There is an important clue in the text known as the Haft Bab (Seven Gates), which says that in 360,000 years the heavens revolve full circle, and - it is implied - history repeats itself. The figure therefore has some connection with astronomy. There was an esoteric society within Islam at this time known as the Brethren of Purity, who also quote this figure. They are said to have obtained it from the Sindhind tables. The Sindhind was an Indian astronomical work composed in 628 by Brahmagupta and brought to Baghdad in 771. We thus have a direct historical link between Ismaili and Indian cosmologies. But where did the Indians get the 360,000 figure?

To discover this we have to look more closely at the number itself. The figure 360,000 is of course a multiple of 60; 60 x 3600 = 216,000, and 216,000 x 2 = 432,000. We have now arrived at a very significant number indeed, which turns up in the most surprising places. For example, Berossos, a Babylonian priest who wrote in Greek in about 280 BCE, gives a list of 10 mythological kings, the sum of whose quoted reigns is 432,000 years. A Sumerian tablet gives a similar though not identical list of kings, the total number of years in this case being 456,000. Fairly similar lists are to be found in Genesis, and it's evident that the authors of all these texts are drawing on a common tradition.

Again, in certain Indian texts (the Mahabharata and the Puranas), which were probably written some time after 400 CE but contain older material, we find the concept of the four yugas - great cycles of mythic time. the duration of the yugas is 1200 Divine Years. One Divine Year is 360 human years, and 1200 x 360 = 432,000.

These coincidences cannot have arisen by chance; there has to be a common source. This appears to be Mesopotamia. The Sumerians were fascinated by astronomy, and astrology may have originated in Sumeria. Sumerian calculation is based on 60, which was called the soss. There were also the ner (600) and the sar (3600); the Great Sar was 3600 x 60 = 216,000, half of 432,000. Evidently there is something very important and special about this figure.

In fact, it is related to the phenomenon known as the precession of the equinoxes. If you spin a top you will nearly always find that it doesn't remain exactly upright; instead, its summit (and its point) describe small circles, so that if the point on which it revolves were dipped in ink it would trace a circle on a sheet of paper. The axis of the earth also describes a circle of this kind, but very slowly, taking about 26,000 years for a complete rotation. This is the Platonic "Great Year".

For observers on the earth this slow rotation results in an apparent shift in the relative positions of the stars and the planets, with a consequent change in the apparent position of the sun in the Zodiac (as determined by noting the Zodiacal sign that rises above the horizon just before sunrise). Over a very long period the sun seems to "move backwards" in the Zodiac. The length of time that the sun spends in each sign is about 2,200 years. At the time of Christ it had just moved from Aries to Pisces (which is almost certainly why in early Christian times the symbol of Christ was taken to be the fish); shortly it will move from Pisces to Aquarius, hence the "Aquarian Age".

Corresponding to this apparent movement of the sun there is a change in the times of the summer and winter solstices and of the equinoxes; hence the description of the phenomenon as the precession of the equinoxes.

The duration of the Great Year is only approximately 26,000 years. More precisely, it is 25,920 years. But 25,920/60 = 432.

In view of all this it's difficult to avoid the conclusion that the Sumerians' interest in 60 (which survives in our 60 seconds in a minute and 60 minutes in an hour) derives from their awareness of the precession of the equinoxes - a phenomenon to which they evidently attached profound significance. All known multiplication tables from Nippur, Sippur, and Ashurbanipal are, according to Joseph Campbell, based on 12,960, which is half the Great Year of 25,920. Moreover, the Sumerian year (excluding the five festival days) consisted of 72 weeks, each of 5 days; and 72 x 360 = 25,920.

That the Mesopotamians knew of the precession of the equinoxes seems from these facts to be a near-certainty. In Hamlet's Mill, Santillana and Von Dechend suggested that the ancients would have been deeply impressed by this knowledge, and though many of their claims seem to me far-fetched, I think they may well have been right about this. At a time when it was widely believed that events on earth were governed by the stars, the discovery would imply that the heavens were apparently revolving slowly and inexorably, like a giant mill grinding out the fate of mankind. "The mills of God grind slowly, but they grind exceeding small." It followed that nothing on earth would endure for ever, for eventually the configuration of the stars would change and with it the fate of mankind. However, it was possible to infer that the sun would in the end traverse the whole Zodiac and return to its starting point, at which time, perhaps, history would repeat itself. Is this the foundation of the ancient belief in eternal recurrence? At any rate, the Ismailis' emphasis on the figure of 360,000 now makes sense; it was not arbitrary or haphazard, but was located in the complex structure of ancient cosmology and astrology.