This ingenuous argument depends, however, on a number of questionable
assumptions. Firstly, it is based on a precise date for the heliacal rising of
Sothis, which is taken to be 20 July (which coincided with the Egyptian civil
New Year, I Akhet 1, in 2781– 2778 BCE).Whether or not this date is astro-
nomically correct, heliacal risings are difficult in practice to observe; we do not
know how accurately the Egyptians, in the early third millenniumBCE, would
have been able to establish its date.^18 Moreover, the date of the heliacal rising
of Sothis varies according to the earthly latitude of the observer by about one
day per degree of latitude; since the kingdom of ancient Egypt extended across
seven degrees of latitude (from Aswan to the Mediterranean), the heliacal
rising of Sothis would have been visible in different parts of Egypt on different
days, and hence could have been dated at any time within a six- to eight-day
period.^19 Sources from the Ptolemaic and Roman periods indicate that the
rising of Sothis was assumed at that time to take place on 19 or 20 July,^20 and it
is on the basis of the latter that scholars have worked their way back to the
precise dates of 2781– 2778 BCE. However, there is no evidence that the heliacal
rising of Sothis was defined in the same way in earlier centuries, as far back as
the third millenniumBCE, as occurring on a date corresponding to 20 July of
the retrojected Julian calendar.
Secondly, the astronomical argument assumes (1) that the heliacal rising of
Sothis was observed and recorded in Egypt as early as the third millennium
BCE, and (2) that it was associated with the beginning of the civil year. Both
points are evidenced in the second millenniumBCE, but not earlier. A letter
from the archive of Illahun (nineteenth centuryBCE) predicts the date of
the rising of Sothis, clearly for cultic purposes:^21 other documents from
(^18) Modern astronomical data actually suggest that in the early third millenniumBCE, a more
accurate date for the apparent heliacal rising of Sothis in lower Egypt would have been 16/17 July
(and in upper Egypt, even earlier): Schaefer (2000) 150.
(^19) Weill (1926) 189–204, (1928) 27–37; Bomhard (1999) 46–7; Belmonte Avilés (2003) 19, 42.
(^20) This date can be treated as fairly stable, because the date of the rising of Sothis varies very
little in relation to the (retrojected) Julian calendar. Sources from the Ptolemaic and Roman
periods assume dates ranging from 19 to 22 July (but mainly 19 July: O’Mara 2003: 18 n. 5). Thus
the decree of Canopus, from 238BCE, gives the date of Sothis for that year as 1 Payni (i.e. II
Shemu 1), which corresponded then to 19 July (ll. 36–7 of the Greek version, cited below; also in
Clagett 1989–99: ii. 326–31, Pfeiffer 2004: 121–3). Much later, in the 3rd c.CE, Censorinus (De
DieNatali21. 9–10) reports that the last coincidence of the New Year (1 Thoth) and the heliacal
rising of Sothis was in 139CE, when 1 Thoth was 20 July (Clagett 1989–99: ii. 333–5; the text
reads 21 July, which is erroneous and may be Censorinus’own error); however, this source
cannot be treated as authoritative, as Censorinus was clearly picking a date of Sothis that would
suit his own chronological purposes (O’Mara 2003). The date of 20 July would appear to suit the
latitude of Alexandria, i.e. the northernmost point of Egypt (see Schaefer 2000; alsoWeill 1926:
195 – 7; Bomhard 1999: 46–7), which is not surprising for the Ptolemaic and Roman periods but
perhaps less appropriate to Old Kingdom Egypt.
(^21) Pap. Berlin 10012 A (Luft 1992: 54–7). On the question of whether the rising of Sothis was
actually observed, or rather calculated on the basis of the civil calendar (i.e. by adding one day to
its civil date every four years), see Clagett (1989–99) ii. 307–15.
The Egyptian Calendar 131