The astrological scheme of the Aztecs was founded less on the planetary influences than on those of the arbitrary signs they had adopted for the months and days. The character of the leading sign in each lunar cycle of thirteen days gave a complexion to the whole; though this was qualified in some degree by the signs of the succeeding days, as well as by those of the hours. It was in adjusting these conflicting forces that the great art of the diviner was shown. In no country, not even in ancient Egypt, were the dreams of the astrologer more implicitly deferred to. On the birth of a child, he was instantly summoned. The time of the event was accurately ascertained; and the family hung in trembling suspense, as the minister of Heaven cast the horoscope of the infant and unrolled the dark volume of destiny. The influence of the priest was confessed by the Mexican in the very first breath which he inhaled.53
We know little further of the astronomical attainments of the Aztecs. That they were acquainted with the cause of eclipses is evident from the representation, on their maps, of the disk of the moon projected on that of the sun.54 Whether they had arranged a system of constellations is uncertain; though that they recognized some of the most obvious, as the Pleiades, for example, is evident from the fact that they regulated their festivals by them. We know of no astronomical instruments used by them, except the dial.55 An immense circular block of carved stone, disinterred in 1790, in the great square of Mexico, has supplied an acute and learned scholar with the means of establishing some interesting facts in regard to Mexican science.56 This colossal fragment, on which the calendar is engraved, shows that they had the means of settling the hours of the day with precision, the periods of the solstices and of the equinoxes, and that of the transit of the sun across the zenith of Mexico.57