Essay on the Theory of the Earth

by baron Georges Cuvier

They took for the point of departure of this period, which they named the Sothiac or great year, a civil year, the first day of which was, or had been, also that of a heliacal rising of Sirius; and it is known, from the positive testimony of Censorinus, that one of these great years had ended in the 138th year of the Christian era[182]. It had consequently commenced in the 1322d before Christ, and that which preceded it in the 2782d. In fact, the calculations of M. Ideler shew, that Sirius was heliacally risen on the 20th July of the Julian year 139, a day which corresponded that year to the first of Thot, or the first day of the Egyptian sacred year[183].

But not only is the position of the sun, with relation to the stars of the ecliptic, or the sidereal year different from the tropical year, on account of the precession of the equinoxes. The heliacal year of a star, or the period of its heliacal rising, especially when it is distant from the ecliptic, differs still from the sidereal year, and differs in various degrees according to the latitudes of the places where it is observed. What is very singular, however, and the observation has already been made by Bainbridge[184] and Father Petau[185], it happens, by a remarkable concurrence in the positions, that, in the latitude of Upper Egypt, at a certain epoch, and during a certain number of ages, the year of Sirius was really within very little of 365 days and a quarter; so that the heliacal rising of this star returned in fact to the same day of the Julian year, the 20th July, in the year 1322 before, and the year 138 after Christ[186].

From this actual coincidence, at this remote period, M. Fourier, who has confirmed all these accounts by new calculations, concludes, that, since the length of the year of Sirius was so perfectly known to the Egyptians, they must have determined it by observations made during a long series of years, and conducted with great accuracy; observations which must be referred to at least 2500 years before the present time, and which could not have been made long before or long after this interval of time[187].

This result would assuredly be very striking, had it been directly, and by observations, made upon Sirius itself, that they had fixed the length of the year of Sirius. But experienced astronomers affirm it to be impossible that the heliacal rising of a star could afford a sufficient foundation for exact observations on such a subject, especially in a climate where the circumference of the horizon is constantly so much loaded with vapours, that, in clear nights, stars of the second or third magnitude can never be seen within some degrees of the verge of the horizon, and that the sun itself is completely obscured at its rising and setting.[188] They maintain, that, if the length of the year had not been otherwise ascertained, there would have been a mistake of one or two days.[189] They have no doubt, therefore, that this duration of 365 days and a quarter, is that of the tropical year inaccurately determined by the observation of the shadow, or by that of the point where the sun rose each day, and through ignorance identified with the heliacal year of Sirius; so that it would be mere chance which had fixed with so much accuracy the duration of this latter for the period of which we speak.[190]

Perhaps it will also be judged, that men capable of making observations so exact, and which they had continued during so long a period, would not have attributed so much importance to Sirius, as to pay him religious homage; for they would have seen that the relations of the rising of this star with the tropical year, and with the inundation of the Nile, were merely temporary, and took place only in a determinate latitude. In fact, according to M. Ideler’s calculations, in the year 2782 before Christ, Sirius appeared in Upper Egypt, on the second day after the solstice; in 1322, on the third; and in the year 139 after Christ, on the twenty-sixth.[191] At the present day, its heliacal rising is more than a month after the solstice. The Egyptians would therefore set themselves by preference to finding the period, which would bring about the coincidence of the commencement of the sacred year, with that of the true tropical year, and then they would discover that their great period must have been 1508 sacred years, and not 1461.[192] Now, we assuredly do not find any traces of this period of 1508 years in antiquity.

In general, we may defend ourselves with the idea, that, if the Egyptians had possessed so long a series of observations, and of accurate observations too, their disciple Eudoxus, who studied among them for thirteen years, would, on his return, have brought into Greece a system of astronomy more perfect, and maps of the heavens less erroneous, and more coherent in their different parts.[193] How should it happen that the precession of the equinoxes was not known to the Greeks, but through the works of Hipparchus, if it had been marked in the registers of the Egyptians, and inscribed in characters so manifest upon the ceilings of their temples? And how comes it that Ptolemy, who wrote in Egypt, should not have deigned to avail himself of any of the observations of the Egyptians?[194]

Farther, Herodotus, who lived so long with them, says nothing of those six hours which they added to the sacred year, nor of that great Sothian period which resulted. On the contrary, he says expressly that the Egyptians, making their year of 365 days, the seasons returned to the same point, so that in his time the necessity of this quarter of a day does not appear to have been suspected.[195] Thalles, who had visited the priests of Egypt, less than a century before Herodotus, did not, in like manner, make known to his countrymen, any other than a year of 365 days only.[196] And, if we reflect that all the colonies which migrated from Egypt, fourteen or fifteen centuries before Christ, the Jews and the Athenians, carried with them the lunar year, it will perhaps be inferred that the year of 365 days itself had not existed in Egypt in these remote ages.

I am aware that Macrobius[197] gives the Egyptians a solar year of 365¼ days; but this author, who is comparatively modern, and who lived at a long period after the establishment of the fixed year of Alexandria, must have confounded the epochs. Diodorus[198] and Strabo[199] only attribute such a year to the Thebans; they do not say that it was in general use, and they themselves did not live till long after Herodotus.

Thus the Sothian or great year must have been a comparatively recent invention, since it results from the comparison of the civil year with this pretended heliacal year of Sirius; and it is for this reason that it is only spoken of in the works of the second and third century after Christ[200], and that Syncellus alone, in the ninth, seems to cite Manetho as having made mention of it.