24-Hour Division of the Day:
One of the two lasting contributions of the Egyptians to astronomy (in the large sense) is the 24-hour division of the day. This convention is the end product of a large number of events which are quite impossible to summarize in a few sentences. However, the basic fact to remember is the following. While at least three different systems for astronomical reference were invented in antiquity–the zodiac by the Mesopotamians, the lunar mansions in India, and the decans in Egypt–it was the decan–lO°-intervals along the ecliptic–which led to the division of the night (period of complete darkness) into 12 equal parts and ultimately the entire sidereal day into 24 hours. These decans, as they were called by the Greeks, were originally constellations rising helically 10 days apart. (Then, after a period of nightly visibility, they were invisible for approximately 70 days, then arose heliacally again, etc.) It was an accident, so to speak, which led to the selection of the specific constellations which were selected. Once the selection had been made, however, Egyptian astronomers were mathematically forced into a 12-part equal division of the night and ultimately the 24-hour day.
A Fixed and Constant Year of 365 Days:
In this instance, a common sense desire for something fixed in the calendaric jungle led (quite early) to the adoption of a 360-day year, to which were added 5 extra days (for feasting), which the Greeks called ‘epagomanal’ days, making a total of 365 days. Since the mean sidereal year is approximately 365 1/4 days, even this fixed calendar fell behind the sun about 1 day every 4 years. Thus the calendar rotated over a period of 1460 years, back to its original position in relation to the position of the sun. Yet, despite this obvious drawback, the idea of a fixed year remains to this day. The obvious advantage of this ‘year’ is that no extra, intercalated months were ever needed, and this is no doubt the principal reason for its attraction.