Sothic cycle
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] The sidereal year of 365.25636 days is valid only for stars on the ecliptic (the apparent path of the Sun across the sky) and having no proper motion, 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.
Censorinus described it in his book De Die Natale, in CE 238, and stated that the cycle had renewed 100 years earlier on the 12th of August.
Isaac Cullimore, an early Egyptologist and member of the Royal Society, published a discourse on it in 1833 in which he was the first to suggest that Censorinus had fudged the terminus date, and that it was more likely to fall in CE 136.
In 1904, seven decades after Cullimore, Eduard Meyer carefully combed known Egyptian inscriptions and written materials to find any mention of the calendar dates when Sirius rose at dawn.
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.
[7]: 52 Gautschy et al. (2017) claimed that a newly discovered Sothis date from the Old Kingdom and a subsequent astronomic study confirms the Sothic cycle model.
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 extended some 20 years.
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.
Throughout the year the star will rise to whatever altitude was chosen near the horizon approximately four minutes earlier each successive sunrise.
The Sothic year is remarkable because its average duration happened to have been nearly exactly 365.25 days, in the early 4th millennium BCE[10] before the unification of Egypt.
The coincidence of a heliacal rising of Sirius and the New Year reported by Censorinus occurred about 20 July, that is a month after the summer solstice.
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.
[citation needed] Because the evidence of dendrochronologists indicates the eruption took place in 1626 BCE, 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 described on the Tempest Stele of Ahmose I[14] have been disputed by writers such as Peter James.
