It may be affirmed of all these ziggurats that they were not tombs like the Egyptian pyramids, but were erected exclusively for astronomical and astrological purposes. The number of stages had always reference to some religious or astronomical fact, as three to symbolize the great triad; five for the five planets; or seven for these and the sun and moon; the number of seven being never exceeded, and the order the same as that adopted for the days of the week, viz. according to the magnitudes of their respective orbits. They were oriented with as much care as the pyramids, which is of itself a proof that they were used as observatories, but with this difference, that their angles instead of their faces were directed towards the true north and south. To this rule there are only two exceptions, probably of late date after Egyptian influences had been introduced, but the original and national ziggurats invariably observe the rule of pointing angles and not sides to the four cardinal points. This is a remarkable fact as showing that the astronomies of Egypt and Chaldæa were not borrowed one from the other, but evolved independently in prehistoric times. An explanation of it has been found in the fact recorded on a geographical tablet, that the Accadians were accustomed to use the terms north, south, east, and west to denote, not the real cardinal points, but countries which lay to the N.W., S.E., and S.W. of them. It is inconceivable, however, that such skilful astronomers should have supposed that the North Pole was in the north-west, and a more probable explanation is to be found in the meaning of the word ziggurat, which is holy mountain. It was a cardinal point in their cosmogony that the heavens formed a crystal vault, which revolved round an exceedingly high mountain as an axis, and the ziggurats were miniature representations of this sacred mountain of the gods. The early astronomers must have known that this mountain could be nowhere but in the true north, as the daily revolutions of the heavenly bodies took place round the North Pole. It was natural, therefore, that they should direct the apex or angle of a model of this mountain rather than its side to the position in the true north occupied by the peak of the world's pivot. Be this as it may, the fact that the ziggurats were carefully oriented, and certainly used as observatories at the earliest dates of Chaldæan history, is sufficient to prove that the priestly astronomers must have already attained an advanced knowledge of science, and kept an accurate record of long-continued observations. This is fully confirmed by the astronomical and astrological treatise compiled for the royal library of Sargon I., date 3800 B.C., which treats of eclipses, the phases of Venus, and other matters implying a long previous series of accurate and refined astronomical observations.