On coming home I found that I had made an error in the first edition of the "Physical Sciences," in giving 365 days 6 hours as the length of the civil year of the ancient Egyptians. My friend Mr. Hallam, the historian, wrote to me, proving from history and epochs of the chronology of the ancient Egyptians, that their civil year was only 365 days. I was grateful to that great and amiable man for copies of all his works while he was alive, and I am obliged to his daughter for an excellent likeness of him, now that he is no more.
FROM HENRY HALLAM, ESQ., TO MRS. SOMERVILLE.
Wimpole Street, March 12th, 1835.
As you will probably soon be called upon for another edition of your excellent work on the "Connexion of the Physical Sciences," I think you will excuse the liberty I take in mentioning to you one passage which seems to have escaped your attention in so arduous a labour. It is in page 104, where you have this sentence:—
"The Egyptians estimated the year at 365 d. 6 h., by which they lost one year in every 14,601, their Sothiac period. They determined the length of their year by the heliacal rising of Sirius, 2782 years before the Christian era, which is the earliest epoch of Egyptian chronology."
The Egyptian civil year was of 365 days only, as we find in Herodotus, and I apprehend there is no dispute about it. The Sothiac period, or that cycle in which the heliacal rising of Sirius passed the whole civil year, and took place again on the same day, was of 1461 years, not 14,601. If they had adopted a year of 365 d. 6 h., this period would have been more than three times 14,601; the excess of the sidereal year above that being only 9' 9", which will not amount to a day in less than about 125 years.
I do not see how the heliacal rising of Sirius in any one year could help them to determine its length. By comparing two successive years they could of course have got at a sidereal year; but this is what they did not do; hence the irregularity which produced the canicular cycle. The commencement of that cycle is placed by ancient chronologers in 1322 A.C. It seems not correct to call 2782 A.C. "the earliest epoch of Egyptian chronology," for we have none of their chronology nearly so old, and in fact no chronology, properly so called, has yet been made out by our Egyptian researches. It is indeed certain that, if the reckoning by heliacal risings of Sirius did not begin in 1322, we must go nearly 1460 years back for its origin; since it must have been adopted when that event preceded only for a short time the annual inundation of the Nile. But, according to some, the year 1322 A.C. fell during the reign of Sesostris, to whom Herodotus ascribes several regulations connected with the rising of the Nile. Certainly, 2782 A.C. is a more remote era than we are hitherto warranted to assume for any astronomical observation.
Believe me, dear Mrs. Somerville,
Very truly yours,
Henry Hallam.
I refer you to Montucla, if you have any doubt about the Egyptian year being of 365 days without bissextile of any kind.