The early astronomy of ancient Egypt is known to us from its practical application to time measurement, in the large sense of a calendar year and in the smaller of the 24 h day. The earliest calendar year was lunar, kept in place in the natural year by the star Sirius. From this lunistellar year evolved the well-known calendar year of 365 days (three seasons of four 30-day months and 5 days added at the end). The division of the 30 day month into three 10-day `weeks', combined with the observation of stars called decans rising at nightfall, eventually resulted in our 24 h day of fixed length. Constellations, except for decanal stars, and planets figured only in mythology. The zodiac was introduced into Egypt apparently in the Ptolemaic period and the decans finally became merely names for thirds of a zodiacal sign. In this latest period true astronomical texts also appear but they cannot be counted Egyptian in origin.