The flooding and receding of the Nile River played a vital role in shaping ancient Egyptian life. The annual inundation cycle, which brought rich, fertile soil and bountiful harvests, was closely observed by the Egyptians. They noticed that the cyclical flooding of the Nile coincided with the appearance of certain stars and celestial events.
Observation of Sirius (Sothis)
One crucial celestial observation made by the Egyptians was the heliacal rising of the star Sirius, also known as Sothis. Sirius is the brightest star in the night sky and its heliacal rising occurs when it emerges above the eastern horizon just before sunrise. The ancient Egyptians observed that the heliacal rising of Sirius coincided with the beginning of the annual Nile flood.
Creation of the Sothic Calendar:
The Egyptians realized the significance of this astronomical phenomenon and based their calendar on it. The Sothic calendar, also known as the Sirius calendar, was developed. Each Sothic year consisted of 365 days and was aligned with the heliacal rising of Sirius.
Adjustment for accuracy
While the Sothic calendar was initially quite accurate, over time, small discrepancies emerged due to the slight difference between the actual solar year (365.242 days) and the 365-day Sothic year. To account for this, the Egyptians incorporated a "leap day" every four years, similar to the concept of leap year in the modern Gregorian calendar.
Fixed calendar
The Egyptian calendar was essentially a fixed calendar, meaning that the months and days were not linked to the lunar phases as in many other ancient calendars. Instead, the calendar was based on the constant and predictable cycle of the Nile's inundation and the heliacal rising of Sirius.
Significance:
The Nile River and its annual inundation cycle were of paramount importance to the ancient Egyptians, and the creation of the calendar based on celestial events related to the Nile allowed them to accurately predict the timing of the flood, plan their agricultural activities, and regulate their societal and religious events.