As we have seen, there is an almost complete lack of scientific texts belonging to the pharaonic period. Therefore, even though Neugebauer and Parker's book is titled Egyptian Astronomical Texts, it is actually a collection of depictions and texts of astronomical content inserted in a religious context, specifically a funerary one: images painted on the inside walls of wooden coffins of the Middle Kingdom, or frescoed on the walls of the tombs during the New Kingdom. In particular, spectacular astronomical ceilings appear in some of the tombs of the Valley of the Kings and in the unfinished tomb of the royal architect of Hatshepsut, Semnut, in Deir el Bahri. These are, therefore, images that the two authors have selected according to a quite subjective criterion, excluding the possibility that astronomical content could be present in the funerary texts of the Old Kingdom as well. As we shall see, this is a major error because such content is indeed there, and transpires clearly in the Pyramid Texts; for the time being, however, we proceed with the Middle Kingdom texts.
The existing material can be classified into three categories of documents with an astronomical content: the decanal lists, the stellar clocks, and the astronomical ceilings. The decans were 36 celestial objects (some were single stars while others were asterisms, that is, groups of stars) with heliacal rising occurring in successive weeks. Since the Egyptian week consisted of 10 days, a calendar could be made associating a decan with each week (36 x 10 = 360); the 5 days needed to bring the calendar to 365 days had decans of their own as well. The first decan was Sirius. The following decans were chosen among the celestial objects that have a period of invisibility comparable to that of Sirius, which means about 70 days. Neugebauer and Parker noted that the celestial objects with this characteristic are located in what they called the decanal belt, a region of the sky south of the ecliptic that includes Sirius and many other stars, for instance Orion. The decans are quoted in the texts with peculiar names, so that it is not easy to associate them with specific celestial objects, to the point that Neugebauer and Parker decided that it was impossible to find them. Instead, very good progress has been made on this matter (Belmonte 2001b), even though it is still debated (cf. Conman 2004).
The decans were also used as a sophisticated watch to measure the nocturnal time. This is how the decanal watch worked: On the specific day of the heliacal rising of a decan, the last hour of the night of that day was found through that decan. The following day the rising happened just before dawn, 2 days later even earlier, and so forth. After 10 days (a week), another decan was designated to indicate the last hour of the night, and the previous decan slipped into the hour list, indicating the previous hour with its rising, and so forth until the 12 hours of the night were all indicated before ending the calculation. The decanal hours did not all last for the same time. This seems strange to us, but the Egyptians would have regarded as strange our way of measuring the nighttime hours. Indeed, for the Egyptians the night, the period when it is dark, always lasted 12 hours, with the hours of variable length, while for us the hours all last the same fixed time and the night (the
period when it is dark) lasts a different number of hours according to the different seasons.
In any case, it was the division of the night into 12 hours that was decided in ancient times and that was certainly already in use around the 9th dynasty (2154 BC) to generate the division of the day into 12 hours. Such division is documented in the New Kingdom by the existence of a solar watch (based on measuring the shadow of a Gnomon) of the period of Tuthmosis III (c. 1490 BC). The use of the 24 hours to divide the day was made stable and filtered by Hellenistic astronomy, which mixed it with the sexagesimal division, and with the 7-day week imported from the Babylonians (see Chapter 5) finally originated our very uncomfortable convention of dividing time in 7 days of 24 hours of 60 minutes each. Because every decan indicated, through its rising, the beginning of a certain hour of a certain week, and the following week it would move on to the next hour, the first time that a decan entered the calculation of the night hours was at its heliacal rising, and the last one was after indicating the 11th decanal hour, which means after 120 days. The decanal lists depicted in many coffins from the Middle Kingdom are then lists of decans organized so as to make it possible to read, from left to right, the subdivision of the night hours. They are, basically, tables that can be schematized in the following way: on the lines are the hours (12 lines) and there are the decan columns (36 columns) with one special column added for the epagomenal days. In any box of the table there is the name of a decan written diagonally. In the central part of the table there is a vertical belt where the Goddess of the Sky Nut, Mes-Bull's Foreleg, Orion, and Sothis-Sirius are depicted, and one horizontal belt with an inscription invoking the intercession of these divinities for the deceased, together with those of the Sun-God King and of other decans.
In the New Kingdom, the decanal lists were substituted by stellar clocks, called ramessides because they were known thanks to the frescos on the walls of the tombs of the pharaohs Rameses VI, VII, and IX. They are depictions of a sitting, somewhat unhappy man (probably because he had to remain perfectly still) and a table with nine columns and 13 lines. The first line represents the beginning of the night, and the others the 12 hours of the night. The seven vertical lines are associated with seven parts of the human body, and the grid was therefore used as reference system, with the central line representing the meridian. A different star is indicated for every single hour, together with the position of the star with respect to the man of reference (presumably the assistant to the astronomer, or a statue). The scheme is repeated using different stars 24 times, one for each of 15 days of the year.
Neugebauer and Parker (1964) criticize the presence of errors and imprecisions both in the decanal lists and in the stellar clocks. I find these criticisms ridiculous; it is obvious that both the decanal lists and the stellar clocks are based, in the funerary context, on developed astronomical knowledge. The astronomical data were inscribed in the tombs for a ritual purpose, to accompany, as it is sometimes said, the journey of the deceased through the night, but they were based on astronomical studies. The artists frescoing the tombs in the Valley of the Kings (even though not at all illiterate, as the excavations in the workers quarters of the Deir el Medina tombs prove) were not astronomers, and it is logical to expect them to have committed some mistakes. It is thus the lack of other sources that forces us to examine depictions copied from preexisting texts, and the one who compiled the originals was an astronomer, possessing precise observations collected through the centuries. Indeed, it is easy to understand that the Egyptian astronomers should have accurately mapped the sky and measured the heliacal rising and the meridian transit of many stars, to be able to choose 36 of them to subdivide the hours of the night. Moreover, it is hard to believe that such an extremely complicated method like the one of the stellar clocks was used in practice to count the hours of the night, and indeed the use of water clocks—containers full of water with a hole allowing the fluid to exit and a series of lines indicating the level reached by the water, from which it was possible to determine the time that passed during the night—is well documented for that period. So the stellar clocks were a method to record astronomical data, rather than a method to count the hours of the night, except, maybe, on some special ritual occasions.
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