that the earth is not a perfect sphere--a suggestion the validity
of which was not to be put to the test of conclusive measurements
until about the close of the eighteenth century. The Arab
measurement was made in the time of Caliph Abdallah al-Mamun, the
son 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