EGYPTIAN A solar year, a lunar month and a day unfortunately are incom mensurable units of time, for a year contains roughly 3651 days and a month roughly 291 days. From this fact arise the difficulties with which early constructors of calendars were con fronted. The Egyptians appear to have begun with a lunar cal endar. We have no contemporary evidence of this, but the writing of the word "month" with the moon-sign, the importance of the monthly and half-monthly festivals in later times, and the adop tion of the month as a unit in the later calendar place it beyond doubt.
At a very early date, however, the Egyptians had begun to observe what is known as the heliacal rising of the star Sirius or Sothis, a conspicuous object in the Egyptian sky. A star is said to rise heliacally on the day on which it first appears again in the sky just before sunrise after being for some time invisible. The Egyptians noted that this rising corresponded very closely with the rise of the Nile, on which the agricultural welfare of the country depended. Small wonder then that they chose this for the first day of the year, and took the period between two such observed risings to form a unit of time which was con venient not only as being much longer than the old month, but as including a whole round of the seasons.
The next step was to subdivide the new unit, and here use was made both of the old months and of the changing seasons. Twelve nominal months of 3o days each gave 36o days, and the missing 5 days were added on at the end under the name of "days additional to the year." The months were grouped into three sets of four, the first four forming the inundation season, the sec ond four the winter or sowing-time and the third four the sum mer or harvest.
Unfortunately the constructors of this calendar either over looked or ignored the fact that every four years, as observation must have shown, Sothis rose a day later, i.e., after a lapse of 366 and not 365 days, the reason being, as we now know, that the star-year, which is virtually identical with the solar year, measures about 3651 days. This error of theirs meant that their calendar got out of gear with the solar year, and consequently with the seasons, to the extent of one day every four years, and the error became greater and greater until eventually, after 1,46o (365X4) solar years, known as a Sothic Period, the calendrical New Year's Day had worked right round the seasons and come back to its correct place again. The Egyptians were not unaware of this absurdity, but it was not until quite late times that they sought to correct it by the insertion of an extra day every four years (Leap Year), and even then the attempt failed.
We know from the Latin writer Censorinus that the first day of the Egyptian calendar year coincided with the rising of Sothis in A.D. 139, and it must therefore have done the same thing 1,46o solar years earlier and so on, i.e., in 1321 B.C., 2781 B.C., 4241 B.C., B.C., etc. Obviously it was at one of these moments that the calendar was introduced. Now the religious texts inscribed in the pyramids of the Fifth and Sixth Dynasties show that the calendar with its five extra days was then already in existence. Egyptologists consequently date the introduction of the calendar to 4241 B.C. or to 2781 B.C., according as they believe the pyramids to be earlier or later than the latter date. A still higher date, e.g., 5701 B.C., is hardly likely.
The Egyptians used no unit of time longer than a year. Con sequently they had no dating by eras in the modern sense. In very early times each year was named of ter some important event in it, e.g., "The year of the first smiting of the East," and was at the same time connected with the reigning king. Later, under the Fifth and Sixth Dynasties, the biennial cattle census was used for time reckoning, and the years of a king's reign were numbered alternately "The Year of the first (second, third, etc.), census" and "The Year after the first (second, third, etc.), census." Later still the years of the reign were numbered straightforwardly I, 2, 3 and so on.
In early Egyptian documents the months bear no special names, being written merely as the first, second, third or fourth month of such and such a season. In the Persian period, however (6th cent. B.c.) , there began to appear month names drawn from fes tivals which took place during the month ; these names may of course have been in use in speech earlier, though they were never written. Considerable uncertainty surrounds the origin of some of them, and the question is complicated by the fact that at some date in or before the Ramesside Age the whole of the names seem to have been thrown one month back in the year.
The week of seven days was totally unknown to the early Egyptians and the evidence brought forward for its existence in very late times is far from convincing.
The day and the night were each divided into 12 hours, but as the day was measured sometimes from sunrise to sunset and sometimes from the appearance of daylight to its disappearance, the length of day and of night varied through the year. Conse quently the Egyptians cannot claim to have established the hour as a fixed unit of time. (T. E. P.)