
in longitude 65 degrees, but Albategnius found it in longitude 82

degrees, a distance too great to be accounted for by inaccuracy of
measurement. The modern inference from this observation is that the
solar system is moving through space; but of course this inference
could not well be drawn while the earth was regarded as the fixed
centre of the universe.
In the eleventh century another Arabian discoverer, Arzachel,
observing the sun to be less advanced than Albategnius had found 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 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. Granada, Toledo, and
Salamanca were also important centres, to which students flocked from
western Europe. It was the proximity of these Arabian centres that
stimulated the scientific interests of Alfonso X. of Castile, at whose
instance the celebrated Alfonsine tables were constructed. A familiar
story records that Alfonso, pondering the complications of the
Ptolemaic cycles and epicycles, was led to remark that, had he been
consulted at the time of creation, he could have suggested a much better
and simpler plan for the universe. Some centuries were to elapse before
Copernicus was to show that it was not the plan of the universe, but
man's interpretation of it, that was at fault.
Another royal personage who came under Arabian influence was
Frederick II. of Sicily--the "Wonder of the World," as he was called by
his contemporaries. The Almagest of Ptolemy was translated into Latin
at his instance, being introduced to the Western world through this
curious channel. At this time it became quite usual for the Italian and
Spanish scholars to understand Arabic although they were totally
ignorant of Greek.
In the field of physical science one of the most important of the
Arabian scientists was Alhazen. His work, published about the year
1100 A.D., had great celebrity throughout the mediaeval period. The
original investigations of Alhazen had to do largely with optics. He
made particular studies of the eye itself, and the names given by him to
various parts of the eye, as the vitreous humor, the cornea, and the
retina, are still retained by anatomists. It is known that Ptolemy had
studied the refraction of light, and that he, in common with his
immediate predecessors, was aware that atmospheric refraction affects
the apparent position of stars near the horizon. Alhazen carried forward
these studies, and was led through them to make the first recorded