(M124) A great festival called the Sed was celebrated by the Egyptians with much solemnity at intervals of thirty years. Various portions of the ritual are represented on the ancient monuments of Hieraconpolis and Abydos and in the oldest decorated temple of Egypt known to us, that of Usirniri at Busiris, which dates from the fifth dynasty. It appears that the ceremonies were as old as the Egyptian civilization, and that they continued to be observed till the end of the Roman period.(410) The reason for holding them at intervals of thirty years is uncertain, but we can hardly doubt that the period was determined by astronomical considerations. According to one view, it was based on the observation of Saturn’s period of revolution round the sun, which is, roughly speaking, thirty years, or, more exactly, twenty-nine years and one hundred and seventy-four days.(411) According to another view, the thirty years’ period had reference to Sirius, the star of Isis. We have seen that on account of the vague character of the old Egyptian year the heliacal rising of Sirius shifted its place gradually through every month of the calendar.(412) In one hundred and twenty years the star thus passed through one whole month of thirty days. To speak more precisely, it rose on the first of the month during the first four years of the period: it rose on the second of the month in the second four years, on the third of the month in the third four years; and so on successively, till in the last four years of the hundred and twenty years it rose on the last day of the month. As the Egyptians watched the annual summer rising of the star with attention and associated it with the most popular of their goddesses, it would be natural that its passage from one month to another, at intervals of one hundred and twenty years, should be the occasion of a great festival, and that the long period of one hundred and twenty years should be divided into four minor periods of thirty years respectively, each celebrated by a minor festival.(413) If this theory of the Sed festivals is correct, we should expect to find that every fourth celebration was distinguished from the rest by a higher degree of solemnity, since it marked the completion of a twelfth part of the star’s journey through the twelve months. Now it appears that in point of fact every fourth Sed festival was marked off from its fellows by the adjective _tep_ or “chief,” and that these “chief” celebrations fell as a rule in the years when Sirius rose on the first of the month.(414) These facts confirm the view that the Sed festival was closely connected with the star Sirius, and through it with Isis.