Sothic cycle

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Sirius (bottom) and Orion (right). The Winter Triangle is formed from the three brightest stars in the northern winter sky: Sirius, Betelgeuse (top right), and Procyon (top left). Hubble heic0206j.jpg
Sirius (bottom) and Orion (right). The Winter Triangle is formed from the three brightest stars in the northern winter sky: Sirius, Betelgeuse (top right), and Procyon (top left).
Sirius as the brightest star in the constellation Canis Major as observed from the Earth (lines added for clarity). CanisMajorCC.jpg
Sirius as the brightest star in the constellation Canis Major as observed from the Earth (lines added for clarity).

The Sothic cycle or Canicular period is a period of 1,461 Egyptian civil years of 365 days each or 1,460 Julian years averaging 365¼ days each. During a Sothic cycle, the 365-day year loses enough time that the start of its year once again coincides with the heliacal rising of the star Sirius (Ancient Egyptian : Spdt or Sopdet , "Triangle"; Greek : Σῶθις, Sō̂this) on 19 July in the Julian calendar. [1] [lower-alpha 1] It is an important aspect of Egyptology, particularly with regard to reconstructions of the Egyptian calendar and its history. Astronomical records of this displacement may have been responsible for the later establishment of the more accurate Julian and Alexandrian calendars.

The Julian calendar, proposed by Julius Caesar in 708 AUC (46 BC/BCE), was a reform of the Roman calendar. It took effect on 1 January 709 AUC (45 BC/BCE), by edict. It was designed with the aid of Greek mathematicians and Greek astronomers such as Sosigenes of Alexandria.

The heliacal rising or star rise of a star occurs annually when it briefly becomes visible above the eastern horizon at dawn just before sunrise, after a period of less than a year when it had not been visible. Historically, the most important such rising is that of Sirius, which was an important feature of the Egyptian calendar and astronomical development. The rising of the Pleiades heralded the start of the Ancient Greek sailing season, using celestial navigation.

Sirius Brightest star in the night sky

Sirius is the brightest star in the night sky. Its name is derived from the Greek word Σείριος Seirios "glowing" or "scorching." With a visual apparent magnitude of −1.46, Sirius is almost twice as bright as Canopus, the next brightest star. Sirius is a binary star consisting of a main-sequence star of spectral type A0 or A1, termed Sirius A, and a faint white dwarf companion of spectral type DA2, termed Sirius B. The distance between the two varies between 8.2 and 31.5 astronomical units as they orbit every 50 years.



The ancient Egyptian civil year, its holidays, and religious records reflect its apparent establishment at a point when the return of the bright star Sirius to the night sky was considered to herald the annual flooding of the Nile. [2] However, because the civil calendar was exactly 365 days long and did not incorporate leap years until 22 BC, its months "wandered" backwards through the solar year at the rate of about one day in every four years. This almost exactly corresponded to its displacement against the Sothic year as well. (The Sothic year is about a minute longer than a solar year.) [2] The sidereal year of 365.25636 days is only valid for stars on the ecliptic (the apparent path of the Sun across the sky), whereas Sirius's displacement ~40˚ below the ecliptic, its proper motion, and the wobbling of the celestial equator cause the period between its heliacal risings to be almost exactly 365.25 days long instead. This steady loss of one relative day every four years over the course of the 365-day calendar meant that the "wandering" day would return to its original place relative to the solar and Sothic year after precisely 1461 civil or 1460 Julian years.

Ancient Egypt ancient civilization of Northeastern Africa

Ancient Egypt was a civilization of ancient North Africa, concentrated along the lower reaches of the Nile River in the place that is now the country Egypt. Ancient Egyptian civilization followed prehistoric Egypt and coalesced around 3100 BC with the political unification of Upper and Lower Egypt under Menes. The history of ancient Egypt occurred as a series of stable kingdoms, separated by periods of relative instability known as Intermediate Periods: the Old Kingdom of the Early Bronze Age, the Middle Kingdom of the Middle Bronze Age and the New Kingdom of the Late Bronze Age.

Egyptian calendar calendar used in ancient Egypt before 22 BC

The ancient Egyptian calendar was a solar calendar with a 365-day year. The year consisted of three seasons of 120 days each, plus an intercalary month of five epagomenal days treated as outside of the year proper. Each season was divided into four months of 30 days. These twelve months were initially numbered within each season but came to also be known by the names of their principal festivals. Each month was divided into three 10-day periods known as decans or decades. It has been suggested that during the Nineteenth Dynasty and the Twentieth Dynasty the last two days of each decan were usually treated as a kind of weekend for the royal craftsmen, with royal artisans free from work.

Flooding of the Nile

The flooding of the Nile has been an important natural cycle in Egypt since ancient times. It is celebrated by Egyptians as an annual holiday for two weeks starting August 15, known as Wafaa El-Nil. It is also celebrated in the Coptic Church by ceremonially throwing a martyr's relic into the river, hence the name, The Martyr's Finger. Ancient Egyptians believed that the Nile flooded every year because of Isis's tears of sorrow for her dead husband, Osiris.


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 at dawn. He found six of them, on which the dates of much of the conventional Egyptian 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. [3] The record actually refers to 21 July AD 140 but is astronomically calculated as a definite 20 July AD 139. This correlates the Egyptian calendar to the Julian calendar. Leap day occurs in AD 140, and so the new year on 1  Thoth is 20 July in AD 139 but it is 19 July for AD 140-142. Thus he was able to compare the day on which Sirius rose in the Egyptian calendar to the day on which Sirius 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. [3] (Official observations were made at Heliopolis or Memphis near Cairo, Thebes, and Elephantine near Aswan, [4] with the rising of Sirius observed at Cairo about 8 days after it is seen at Aswan.) [4] Meyer concluded from an ivory tablet from the reign of Djer that the Egyptian civil calendar was created in 4241 BC,[ citation needed ] 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 19 July 4241 BC 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 Sirius to 19 July 2781 BC or reject the assumption that the document in question indicates a rise of Sirius at all. [5]

Eduard Meyer German historian of antiquity

Eduard Meyer was a German historian. He was the brother of Celticist Kuno Meyer (1858–1919).

Censorinus was a Roman grammarian and miscellaneous writer from the 3rd century AD.

Memphis, Egypt Ancient capital of Inebu-hedj, Egypt

Memphis was the ancient capital of Inebu-hedj, the first nome of Lower Egypt. Its ruins are located near the town of Mit Rahina, 20 km (12 mi) south of Giza.

Chronological interpretation

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 Sirius on the same day as the new year. If this does indicate the beginning of a Sothic cycle, it must date to about 17 July 2773 BC. [6] However, this date is too late for Djer's reign, so many scholars believe that it indicates a correlation between the rising of Sirius and the Egyptian lunar calendar, instead of the solar civil calendar, which would render the tablet essentially devoid of chronological value. [5] In 2017 it was claimed that a newly discovered Sothis date from the Old Kingdom and a subsequent astronomic study confirms the Sothic cycle model. [7]

Djer ancient Egyptian pharaoh of the 5th Dynasty of the Middle Empire

Djer is considered the third pharaoh of the First Dynasty of ancient Egypt in current Egyptology. He lived around the mid-thirty-first century BC and reigned for c. 40 years. A mummified forearm of Djer or his wife was discovered by Flinders Petrie, but was discarded by Émile Brugsch.

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. [3] The Ramses or Turin Papyrus Canon says 213 years (1991-1778 BC), Parker reduces it to 206 years (1991-1785 BC), based on 17 July 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 21 July 1888 BC as the 120th year, and then as 2003-1790 BC perceiving the date as 20 July 1884 BC as the 120th year.

Senusret III Pharaoh of Egypt

Khakaure Senusret III was a pharaoh of Egypt. He ruled from 1878 BC to 1839 BC during a time of great power and prosperity, and was the fifth king of the Twelfth Dynasty of the Middle Kingdom. He was a great pharaoh of the Twelfth Dynasty and is considered to be, perhaps, the most powerful Egyptian ruler of the dynasty. Consequently, he is regarded as one of the sources for the legend about Sesostris. His military campaigns gave rise to an era of peace and economic prosperity that reduced the power of regional rulers and led to a revival in craftwork, trade, and urban development. Senusret III was among the few Egyptian kings who were deified and honored with a cult during their own lifetime.

Richard Anthony Parker was a prominent Egyptologist and professor of Egyptology. Originally from Chicago, he attended Mt. Carmel High School with acclaimed author James T. Farrell. He received an A.B. from Dartmouth College in 1930, and a Ph.D. in Egyptology from the University of Chicago in 1938. He then went to Luxor, Egypt to work as an epigrapher with the University of Chicago’s Epigraphic and Architectural Survey, studying the mortuary temple of Ramses III. When World War II necessitated a temporary halt to the project, Parker came back to Chicago to teach Egyptology at the university. In 1946, he returned to Egypt to continue his work on the epigraphic survey, and soon rose to the position of field director.

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 18th Dynasty needs to be expanded by some 20 years. [8]

Amenhotep I Second Pharaoh of the Eighteenth dynasty of Egypt

Amenhotep I or Amenophis I, (,), from Ancient Greek Ἀμένωφις, additionally King Zeserkere, was the second Pharaoh of the 18th dynasty of Egypt. His reign is generally dated from 1526 to 1506 BC. He was a son of Ahmose I and Ahmose-Nefertari, but had at least two elder brothers, Ahmose-ankh and Ahmose Sapair, and was not expected to inherit the throne. However, sometime in the eight years between Ahmose I's 17th regnal year and his death, his heir apparent died and Amenhotep became crown prince. He then acceded to the throne and ruled for about 21 years. Although his reign is poorly documented, it is possible to piece together a basic history from available evidence. He inherited the kingdom formed by his father's military conquests and maintained dominance over Nubia and the Nile Delta but probably did not attempt to maintain Egyptian power in the Levant. He continued the rebuilding of temples in Upper Egypt and revolutionized mortuary complex design by separating his tomb from his mortuary temple, setting a trend in royal funerary monuments which would persist throughout the New Kingdom. After his death, he was deified as a patron god of Deir el-Medina.

Observational mechanics and precession

The Sothic cycle is a specific example of two cycles of differing length interacting to cycle together, here called a tertiary cycle. This is mathematically defined by the formula 1/a + 1/b = 1/t or half the harmonic mean. In the case of the Sothic cycle the two cycles are the Egyptian civil year and the Sothic year.

The Sothic year is the length of time for the star Sirius to visually return to the same position in relation to the sun. Star years measured in this way vary due to axial precession, [9] the movement of the Earth's axis in relation to the sun. The length of time for a star to make a yearly path can be marked when it rises to a defined altitude above a local horizon at the time of sunrise. This altitude does not have to be the altitude of first possible visibility. Throughout the year the star will rise approximately four minutes earlier each successive sunrise. Eventually the star will return to its same relative location at sunrise. This length of time can be called an observational year. Stars that reside close to the ecliptic or the ecliptic meridian will on average exhibit observational years close to the sidereal year of 365.2564 days. The ecliptic and the meridian cut the sky into four quadrants. The axis of the earth wobbles around slowly moving the observer and changing the observation of the event. If the axis swings the observer closer to the event its observational year will be shortened. Likewise, the observational year can be lengthened when the axis swings away from the observer. This depends upon which quadrant of the sky the phenomenon is observed.

The Sothic year is remarkable because its average duration was exactly 365.25 days in the early 4th millennium BC [10] before the unification of Egypt. The slow rate of change from this value is also of note. If observations and records could have been maintained during predynastic times the Sothic rise would optimally return to the same calendar day after 1461 calendar years. This value would drop to about 1456 calendar years by the Middle Kingdom. The 1461 value could also be maintained if the date of the Sothic rise were artificially maintained by moving the feast in celebration of this event one day every fourth year instead of rarely adjusting it according to observation.

It has been noticed, and the Sothic cycle confirms, that Sirius does not move retrograde across the sky like other stars, a phenomenon widely known as the precession of the equinox. Professor Jed Buchwald wrote "Sirius remains about the same distance from the equinoxes—and so from the solstices—throughout these many centuries, despite precession." [11] For the same reason, the helical rising or zenith of Sirius does not slip through the calendar at the precession rate of about one day per 71.6 years as other stars do but much slower. [12] This remarkable stability within the solar year may be one reason that the Egyptians used it as a basis for their calendar. The coincidence of a heliacal rising of Sirius and the New Year reported by Censorinus occurred about the 20th of July, that is a month later after the summer solstice.

Problems and criticisms

Determining the date of a heliacal rise of Sirius has been shown to be difficult, especially considering the need to know the exact latitude of the observation. [3] 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 imprecise. [3]

A number of criticisms have been leveled against the reliability of dating by the Sothic cycle. Some are serious enough to be considered problematic. Firstly, none of the astronomical observations have dates that mention the specific pharaoh in whose reign they were observed, forcing Egyptologists to supply that information on the basis of a certain amount of informed speculation. Secondly, there is no information regarding the nature of the civil calendar throughout the course of Egyptian history, forcing Egyptologists to assume that it existed unchanged for thousands of years; the Egyptians would only have needed to carry out one calendar reform in a few thousand years for these calculations to be worthless. Other criticisms are not considered as problematic, e.g. there is no extant mention of the Sothic cycle in ancient Egyptian writing, which may simply be a result of it either being so obvious to Egyptians that it didn't merit mention or to relevant texts being destroyed over time or still awaiting discovery.

One recent popular history of the Ancient Near East by Marc Van de Mieroop, in his discussion of chronology and dating, does not mention the Sothic cycle at all, and believes that the bulk of historians nowadays would consider that it is not possible to put forward exact dates earlier than the 8th century BCE. [13]

Some have recently claimed that the Theran eruption marks the beginning of the Eighteenth dynasty due to Theran ash and pumice discoveries in the ruins of Avaris in layers that mark the end of the Hyksos era[ citation needed ]. Because the evidence of dendrochronologists indicates the eruption took place in 1626 BC, this has been taken to indicate that dating by the Sothic cycle is off by 50–80 years at the outset of the 18th dynasty[ citation needed ]. Claims that the Thera eruption is the subject of the Tempest Stele of Ahmose I [14] have been disputed by writers such as Peter James. [15]


  1. The date slowly varies within the Gregorian calendar, owing to its omission of three leap years every four centuries. It presently occurs on 3 August. [1]

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  1. 1 2 "Ancient Egyptian Civil Calendar", La Via , retrieved 8 February 2017.
  2. 1 2 Tetley (2014) , p.  42.
  3. 1 2 3 4 5 Kitchen, K. A. The Chronology of Ancient Egypt. p.205. World Archaeology, Vol. 23, No. 2 (October 1991).
  4. 1 2 Tetley, M. Christine (2014), The Reconstructed Chronology of the Egyptian Kings, Vol. I, p.  43, archived from the original on 2017-02-11.
  5. 1 2 Grimal, Nicolas. A History of Ancient Egypt. p.52. Librairie Arthéme Fayard, 1988.
  6. Grimal, Nicolas. A History of Ancient Egypt. p.51. Librairie Arthéme Fayard, 1988.
  7. Gautschy et al. 2017 A New Astronomically Based Chronological Model for the Egyptian Old Kingdom
  8. Grimal, Nicolas. A History of Ancient Egypt. p.202. Librairie Arthéme Fayard, 1988.
  9. Ingham, M. F. "The Length of the Sothic Cycle", The Journal of Egyptian Archaeology, 55 (1969), p. 36-40.
  10. SkyCharts III
  11. Buchwald, Jed Z. (2003), "Egyptian Stars under Paris Skies", Engineering and Science (Caltech), 66 (4) (2003) 20–31.
  12. One day per 120 years, see Winlock H., Origin of the Ancient Egyptian Calendar, Proc. of the Am. Philosophical Soc., 83 (1940): 447-64.
  13. Marc Van de Mieroop A History of the Ancient Near East, ca. 3000-323 BC (2015) Wiley-Blackwell, Oxford. ISBN   978-1118718162
  14. Ritner, Robert K; Moeller, Nadine (2014). "The Ahmose 'Tempest Stela', Thera and Comparative Chronology". Journal of Near Eastern Studies. 73 (1): 1–19. doi:10.1086/675069. JSTOR   10.1086/675069.
  15. James, Centuries of Darkness (London, 1991: ).