To raise heliacal of Sirius

In the night sky of Egypt, the most brilliant star of all is Sirius (or Sothis if one uses the hellenized shape of the Egyptian). It is a star doubles represented in the shape of a small bitch ( canicula in Latin) pertaining to the Constellation of the Large Dog ( canis in Latin).

The Sun approaches Sirius, until entering in conjunction with it at the beginning of July. As from this day, Sirius is invisible with any terrestrial observer during a few tens of days, until it carries out its Lever heliacal , i.e. its reappearance in the gleams of the paddle.

During the predynastic period, this heliacal rising coincided with the beginning of the Crue of the the Nile observed with Thèbes towards the 20 June 25th, therefore with the summer solstice in our hemisphere. The simultaneous reappearance of the most brilliant star and water had a significance highly symbolic system. It is also the hottest period of the year, from where the name of heat wave (derivative of canicula) given by the Romans, and which we use to define one period of hot summer days.

Thanks to the development of the atmospheric models, one can today go back with precision the occurrence from the heliacal rising to any star visible with the naked eye according to the site chosen for the observation.

If to raise it heliacal of the star Sothis is a more or less fixed phenomenon (makes some slightly variable according to the Précession of the equinoxes with delay of 14 days per millenium) during one solar year (year of approximately 365,25 days), the new year of the Egyptian calendar (365 days year), 1st Thôt , is mobile. The shift between the solar year and the calendar year was of approximately a day every four years. It thus took 1460 years (365 X 4) so that an event of the Egyptian calendar coincides again with an event of the solar calendar.

Thus, theoretically, every 1460 years, the new year of the Egyptian calendar coincided with the heliacal rising of the Sothis star. These years, as those where the new year coincided with the flood, were regarded as extremely beneficial and were often immortalisées on the low-reliefs. These facts make it possible to rather precisely go back the reigns to certain Pharaons. This 1460 years period is called sothiaque period .

Consequently, the indication of a date of raising heliacal of Sirius allows its absolute dating, knowing that, according to the grammairien and chronologist Censorin, such a coincidence having taken place in 139, one can calculate that the preceding ones took place in -1317, -2775 and -4235. It is then enough to know the date of the heliacal rising in the Egyptian civil calendar to know at which temporal distance one is forthcoming coincidence.

For example, a text of announces a heliacal rising, the 16 of the 8th month of year 7 of the reign of Sésostris {{III}}. The advance to be achieved until next coincidence is thus composed the 14 days remaining of the 8th month, 120 the last 4 months days, the 5 days epagomenes and of the day of apokatastasis, that is to say 140 days in all. At a rate of one day every 4 years, one thus finds that year 7 of the reign of Sésostris was at 560 years (140 X 4) of next coincidence, that of -1320, that is to say the year -1880, in perfect agreement with the other estimates of the dates of this reign.

Nowadays a certain number of criticisms are formulated against the dating via the sothiaque period. One is thus not sure that the civil calendar remained unchanged lasting of the centuries, one did not find explicit mention of this cycle in the writings predynastic and the date of -4235 as creation date of the calendar seems contradicted by archeology.

Today with the precession of the equinoxes, to raise it heliacal of Sirius has in August place with the latitude of the Cairo whereas the rising (without the Barrage of Aswan) always takes place about on June 25th. The heliacal rising of Sirius thus has six good weeks of delay on the rising of the Nile, and the bond between the two events is broken.

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