The Star That Announced the Flood

For about seventy days each year, Sirius, the brightest star in the night sky, vanishes from view, lost in the glare of the sun. Egyptians called it Sopdet. Its return each summer, rising just before dawn for the first time in over two months, came just as the Nile began its annual flood.

That flood was the foundation of Egyptian farming: it left behind a layer of rich black silt, and once the water receded, farmers sowed their fields in it. It is little surprise, then, that Sopdet's reappearance became tied to the Egyptian New Year, with the civil calendar apparently set up in a year when the star's return coincided with New Year's Day itself.

That calendar counted 365 days: twelve months of thirty days, arranged in three seasons of four months each, plus five extra days added at the end, said to be the birthdays of five gods. It is strikingly close to the 365-day year still in use today. The one thing it lacked was a leap day, so over the centuries it slowly drifted out of step with the star that had inspired it, sliding roughly one day off course every four years.

A modern watch with a calendar function still counts the same 365 days Egypt did four and a half thousand years ago. The finest ones also carry the correction ancient Egypt's calendar never had.