Sopdet, better known under the name the Greeks gave her, Sothis, was the personification of the brightest star in the night sky, usually referred to as Sirius, the Dog Star. She is depicted as a woman with a star on her head, sometimes also with the horns of a cow.
Every year she was absent from the sky for 70 days. The first day of her reappearance (becoming visible just before sunrise after having moved far enough away from the glare of the sun, the so-called heliacal rising, taking place on July 19 or 20) marked the beginning of the new year and gave the goddess the title Lady of the New Year.
The goddess is depicted as a cow on an ivory tablet from the first dynasty; here she has a plant between her horns (which, as a hieroglyph, can be read as "opening of the year"). Therefore it has been concluded that already at the beginning of history she was associated with the new year.
Sopdet was member of a triad with her husband Sah (the personification of the constellation of Orion) and their son Soped, and these were often associated with Osiris, Isis and Horus. Her absence from the sky was linked to the period of 70 days during which Osiris was embalmed or disappeared before his resurrection, a period in which, as some texts state, Isis disappeared with him to mourn at his bier.
The fact that the Egyptian based the beginning of the year on the reappearance of the goddess caused serious problems. The Egyptian calendar consisted of 12 months of 30 days plus 5 extra days, so a total of 365 days. But the exact solar year is a bit longer: 365 days, 5 hours, 48 minutes and 46 seconds. This means that the theoretical or civil year, as used by the Egyptians, advanced circa ¼ of a day each year, compared to the solar year. In other words, every 4 years they would be ahead of the solar year 1 full day, after 120 years they would be ahead 30 days etcetera (texts are known in which the writer complains that it is winter in the middle of summer). Only after 1460 years the heliacal rising of Sothis and New Year's day would coincide again. This period is known as the Sothic cycle.
We know from dated documents that the heliacal rising and New Year’s day coincided in 139 AD, during the reign of Antoninus Pius. This means that previous occurrences were in 1322 BC and 2782 BC (accounting for a slight difference between a solar year and a Sothic year). We do have several records from ancient Egypt about heliacal risings:
- In one of the records Sopdet is said to have appeared during the reign of Thutmoses III on day 28 of the 11th month; the year is not mentioned; it can be calculated that the text describes the situation in 1458 BE (if the rising was observed in Memphis) or 1438 BE (if seen from Thebes).
- Another record is a medical papyrus from the early 18th Dynasty. The reappearance of Sopdet is here said to have occurred on the 9th day of the 11th month of year 9 of Amenhotep I, which corresponds to either 1538 BC (if the observation was done at Memphis) or 1518 BC (if done at Thebes).
- A third record is a letter to a priest in Kahun, stating that the goddess will rise on the 16th day of the 8th month of year 7 of king Sesostris III, which corresponds to 1866 BE.
These dates, and information from other sources, help us to establish a more precise time table of Egyptian history.