Astronomy in Ancient Egypt

Astronomy in Ancient Egypt

The Egyptians observed that the stars make a complete turn in little more than 365 days. In addition, this cycle of 365 days of the Sun agrees with that of the seasons, and already before 2500 BC the Egyptians used a calendar based on that cycle, so presumably, they used astronomical observation systematically since the fourth millennium.

The Egyptian calendar year had 12 months of 30 days, plus 5 days called epag√≥menos. The difference, then, was ¼ day compared to the solar year. They did not use leap years: 120 years later one month went ahead, in such a way that 1456 years later the calendar year and the astronomical one came back to coincide again.

The Nile began its flood more or less at the moment in which the star Sothis, our Sirius, (the Sepedet of the Egyptians), after having long been invisible under the horizon, could be seen again shortly before the sunrise.

The Egyptian calendar had three seasons of four months each: 
  • Flood or Akhet.
  • Winter or Peret, that is, "exit" from the lands out of the water. 
  • Summer or Shemu, that is, "lack of water".
The opening of the Egyptian year occurred on the first day of the first month of the Flood, approximately when the star Sirius began again to be observed a little before sunrise.

From the end of the Egyptian era (144 AD) are the so-called Carlsberg papyri, which contains a method to determine the phases of the Moon, from very ancient sources. In them, a cycle of 309 lunations is established for every 25 Egyptian years, in such a way that these 9,125 days are arranged in groups of lunar months of 29 and 30 days. The knowledge of this cycle allowed the Egyptian priests to place lunar mobile parties in the civil calendar.

The orientation of temples and pyramids is another proof of the type of astronomical knowledge of the Egyptians. Pyramids were built like the one at Gizeh, aligned with the polar star, with which it was possible to determine the start of the seasons using the position of the shadow of the pyramid. They also used the stars to guide navigation.

The legacy of Egyptian astronomy reaches our days 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 it from the stars."

The insightful observation of stellar and planetary movement allowed the Egyptians to draw up two calendars, one lunar and the other civil. The Julian calendar and, later, the Gregorian calendar - the one we are currently using - are nothing more than a modification of the Egyptian civil calendar.

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