The Egyptian calendar

The earliest Egyptian calendar was based on the moon's cycles, but the lunar calendar failed to predict a critical event in their lives: the annual flooding of the Nile river. The Egyptians soon noticed that the first day the "Dog Star," which we call Sirius, in Canis Major was visible right before sunrise was special. The Egyptians were probably the first to adopt a mainly solar calendar. This so-called 'heliacal rising' always preceded the flood by a few days. Based on this knowledge, they devised a 365-day calendar that seems to have begun in 4236 B.C., the earliest recorded year in history.