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These different observations were afterwards confirmed, and other astronomers profited by them. Ptolemy in the second century, and the Arabian Aboul Wefa in the tenth, completed the remarks of Hipparchus on the inequalities that the moon undergoes whilst following the undulating line of its orbit under the action of the sun.
Averroes, in quality of commentator on Aristotle, became his rival in the minds of the mediaeval schoolmen; Avicenna, in quality of commentator on Hippocrates and Galen, was for centuries the text-book of all European physicians; while Albatani and Aboul Wefa, as astronomers, commented on Ptolemy, not however without making a few important additions to his knowledge; for Aboul Wefa discovered a third inequality of the moon's motion, in addition to the two mentioned by Ptolemy, which he did, according to Professor Whewell, in a truly philosophic manner an apparently solitary instance, and one which, in its own day, had no effect; for the fact was forgotten, and rediscovered centuries after by Tycho Brahe.
In Alexandria the memory of the illustrious old times was to be recalled by the discovery of the motion of the sun's apogee by Albategnius, and the third inequality of the moon, the variation, by Aboul Wefa; to be discovered six centuries later in Europe by Tycho Brahe.
The water of the Nile, when at its lowest, covers 7 cubits of the Nilometer, and when it reaches a height of 15-2/3 cubits the Sheikh of the Nile proclaims the Wefa, i.e., that the height of the water necessary for irrigating every part of the Nile valley has been attained. The signal is then given for the cutting of the embankment.
In the middle of the ninth century Albategnius, a Syrian prince, improved the value of excentricity of the sun's orbit, observed the motion of the moon's apse, and thought he detected a smaller progression of the sun's apse. His tables were much more accurate than Ptolemy's. Abul Wefa, in the tenth century, seems to have discovered the moon's "variation."
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