Notwithstanding all that is said to the contrary, the same opinion must be formed of the astronomical knowledge of the Chaldeans. It is natural enough to think, that a people who inhabited vast plains, under a sky perpetually serene, must have been led to observe the course of the stars, even at a period when they still led a wandering life, and when the stars alone could direct their courses during the night; but since what period were they astronomers, and to what perfection did they carry the science? Here rests the question. It is generally allowed that Callisthenes sent to Aristotle observations made by them, and which referred to a period of 2200 years before Christ; but this fact is related only by Simplicius[201], as stated upon the authority of Porphyry, and 600 years after Aristotle. Aristotle himself says nothing on the subject, nor has any creditable astronomer spoken of it. Ptolemy mentions and makes use of ten observations of eclipses really made by the Chaldeans; but they do not refer to an earlier period than that of Nabonassar (721 years before Christ); they are inaccurate also; the time is expressed only in hours and half-hours, and the shadow only in halves or fourths of the diameter. Notwithstanding, as they had fixed dates, the Chaldeans must have had some knowledge of the true length of the year, and some means of measuring time. They appear to have known the period of eighteen years, which brings back the eclipses of the moon in the same order; a piece of knowledge which the mere inspection of their registers would promptly afford them; but it is certain that they could neither explain nor predict eclipses of the sun. It is from not having sufficiently understood a passage of Josephus, that Cassini, and after him Bailly, have imagined that they discovered in it a luni-solar period of 600 years, which had been known from the time of the first patriarchs[202]. Thus every thing leads us to believe that the great reputation of the Chaldeans was given them at a more recent period, by their unworthy successors, who, under the same name, sold their horoscopes and predictions throughout the whole Roman empire, and who, in order to procure themselves more credit, attributed to their rude ancestors the honour of the discoveries of the Greeks. With regard to the Indians, every body knows that Bailly, believing that the epoch which is used as a period of departure in some of their astronomical tables had been actually observed, has attempted to draw from thence a proof of the great antiquity of the science among this people, or at least among the nation which had bequeathed them its knowledge. But the whole of this system, invented with so much labour, falls to the ground of itself, now that it is proved that this epoch has been adopted but of late, from calculations made backwards, and even false in their results.[203]