Founded upon a sexagesimal numeration, the metrical system of Babylon and Nineveh was "the most scientific of all those known and practised by the ancients: until the elaboration of the French metrical system, it was the only one whose every part was scientifically co-ordinated, and of which the fundamental conception was the natural development of all measures of superficies, of capacity, or of weight, from one single unit of length, a conception which was adopted as a starting point by the French commission of weights and measures." The cubit of 525 millimetres was the base of the whole system.[97] We shall not here attempt to explain how the other measures--itinerary, agrarian, of capacity, of weight--were derived from the cubit; to call attention to the traces left in our nomenclature by the duodecimal or sexagesimal system of the Babylonians, even after the complete triumph of the decimal system, is sufficient for our purposes. It is used for instance in the division of the circle into degrees, minutes, and seconds, in the division of the year into months, and of the day into hours and their fractions. This convenient, exact, and highly developed system of arithmetic and metrology enabled the Chaldaeans to make good use of their observations, and to extract from them a connected astronomical doctrine. They began by registering the phenomena. They laid out a map of the heavens and recognized the difference between fixed stars and those movable bodies the Greeks called planets--among the latter they naturally included the sun and the moon, the most conspicuous of them all both in size and motion, whose courses were the first to be studied and described. The apparent march of the sun through the crowded ranks of the celestial army was defined, and its successive stages marked by those twelve constellations which are still called the _Signs of the Zodiac_. In time even these observations were excelled, and it now appears certain that the Chaldaeans recognized the annual displacement of the equinoctial point upon the ecliptic, a discovery that is generally attributed to the Greek astronomers. But, like Hipparchus, they made faults of calculation in consequence of the defects of their instruments.[98] It was the same with the moon. They succeeded in determining its mean daily movements, and when they had established a period of two hundred and twenty-three lunations, they contrived to foretell its eclipses. Eclipses of the sun presented greater difficulties, and the Chaldaeans were content with noting their occurrence. They were acquainted with the solar year of three hundred and sixty-five days and a quarter; they used it in their astronomical calculations; but their religious and civil year was one composed of twelve lunar months, alternately full and short, that is, of twenty-nine and thirty days respectively. The lunar and solar years were brought into agreement by an intercalary cycle of eight years.[99]