Astronomy & The Calendar
In the Life of Ancient Egypt
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.
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