How was this? The astronomical answer is very simple.
I have already referred to the common practice of all ancient peoples that we know of to make sacrifices at dawn, and have shown how, in order to do this, they took their time from a star rising before the sun. An observation of the so-called "heliacal rising" of a star—if the star were properly chosen—would give them the interval necessary for their preparations before the sun itself appeared; and, as the highest festival of all was that of New Year's Day, it was especially important that the work should be well done then.
Now, if the stars had no precessional movement, the sun and stars, after each interval of a true year, would be in exactly the same position; but in consequence of the stars having the precessional movement to which I have before referred, the star so observed and the sun will not be in exactly the same position after the interval of a true year. On this account, then, the difference of time between the heliacal risings will not represent the length of a true year. But, further, the heliacal rising of the star will not take place on the same day for the whole of Egypt, the difference between Thebes and Memphis, depending upon their latitudes, amounting to about four days; and, further still, the almost constant mists in the mornings in the Nile valley prevent accurate observations of the moment of rising.
Still, as a matter of fact, the Egyptians defined their new year by the rising of a star, and the length of it by the interval separating two heliacal risings. Such a year could not be accurate; and again, as a matter of fact, their correction was not accurate, for the year was defined now as consisting of 365 days. It seems clear from this that the correction was made before the solar temples were in use.
In any case the year of 360 days had naturally to give way, and it ultimately did so, in favour of one of 365. The precise date of the change is, as we have seen, not known.[71] The five days were added as epacts or epagomena; the original months were not altered, but a "little month" of five days was interpolated at the end of the year between Mesori of one year and Thoth of the next, as already stated.
When the year of 365 days was established, it was evidently imagined that finality had been reached; and, mindful of the confusion which, as we have shown, must have resulted from the attempt to keep up a year of 300 days by intercalations, each Egyptian king, on his accession to the throne, bound himself by oath before the priest of Isis, in the temple of Ptah at Memphis, not to intercalate either days or months, but to retain the year of 365 days as established by the Antiqui.[72] The text of the Latin translation preserved by Nigidius Figulus cannot be accurately restored; only thus much can be seen with certainty.
To retain this year of 365 days, then, became the first law for the king, and, indeed, the Pharaohs thenceforth throughout the whole course of Egyptian history adhered to it, in spite of their being subsequently convinced, as we shall see, of its inadequacy. It was a Macedonian king who later made an attempt to replace it by a better one.
We may reckon upon the conservatism of the priests of the temples retaining the tradition of the old rejected year in every case. Thus even at Philæ in late times, in the temple of Osiris, there were 360 bowls for sacrifice, which were filled daily with milk by a specified rotation of priests. At Acanthus there was a perforated cask into which one of the 360 priests poured water from the Nile daily.
Indeed, these temple ceremonials are an evidence of their antiquity, and the further we put back the change from the 360 to 365 days, the greater the antiquity we must assign to them, and therefore to the temples themselves.