The Sothic cycle
or Canicular period
is a period of 1461 ancient Egyptian
years (of 365 days each) or 1460 Julian
years (averaging 365.25 days each). During a Sothic cycle, the 365-day year loses enough time that the start of the year once again coincides with the heliacal rising
of the star Sirius
name for Greek
Σείριος, a star called Sopdet
by the Egyptians, in Greek transcribed as Sothis
; a single year between heliacal risings of Sothis is a Sothic year
). This rising occurred within a month or so of the beginning of the Nile
flood, and was a matter of primary importance to this agricultural
society. It is believed that Ancient Egyptians followed both a 365-day civil calendar and a lunar religious calendar.
Mechanics and discovery
The ancient Egyptian civil year was 365 days long, and apparently did not have any intercalary
days added to keep it in alignment with the Sothic year, a kind of sidereal year
. Normally, a sidereal year is considered to be 365.25636 days long, but that only applies to stars on the ecliptic
, or the apparent path of the Sun
. Because Sirius is not on the ecliptic, the wobbling of the celestial equator and hence of the horizon at the latitude of Egypt causes the Sothic year to be slightly smaller. Indeed, it is almost exactly 365.25 days long, the average number of days in a Julian year.
This cycle was first noticed by Eduard Meyer in 1904, who then carefully combed known Egyptian inscriptions and written materials to find any mention of the calendar dates when Sirius rose. He found six of them, on which the dates of much of the conventional chronology are based. A heliacal rise of Sirius was recorded by Censorinus as having happened on the Egyptian new year's day, between AD 139 and 142 The record actually refers to July 21 of 140 AD but is astronomically calculated as a definite July 20 of 139 AD. This correlates the Egyptian calendar to the Julian calendar. Leap day occurs in 140 AD, and so the new year, Thoth 1, is July 20 in 139 AD but it is July 19 in 140-142 AD. Thus he was able to compare the day on which Sothis rose in the Egyptian calendar to the day on which Sothis ought to have risen in the Julian calendar, count the number of intercalary days needed, and determine how many years were between the beginning of a cycle and the observation. One also needs to know the place of observation, since the latitude of the observation changes the day when the heliacal rising of Sirius occurs, and mislocating an observation can potentially change the resulting chronology by several decades. Meyer concluded from an ivory tablet from the reign of Djer that the Egyptian civil calendar was created in 4241 BC, a date that appears in a number of old books. But research and discoveries have since shown that the first dynasty of Egypt did not begin before c.3100 BC, and the claim that 4241 BC (July 19) is the "earliest fixed date" has since been discredited. Most scholars either move the observation upon which he based this forward by one cycle of Sothis to 2781 (July 19), or reject the assumption that the document in question indicates a rise of Sothis at all.
Three specific observations of the heliacal rise of Sirius are extremely important for Egyptian chronology. The first is the aforementioned ivory tablet from the reign of Djer
which supposedly indicates the beginning of a Sothic cycle, the rising of Sothis on the same day as the new year. If this does indicate the beginning of a Sothic cycle, it must date to about 2773 BC (July 17). However, this date is too late for Djer's reign, so many scholars believe that it indicates a correlation between the rising of Sothis and the lunar calendar, instead of the solar calendar, which would render the tablet essentially devoid of chronological value.
The second observation is clearly a reference to a heliacal rising, and is believed to date to the seventh year of Senusret III
. This observation was almost certainly made at Itj-Tawy
, the Twelfth Dynasty capital, which would date the Twelfth Dynasty from 1963 to 1786 BC. The Ramses (Turin) Papyrus Canon says 213 years (1991-1778 BC), Richard Parker reduces it to 206 years (1991-1785 BC), based on July 17 of 1872 BC as the Sothic date (120th year of 12th dynasty, a drift of 30 leap days). Prior to Parker's investigation of lunar dates the 12th dynasty was placed as 213 years of 2007-1794 BC perceiving the date as July 21 of 1888 BC as the 120th year, and then as 2003-1790 BC perceiving the date as July 20 of 1884 BC as the 120th year.
The third observation was in the reign of Amenhotep I
, and, assuming it was made in Thebes, dates his reign between 1525 and 1504 BC. If made in Memphis, Heliopolis, or some other Delta site instead, as a minority of scholars still argue, the entire chronology of the Eighteenth dynasty needs to be expanded by some 20 years.
For 12th dynasty:
31 degree Alexandria absence 72 days until July 18
30 degree Memphis absence 70 days until July 17
25 degree Thuban absence 60 days until July 12
23 degree Fort Quban absence 56 days until July 10 solstice
Constant since 139 AD:
31 degree Alexandria absence 72 days until July 21
30 degree Memphis /Kuwait absence 70 days until July 20 current Gregorian Aug 2 (1990 AD)
25 degree Thuban absence 60 days until July 15
23 degree Fort Quban absence 56 days until July 13
Our August 2 is the current (July 20) Sothic new year while the fixed new year (Aug 29) with leap day is September 11.
Problems and criticisms
Determining the date of a heliacal rise of Sothis has been shown to be difficult, especially considering the need to know the exact latitude of the observation. Another problem is that because the Egyptian calendar loses one day every four years, a heliacal rise will take place on the same day for four years in a row, and any observation of that rise can date to any of those four years, making the observation not extremely precise.
A number of criticisms have been leveled against the reliability of dating by the Sothic cycle. Some are serious enough to warrant consideration (for example, was the civil calendar unchanged through the thousands of years of Egyptian history?), while others are not (for example, there is no explicit mention of the Sothic cycle in ancient Egyptian writing). Some have recently claimed that the Theran eruption marks the beginning of the XVIII dynasty in Egypt, and because dendrochronologists believe it took place in 1626 BC, that the Sothic cycle is off by 50–80 years. The claim that the Thera eruption is in fact the subject matter of the Tempest Stele of Ahmose I is disputed, and most Egyptologists reject this problem outright. Egypt maintained diplomatic ties with other foreign powers, which allows for comparative chronologies to be made, and it is virtually impossible to move the entire chronology of the Hittites, Mitanni, Assyria, and Babylon earlier by over half a century. Peter James' book Centuries of Darkness (London, 1991: ) is a good introduction to attacks on Sothic dating from the perspective of the revisionists.