For a period of over two millennia, from the building of the Giza pyramids till the suicide of Cleopatra, the Ancient Egyptian calendar was in continual operation, the dates of significant events recorded assiduously by a priestly bureaucracy. No other ancient calendar can claim such longevity and astronomers continued to use it up to the time of Copernicus to determine the times of Astronomical events.
Such fastidious reckoning has been of enormous benefit to archeologists intent on determining a chronology of the period, though problems of interpretation remain. The priestly scribes did not have a single reference point from which to count the years but counted them afresh from the time of accession of each new pharaoh. Archeologists have therefore had to rely on the records of Pharaonic succession to establish continuity and these may have been subject to later political tampering and revisionism, while during periods of civil collapse, the records are fragmentary.
The calendar was based on 12 months, each of 30 days divided into three weeks of 10 days, with five extra days appended at the end of each year to give 365 days in all. The months were given the names of presiding deities so that the year began on the first day of the month of Thoth.
However, over the large periods of time reckoned by this calendar, seasonal events such as the vernal equinox or the flooding of the Nile were seen to track back slowly through the months of the calendar.
This discrepancy arises from the fact that our planet takes longer than 365 days to complete its orbit round the sun: it needs an extra ¼ day to return to its original position relative to the sun and stars. In 4 years, it is therefore a day away; in 30 X 4 years, ie. 120 years, a whole month away. Only after 365¼ X 4 years, ie 1461 years will the seasonal year start again on the first day of the month of Thoth.
This failure of the civil calendar to tally with the changing seasons meant that the priests had to use another method to track seasonal events. They did this by observation of the stars. As our planet spins, the stars seem to rise in the East, move across the celestial sphere and set in the West. The moment when a star first appears over the Eastern horizon at the end of the night, just before the sun follows and obliterates it with daylight brilliance, is called the heliacal rising of the star – at this moment the star is about 6º above the horizon, and the sun is following about 5º below the horizon. On subsequent nights, due to the Earth’s orbit round the sun, the star will seem to distance itself progressively from the sun at sunrise and approach the sun at sunset until it finally disappears from view for a period of 70 days and then reappears again on the other side of the sun just before sunrise, it’s heliacal rising once again.
The heliacal rising of Sirius, by far the brightest star in the sky, known as Sopdet to the Egyptians and Sothis to the Ancient Greeks, took on particular significance as it occurred just before the seasonal flooding of the Nile. It became associated with Isis, goddess of rebirth, rising just after the constellation of Orion, associated with Osiris, god of the underworld, reborn through the agency of Isis, his consort. Both straddle the Milky Way, symbol of the life generating Nile, fed by the tears of a grieving Isis. The temples of Isis in Philae and Hathor in Dendera are both believed to have been oriented in line with the heliacal rising of Sirius. The 70 day period during which Sirius disappeared from the night sky before its heliacal rising again was also the period of time between mummification and entombment, when through the ceremony of the opening of the mouth, the dead person was believed to reawake to eternal life in the underworld.
The date of the helical rising of Sirius was taken as the beginning of the Egyptian calendar on the first day of Thoth, but because of the discrepancy between the civil calendar and the Earth’s orbit with respect to the sun and stars, this moment tracked back through the calendar in a cycle of 1461 years called the Sothic cycle. A moment when it synchronized with the civil calendar was recorded in 139 CE, which acts as a fixed point to anchor the chronology of Ancient Egypt; tracking back we can calculate that other synchronizations must have occurred in 1332 BCE and 2782 BCE.
Thus the helical rising of Sirius was an accurate reference point for the Earth’s orbit round the sun and could be used to predict seasonal events, while knowledge of the Sothic cycle allowed this siderial calendar to be synchronized with the bureaucratic calendar with its discrepancy of ¼ day .
Although there is a small shift in the heliacal rising of Sirius with respect to the seasons, the cycle is far too slow to be detected by the ancient Egyptians. It is, however, significant enough to cause a difference between their time and ours. This is due to precession of the Earth which creates a wobble in its spin, taking 25 800 years to complete. In the first century CE the heliacal rising in Memphis took place around the 17th July; it now takes place around the 4th August.