The astronomy of ancient Egypt
If only the Library of Alexandria had withstood wars and conquests! Unfortunately, our knowledge of astronomy of ancient Egypt are very limited and we just few papyrus and a few inscriptions on tombs or temples.
In Egyptian mythology, the goddess Nut represents the sky, the air Shou and Geb the earth. This image shows part of the Greenfield Papyrus (1025 BC) where we see the body of Nut, held in place by Shou, and that of Geb, the ground.
A solar year of 365 days
At the time of ancient Egypt, The Nile flood occurred every year around July 19. Coincidentally, it is also at this time that the brightest star in the sky, Sirius, called Sothis in Greek and Egyptian Sopdet had its helical rising and thus made his first appearance of the year. As the Nile flood would fertilize the land and feed the people, observing the helical rising of Sirius, and more generally of the night sky, became a key element of the Egyptian civilization.
Based on their time measuring the apparent movement of the Sun, rather than the cycles of the moon, the Egyptians invented the solar calendar. As the helical rising of Sirius occurred approximately every 365 days and nights, they divided the year into 365 days. As the cycle of the moon lasted about 30 days and nights, they divided the year into 12 months of 30 days each month is still divided into three decades of 10 days.
Finally, to reach a total of 365, they added five additional days, called epagomenal days, which became day celebration of the gods Osiris, Seth, Isis, Nephthys and Horus.
As the astronomical year does not last exactly 365 days, the Egyptian calendar gently drifting over the cycle of the sky, about one day every four years. The Nile flood therefore coincided with the official start of the year that every 1,460 years, a length of time we named the Sothic period.
It was not until Julius Caesar established the Julian calendar and leap year in 45 BC that the timeframe be better aligned with the stars.
A 24-hour day
Egyptians also invented division of the day into 24 hours. To better get into the sky and measure the passage of time, they cut the sky in small groups recognizable stars who got up one after the other during the night. To coincide with the decades of 10 days, each group of stars was chosen so that its helical rising is separated from the previous 10 days. So we had 36 groups of stars, which named the decans.
Since the length of the night depends on the season, the number of observable decanates overnight varies. But at the beginning of the summer, at the time of the heliacal rising of Sirius, the night lasts about 8 hours and only 12 decans are observable. This number was taken - in somewhat arbitrary - as the basis of the new system. The principle was extended to the day itself divided into 12 hours. Thus the Egyptians established the 24-hour day that we still use.
Egyptian myths
The Egyptians had a very rich and often associated with celestial phenomena mythology. In some texts, the world was a big rectangular box whose north and south sides were longer. This box was a flat ceiling supported by four pillars. These were connected by a mountain range and a celestial river flowed quietly in a projection along this chain. Boats plying the celestial river and carried the Moon, Sun and planets.
In another interpretation, it was the body of the goddess Nut, deployed on top of the world, arms and legs spread, which formed the heavens. Her uterus engendered every morning the sun is at its mouth and swallowed the sunset to the west. Geb, god of the Earth, was lying under Nout, his wife and sister.
All celestial bodies were usually associated with deities. The Sun representing different gods depending on its position in the sky, Khepri at dawn, noon and Atum Re at night. Moon represented herself as several deities, Aah, Thoth and Khonsu.
The constellation of Orion was of particular importance and evoked Osiris, the first child of Nut and Geb, god of death and renewal. The death of Osiris and his rebirth were powerful symbols of annual drought Egypt always followed the Nile River and fertilization of the land.
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