The significance of the crescent being shown as lying on its back is seen at once when it is remembered that the new moon is differently inclined to the horizon according to the time of the year when it is seen. It is most nearly upright at the time of the autumn equinox; it is most nearly horizontal, "lying on its back," at the spring equinox. It is clear from this symbol, therefore, that the Babylonians began their year in the spring. [Illustration: POSITION OF THE NEW MOON AT THE EQUINOXES.] This method, by which the new moon was used as a kind of pointer for determining the return of the sun to the neighbourhood of a particular star at the end of a solar year, is quite unlike anything that commentators on the astronomical methods of the ancients have supposed them to have used. But we know from the ancient inscription already quoted that it was actually used; it was eminently simple; it was bound to have suggested itself wherever a luni-solar year, starting from the observed new moon, was used. Further, it required no instruments or star-maps; it did not even require a knowledge of the constellations; only of one or two conspicuous stars. Though rough, it was perfectly efficient, and would give the mean length of the year with all the accuracy that was then required. [Illustration: BOUNDARY-STONE IN THE LOUVRE; APPROXIMATE DATE, B.C. 1200. (From a photograph by Messrs. W. A. Mansell.)] But it had one drawback, which the ancients could not have been expected to foresee. The effect of "precession," alluded to in the chapter on "The Origin of the Constellations," p. 158, would be to throw the beginning of the year, as thus determined, gradually later and later in the seasons,--roughly speaking, by a day in every seventy years,--and the time came, no doubt, when it was noticed that the terrestrial seasons no longer bore their traditional relation to the year. This probably happened at some time in the seventh or eighth centuries before our era, and was connected with the astronomical revolution that has been alluded to before; when the ecliptic was divided into twelve equal divisions, not associated with the actual stars, the Signs were substituted for the Constellations of the Zodiac, and the Ram was taken as the leader instead of the Bull. The equinox was then determined by direct measurement of the length of the day and night; for a tablet of about this period records-- "On the sixth day of the month Nisan the day and night were equal. The day was six double-hours (_kasbu_), and the night was six double-hours."