Like most early civilizations, the ancient Egyptians measured the passing of time by the phases of the moon. Their first calendar was based on the lunar cycle – the 29 or 30 days from one new moon to the next. A year consisted of three four-month seasons, and the new year was heralded by the star they called Sopdet that appeared just above the horizon at dawn around the time of the annual Nile flood.
The lunar calendar was not without problems: The first day of each new month was unpredictable, and no one knew in advance exactly how many days a particular month would have. Days or even weeks might pass between the end of the last lunar month of the year and the reappearance of Sopdet. For most people in this agricultural society, these were minor inconveniences, but the civil bureaucracy needed a more consistent system – a year with a fixed number of days, not subject to the variations of moon and stars.
Around 2900 BC, a civil calendar was adopted based on a solar year of 365 days. It had 12 months of 30 days each – with three 10-day weeks – plus five days between the old and new years set aside for religious feasts. Years were numbered consecutively within the reign of each pharaoh.
But like any calendar divided into days, it missed the sun’s exact annual cycle by about six hours. The result was that, over the course of four years, the civil calendar crept one full day ahead of the true solar year. Once its inaccuracy became obvious, the civil calendar was probably just ignored by farmers and other simple folk, even though the government was tied to its errant schedule. Eventually, about 2500 BC, an official lunar calendar was installed side by side with the civil calendar. It served mainly to schedule religious events and the lunar feast day that gave each month its name.
To measure the passage of time during daylight hours, the Egyptians had sun clocks, similar to a sundial. Some Middle Kingdom coffins were decorated with star clocks – a list of stars known as decans that were identified with different hours of the night at various times of the year.
During the reign of the New Kingdom pharaoh Amenhotep I, clocks that kept time without reliance on the sun or stars made their appearance. These clocks consisted of a water-filled vase with a very small perforation in the bottom. The inside of the vase was inscribed at different levels to mark the hours. These water clocks were often carved in the shape of a baboon, an animal representing the god Thoth, who was associated with the measurement of time. It is interesting to note that except for the abstract concept of at or moment, the Egyptians – for all their skill as timekeepers – had no name for a unit of time shorter than an hour.