The Egyptians observed the stars made a complete turnaround in just over 365 days. Furthermore, this 365-day cycle of the Sun agreed or was in sync with the seasons, and already before 2500 BC the Egyptians used a calendar based on that cycle, so presumably used astronomical observations to systematically since the fourth millennium.
Egyptian civil year had 12 months of 30 days plus 5 days called epagómenos. The difference, then, was about ¼ day solar year. Not use leap years: 120 years later, one month ahead, so that 1456 years after the calendar year and returned to the astronomical match again.
The Nile began his rise more or less at the time the star Sothis, our Sirius (the Sepedet of the Egyptians), having long been invisible under the horizon, could be back shortly before leaving the Sun
The Egyptian calendar had three seasons of four months each: -Flood or Akhet. -Winter or Peret, ie, 'exit' of the land outside the Summer or Shemu water.-ie, "lack of water."
Cycles in the Egyptian Calendar
The opening of the Egyptian year occurred the first day of first month of the Flood, about the time the star Sirius began again to be seen just before sunrise
In the late Egyptian period (144 AD) are called Carlsberg Papyri, which includes a method for determining the phases of the moon, from ancient sources. They establish a cycle of 309 lunations per 25 Egyptian years, so these 9125 days are arranged in groups of 29 lunar months and 30 days. Understanding this cycle allows the Egyptian priests put on the civil calendar of movable feasts moles.
The orientation of temples and pyramids is another evidence of the kind of astronomical knowledge of the Egyptians: the faces of pyramids were built of Giza, aligned with the North Star, with which they could determine the onset of the stations used for this the position of the shadow of the pyramid. They also used the stars to guide navigation.
The legacy of Egyptian astronomy to this day in the form of the calendar. Herodotus in his Histories says: "The Egyptians were the first of all the men who discovered the year, and said that they found from the stars."
The keen observation of stellar and planetary motion allowing Egyptians to the development of two calendars, a lunar and a civilian. The Julian calendar, and later the Gregorian - the one we use today - are but a modification of the Egyptian civil calendar.