of the famous Harun-al-Rashid. Both father
and son were famous for their interest in science. Harun-al-Rashid was,
it will be recalled, the friend of Charlemagne. It is said that he sent
that ruler, as a token of friendship, a marvellous clock which let fall
a metal ball to mark the hours. This mechanism, which is alleged to
have excited great wonder in the West, furnishes yet another instance of
Arabian practicality.

Perhaps the greatest of the Arabian astronomers was Mohammed ben Jabir
Albategnius, or El-batani, who was born at Batan, in Mesopotamia, about
the year 850 A.D., and died in 929. Albategnius was a student of the
Ptolemaic astronomy, but he was also a practical observer. He made the
important discovery 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 pla