The foundations for the study of astronomy in the ancient Egypt were laid by Otto Neugebauer together with Richard Parker. The whole body of what they considered to be the known texts of Egyptian astronomy was published in four large volumes (Neugebauer and Parker 1964). These books are difficult to find today, but fortunately a copy belongs to the architecture library of my university, the Politecnico of Milan.
In these volumes, Neugebauer and Parker state:
The study of all available Egyptian astronomical texts nowhere reveals the existence of such a level of theoretical understanding as was reached in Babylonian and Greek astronomy in the fifth or fourth century BC. This is not surprising since the mathematical tools developed in Egypt were by far too primitive to be of use in astronomical investigations.
Even worse, the work starts with the Middle Kingdom because, according to the authors, the earlier age of the pyramids, one of the most intense and wonderful periods in the history of mankind, did not leave any record of astronomical knowledge. As I discuss in later chapters, this assertion is blatantly incorrect. For the moment, however, let's look at what is certain and universally accepted. The only astronomical concept that is universally accepted as dating from the Old Kingdom is the codification of the Egyptian calendar. Maybe in predynastic times a lunar calendar already existed; nevertheless, very soon, during the first dynasties, a solar civil calendar of 365 days—without a leap year, thus slipping approximately one day every 4 years with respect to the solar cycle—was introduced as well. In Egypt, though, the only significant natural event was the cycle linked to the annual Nile flooding, rather than to the alternation of the seasons. Therefore, the year was divided not into four seasons but into three: Akhet (flooding), Peret (growing of the crop), and Shemu (the dry season). Every season was divided into months of 30 days, and every month into weeks of ten days. Five days (usually called epagomenal days) were added to the calendar to reach a total of 365.
The calendar was codified in a period in which the day of the heliacal rising of Sirius (the day in which it becomes visible again just before dawn after a period of invisibility), the brightest celestial object after the Sun, the Moon, and Venus, roughly coincided with the flooding of the Nile (which is a gradual event that happens over 30 to 40 days) and with the summer solstice, chosen as New Year's Day. The likely reason behind this choice is linked to a cult of "stellar rebirth''; a trace of this cult is, for instance, the fact that the mummification process was ritually fixed to last 70 days, which is a good approximation of the period of invisibility of Sirius at Memphis (it is impossible to define with high precision the period of invisibility of a star; see Appendix 1). In any case, the duration of the cycle of Sirius, or sothic cycle, is of 365.25 days, and as a consequence of that, New Year's Day started wandering with respect to the heliacal rising of Sirius, completing a full cycle after 1461 years, when the heliacal rising of Sirius and the beginning of the year coincided again. Such a long period is known as the sothic year; in a way, the existence of this "double geared'' count of time is similar to what happened among the Maya, who had two calendars running simultaneously and commensurable through a "long year'' lasting 52 solar years, as we shall see in Chapter 7.
From the work of writers from the classic age, we know that in the year 139 AD the heliacal rising of Sirius coincided with the beginning of the new year; therefore, the calendar could have been codified 1461 years before, around 1321 BC, a date that is actually too late according to all sources. The consequence is that the calendar must have been adopted not one but two sothic years before the year 139 AD, which means around the year 2781 BC, right at the time of the Old Kingdom, just before the age of the pyramids (therefore we can at least be sure that, at that time, heliacal risings of bright stars were observed).
The fact that the Egyptian astronomers recorded the heliacal risings of Sirius for millennia is of extreme importance for Egyptology. Indeed, it is obviously possible to calculate the days in which the heliacal rising of Sirius happened through the centuries. In 1870, the Egyptologist Goerg Ebers bought a papyrus in Thebes where the event is recorded to occur on the ninth day of the third month Shemu during the ninth year of Amenhotep I, and other heliacal risings are recorded by other sources under Sesostri III and Tuthmosis III. It is then possible to link the reigns of such pharaohs to dates on our calendar and therefore identify, for instance, to which Gregorian year the ninth year of Amenhotep I corresponds. Further, thanks to some sources that record the durations of the reigns of the single pharaohs (for example the list of the kings, existing in the temple of Seti I in Abydos), it is thus possible to pinpoint the chronology of the pharaonic dynasties (it is a complicated and risky operation; for instance, if the observation recorded in the Ebers Papyrus happened in Memphis, it would have been in the year 1537 BC, while if it happened in Thebes, it would have been the year 1517 BC). Unfortunately, we did not get any recordings of heliacal rising of Sirius prior to the reign of Sesostri III (circa 1830 BC); therefore, the chronology of the Old and Middle kingdoms is much less accurate than that of the New Kingdom (for a recent and thorough approach to the interesting problems raised by the Egyptian calendar, which are only addressed in brief here, see Belmonte and Edwards 2004).
The "wandering" of the civil calendar generated in some writers the crazy idea that the ancient Egyptians struggled to put up with the continuous problems caused by the progressive shifting of the dates with respect to the seasons, and some even wrote that the Egyptians' stupidity prevented them from reforming the calendar during the three thousand years of their history. Probably, instead, since it was the flooding of the Nile that dictated the yearly cycle of farming rather than the solar cycle, they simply did not feel the necessity of adjusting the calendar. In any case, the Egyptians knew the solar cycle perfectly well. To verify this, all we need to do is to visit one of the most magnificent temples in human history.
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