The annual flooding of the Nile was the foundation of Egyptian civilization and agriculture, so predicting this occurrence with accuracy was the driving force behind the development of Egyptian astronomy. Once again, their studies of the heavens became intertwined with religion, esoterica and the priesthood.

EARLY EGYPTIAN ASTRONOMY

The history of Egyptian astronomy begins in the depths of prehistory and the discovery of stone circles at Nabta Playa, dating from the 5th Millennium BC, show that the Egyptians had already developed a calendar. The stone circle shows that they were accomplished at marking time and, it can be assumed, predicting the coming of the floods. They also developed a system of constellations that appear to be of native origin and independent from the work of the Greeks and the Mesopotamians.

Stellar Clock, used to plot stellar time. (Public Domain)

The Egyptians were fully aware that the year was about 365 days, and divided it into 12 months of 30 days, with five ceremonial, intercalary days, called the Epagomenal days, added. This calendar lost one day every four years, and they made little attempt to correct this. Instead, they developed another calendar based around the star Sirius, which also consisted of 365 days but which included the extra quarter day. Strangely, they also kept a 360 day ceremonial calendar, running concurrently with the others, and these calendars coincided every 1461 years, which was seen as a time of great celebration and the start of a new age.

Egyptian astronomers used sundials to tell the time, dividing the days into 24 hours, although the length of these hours was not fixed, ensuring that day and night both consisted of twelve hours, whatever the time of year. At night, they divided the night sky into 36 groups of stars, called Decans, which could be used to tell the time.

Of Egyptian physics and chemistry we know nothing, and almost as little of Ancient Egypt Astronomy.

The stargazers of the temples seem to have conceived the earth as a rectangular box, with mountains at the corners upholding the sky.

They made no note of eclipses, and were in general less advanced than their Mesopotamian contemporaries.

Nevertheless they knew enough to predict the day on which the Nile would rise, and to orient their temples toward that point on the horizon where the sun would appear on the morning of the summer solstice.

Perhaps they knew more than they cared to publish among a people whose superstitions were so precious to their rulers; the priests regarded their astronomical studies as an esoteric and mysterious science, which they were reluctant to disclose to the common world.

For century after century they kept track of the position and movements of the planets, until their records stretched back for thousands of years.

They distinguished between planets and fixed stars, noted in their catalogues stars of the fifth magnitude (practically invisible to the unaided eye), and charted what they thought were the astral influences of the heavens on the fortunes of men.

From these observations they built the calendar which was to be another of Egypt's greatest gifts to mankind.

They began by dividing the year into three seasons of four months each: first, the rise, overflow and recession of the Nile; second, the period of cul¬tivation; and third, the period of harvesting.

To each of these months they assigned thirty days, as being the most convenient approximation to the lunar month of twenty-nine and a half days; their word for month, like ours, was derived from their symbol for the moon.

At the end of the twelfth month they added five days to bring the year into harmony with the river and the sun.

As the beginning of their year they chose the day on which the Nile usually reached its height, and on which, originally, the great star Sirius (which they called Sothis) rose simultaneously with the sun.

Since their calendar allowed only 365, instead of 365 '/$, days to a year, this "heliacal rising" of Sirius (i.e., its appearance just before sunrise, after having been invisible for a number of days) came a day later every four years; and in this way the Egyptian calendar diverged by six hours annually from the actual calendar of the sky.

The Egyptians never corrected this error. Many years later (46 B.C.) the Greek astronomers of Alexandria, by direction of Julius Caesar, improved this calendar by adding an extra day every fourth year; this was the "Julian Calendar.

Under Pope Gregory XIII (1582) a more accurate correction was made by omitting this extra day (February zpth) in century years not divisible by 400; this is the "Gregorian Calendar" that we use today.

Our calendar is essentially the creation of the ancient Near East, astronomy.

Source: http://www.love-egypt.com/

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.

Bernard R. Ortiz de Montellano

In 1976 Robert Temple published the Sirius Mystery claiming that the extraordinary astronomical knowledge of the Egyptians and the Dogon of Mali(1) was due to visitations from inhabitants of the Sirius system. These claims were dealt with in a article in The Skeptical Inquirer (Ridpath 1978). Since that time, however, the Afrocentrist movement has revived and expanded these claims (Adams 1983a; 1983b; 1990; Van Sertima 1983; Ortiz de Montellano 1991), and they have been naively parroted in more mainline publications (Gebre-Egziabher 1993/1994; Harding 1991). Adams (1990: 60) briefly presents the current claims:

They knew of the rings of Saturn, and the moons of Jupiter, the spiral structure of the Milky Way, where our star system lies. They claimed that billions of stars spiral in space like the circulation of blood in the human body... Perhaps the most remarkable facet of their knowledge is their knowing intricate details of the Sirius star system, which presently can only be detected with powerful telescopes. The Dogon knew of the white dwarf companion star of Sirius, the brightest star in the sky. They knew its approximate mass ("it is composed of 'sagala,' an extremely heavy, dense metal such that all the earthly beings combined cannot lift it') its orbital period (50 years) and its axial rotation period (one year). Furthermore, they knew of a third star that orbits Sirius and its planet [sic]. The X-ray telescope aboard the Einstein Orbiting Observatory recently confirmed the existence of the third star. (2) The Dogon with no apparent instrument at their disposal, appear to have known these facts for at least 500 years.

Claims that the Dogon knew these things for at least 700 years (not 500) and that the ancient Egyptians also possessed this knowledge were first made in Adams (1983a) and endorsed by Van Sertima (1983). The sole source this information about Dogon astronomical knowledge is the research of two French anthropologists, Marcel Griaule and Germaine Dieterlen (1950; 1965), and more directly the book by Temple (1976).