The number of astronomical facts with which the Chaldaeans had made themselves acquainted was considerable.

[Illustration:  340.jpg CHALDAEAN MAP OF THE WORLD.]

     Drawn by Faucher-Gudin, from a sketch by Peiser.

It was a question in ancient times whether they or the Egyptians had been the first to carry their investigations into the infinite depths of celestial space:  when it came to be a question as to which of the two peoples had made the greater progress in this branch of knowledge, all hesitation vanished, and the pre-eminence was accorded by the ancients to the priests of Babylon rather than to those of Heliopolis and Memphis.*

* Clement of Alexandria, Lucien, Diogenes Laertius, Macrobius, attribute the origin of astronomy to the Egyptians, and Diodorus Sioulus asserts that they were the teachers of the Babylonians; Josephus maintains, on the contrary, that the Egyptians were the pupils of the Chaldaeans.

[Illustration:  340.jpg ASTRONOMICAL TABLE]

The Chaldaeans had conducted astronomical observations from remote antiquity.* Callisthenes collected and sent to his uncle Aristotle a number of these observations, of which the oldest had been made nineteen hundred and three years before his time—­that is, about the middle of the twenty-third century before our era:  he could have transcribed many of a still earlier date if the archives of Babylon had been fully accessible to him.

* Epigenes asserts that their observations extended back to 720,000 years before the time of Alexander, while Berossus and Critodemus limit their antiquity to 490,000 years, which was further reduced to 473,000 years by Diodorus, to 470,000 by Cicero, and to 270,000 by Hipparchus.

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.