The 5th millennium has become a startpoint for calendars and chronologies, though only one has any basis in reality. The year 4750 BC is the retrospective startpoint for the Assyrian calendar, marking the traditional date for the foundation of Assur, some 2,000 years before it actually happened.
Another traditional date is 19 July 4241 BC, marking the supposed beginning of the Egyptian calendar, as calculated retrospectively by Eduard Meyer. The more likely startpoint is 19 July 2781 BCE, one Sothic cycle later. It has generally been believed that the calendar was based on a heliacal (dawn) rising of Sirius but that view is now being questioned.
According to the Ussher chronology, the creation of Earth happened on 22/23 October 4004 BC. This chronology was the work of James Ussher, whose basis was the dates in the Old Testament of the Bible. He estimated that the universe was created by God at either 18:00 on the 22nd (Jewish calendar) or 09:00 on the 23rd (Ussher-Lightfoot-Chronology).
The only exact date in the 5th millennium is Monday, 1 January 4713 BC, the beginning of the current Julian Period, first described by Joseph Justus Scaliger in the sixteenth century. This Julian Period lasts 7,980 years until the year 3268 CE in the next millennium. It is a useful device for date conversions between different calendars. The date of origin has the integer value of zero in the Julian Day Count: i.e., in the Julian Calendar; the equivalent date in the Gregorian Calendar is 24 November 4714 BC
Sothic Cycle of Sirius
Sopdet is the ancient Egyptian name of the star Sirius and its personification as an Egyptian goddess. Known to the Greeks as Sothis, she was conflated with Isis as a goddess and Anubis as a god.
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. 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.
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. 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. 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.
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 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.
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