The Chaldaean priests had been accustomed from an early date to record on their clay tablets the aspect of the heavens and the changes which took place in them night after night, the appearance of the constellations, their comparative brilliancy, the precise moments of their rising and setting and culmination, together with the more or less rapid movements of the planets, and their motions towards or from one another. To their unaided eyes, sharpened by practice and favoured by the transparency of the air, many stars were visible, as to the Egyptians, which we can perceive only by the aid of the telescope. These thousands of brilliant bodies, scattered apparently at random over the face of the sky, moved, however, with perfect regularity, and the period between their departure from and their return to the same point in the heavens was determined at an early date: their position could be predicted at any hour, their course in the firmament being traced so accurately that its various stages were marked out and indicated beforehand. The moon, they discovered, had to complete two hundred and twenty-three revolutions of twenty-nine days and a half each, before it returned to the point from which it had set out. This period of its career being accomplished, it began a second of equal length, then a third, and so on, in an infinite series, during which it traversed the same celestial houses and repeated in them the same acts of its life: all the eclipses which it had undergone in one period would again afflict it in another, and would be manifest in the same places of the earth in the same order of time.* Whether they ascribed these eclipses to some mechanical cause, or regarded them as so many unfortunate attacks made upon Sin by the seven, they recognized their periodical character, and they were acquainted with the system of the two hundred and twenty-three lunations by which their occurrence and duration could be predicted. Further observations encouraged the astronomers to endeavour to do for the sun what they had so successfully accomplished in regard to the moon. * This period of two hundred and twenty-three lunations is that described by Ptolemy in the fourth book of his “Astronomy,” in which he deals with the average motion of the moon. The Chaldaeans seem not to have been able to make a skilful use of it, for their books indicate the occurrence of lunar eclipses outside the predicted periods.