With no doubt the Egyptians have been the first to invent a solar and rational calendar.
Their year was made of 365 days and divided in 12 months of 30 days each, to which there were added, at the end of the yearly cycle, 5 supplementary days or “epagomeni”. The months were divided in three groups of four months that coincided with the three seasons.
The flood season akhet
The germination season pert
The heat season shem
Only in the low epoch a name was given to each month:
Thot, Paophi, Athyr, Choiak for the flood season;
Tybi, Meshir, Phmenoth, Pharmuthi for the germination season
Pachons, Payni, Epiphi, Mesori for the third season, the summer.
So, to appoint a certain date, they gave the year of the reign, the month of the season and the day.
For example: IX year of Zoserkare Amenhotep (I), third month of heat (Epiphi), 9th day, is according to Eber’s medical papyrus, the day of the eliaca rise of Sothis (Sepet), which is the star Sirio.
Actually the Egyptian year began the day when Sirio comes out of the horizon at dawn. This phenomenon, called the “eliaca rise of Sothis” corresponds approximately with the flood of the Nile, which was very important for the economy of these people of farmers, because it depended on the floods. This date in fact (the 19th of July in the giuliano calendar or the 15th of June of our calendar, at the latitude of Memphis) marked the beginning of the year.
As the Egyptian year had only 365 days, against the 365 days and 6 hours of the cycle of the sun and of Sirio, the conventional beginning of the year was moved ahead of a day every 4 years.
So, 1461 years were necessary, so the cosmic and official year coincided again.
Of course the Egyptians had noticed this displacement that brought the summer marked on the calendar in the middle of winter and so for this reason they called the official year “vague year”.
Anyway, for the work in the fields, the farmers always referred only to the natural cycle of the seasons. The cycles of 1461 days, called “sotiaci periods” have helped the specialists in the research of the precise date of the introduction of the calendar. We know for sure that the eliaca rise of Sothis coincided with the first day of the official calendar in 139 AC. A simple calculation proves that this coincidence had already happened in 1322 BC (beginning of the XIX dynasty), in 2783 BC (the end of the tinita period and the beginning of the Ancient Reign) and in 4244 BC. The first date is too far, regarding the second, the Texts in the Pyramids in Saqqara let us suppose that in 2783 BC, the calendar had already been made.
The qualified scientists have so accepted the date of 4244 BC, that, according to some of them, should be the first historical certain date.
The invention of the calendar is attributed to the priests in Eliopoli who, according to the experts, have made it in the pre-dynastic period, in an epoch when the city was the capital of the unified reign.
The German mathematician Naugebauer has established the unacceptability of this date. The carbon 14 analysis in fact, has showed that actually it takes us back to the Neolithic of Fayyum and it’s not possible to admit the invention of the calendar at that time. So, if there was an eliopolitan domination in the medium predynastic period, we have to lower of a millennium the presumed date of the introduction of the calendar, definitely excluding the 4244 BC.