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Egyptian Calendar
| Egypt assignment index page | Intro | Pyramids | Calendar | Mathematics | Medicine |
The ancient Egyptians set the first day of their calendar year by the heliacal rising of Sirius -- the brightest star in the sky (which they called Sothis). Around 2000 B.C., Sirius rose heliacally around June 7, a time of year which happened to just precede the usual annual flooding of Nile (the main event in their agricultural cycle). Reminder: You learned about heliacal rising in last week's assignment, Stars over Ancient Babylon.
The Egyptians developed a 365-day solar calendar, fashioned on practical agricultural grounds rather than astronomical complexities. Each year consisted of exactly 365 days, with no intercalations of extra days or months added here or there to coordinate the calendar with lunar months. Unlike the Babylonian luni-solar calendar (see Stars over Ancient Babylon), it didn’t seem to bother the Egyptians when the months wandered through the seasons of the year. Yet the absence of intercalation, because it was practiced unevenly by Babylonians and Greeks, made the Egyptian calendar the standard of later astronomers throughout antiquity. Ptolemy employed it in 150 A.D., and Copernicus still used it in the sixteenth century A.D.!
How many "decades" were in an Egyptian year? The Egyptians divided the 365 days of the year into 36 periods of 10 days each. These 10-day periods were called “decades” (not weeks). With the days left over out of a total of 365 days per year, they added an end-of-the-year five-day celebration. So you might say that the Egyptians invented vacations!
Sirius thus rose heliacally on the morning of the first day of the first decade of each new year.
The region of Osiris (our Orion) and Sothis (our Sirius), besides their significance we have just noted in the Pyramid Texts, also comprised the first two of the thirty-six “decans,” or regions of the sky which the Egyptians initially used as night-time star clocks. These star-clocks were later painted on coffin-lids, although by then they had become obsolete except for symbolic ornamentation.
Decans and Star Clocks
“Decans” are 36 sets of stars, corresponding to each decade.
In the Middle Kingdom period (ca. 1800–1200 B.C.), long after the great pyramids were built and the civil calendar had been established, Egyptian coffin lids were decorated with “diagonal star clocks.” From analysis of these coffin lids it appears that each of the 36 decades or ten-day periods of the year was associated with a particular star, which rose heliacally on the first day of that decade (marking the last hour of nighttime before sunrise). These 36 stars, and the constellations associated with them, constituted the “decans,” with Sirius being the decan for the first ten-day period, Orion the decan for the second ten-day decade, etc.
The rising of Sirius marked the first decade of the year. The heliacal rising of the first star of each decan marked the first day of that decade.
The 36 decans were inscribed on the coffin-lids in 36 parallel columns. Each decan column contained 12 lines or rows representing the hours of the night, and listing the particular decan which would rise at a given hour. That is, while a given decan would be associated with the hour of sunrise for a given ten-day period, the decans for the twelve or so previous ten-day periods would precede it in the sky, being visible before sunrise at various heights (the more recent decans lying behind or to the east of earlier decans; the tenth or eleventh preceding decans appearing across the meridian and over into the western sky).
Star-clocks had 36 columns, one for each decade (plus one extra column for the last 5 days). Each row corresponded to an hour at night, during which a given decan would be rising.
From the position of a given decan in the sky and with reference to such a diagonal star clock one could determine the time of night. For a given decade, the night hour was determined by observing how many of the decans had already risen.
It is an odd contingency of history that the modern measure for the length of the day derives from these 36 decans. This is odd because one would expect that if the Egyptians had 36 decans to divide the nighttime sky, that they would have divided the passing hours of the night into, say, half this number of periods, that is, 18 hours. Right? So why did the Egyptians divide the nighttime into 12 hours when they had 36 decans?
It would seem more likely, perhaps, that they would have divided the day evenly -- 18 decans for the nighttime and 18 for the daylight; thus creating a 36 hour day. The reason we have a 12-hour night and 24 hours in a day (instead of 36) is that Sirius’ heliacal rising -- and the beginning of the new year -- occurred in June, near the summer solstice, when nighttime was near its minimal length. At that time of year, only 12 decans are visible during the night, excluding the long duration of daylight and twilight. If daylight is divided into a corresponding number of hours, twice 12 equals a 24 hour day.
Night hours were always equally divided. Night hours did not always equal day hours.
Thus, oddly enough, 24-hour days derive not from Babylonian sexagessimal mathematics, but from the decimal Egyptian calendar with its 36 ten-day periods or decades.
Greek and Babylonian astronomers later established a means for converting these “seasonal hours” -- which vary in length depending on the time of year -- into “equinoctial hours” of equal duration. At that time the equinoctial hours were divided into 60 minutes of 60 seconds each, a clearly Babylonian or sexagessimally based division.
| Egypt assignment index page | Intro | Pyramids | Calendar | Mathematics | Medicine |
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HSCI 3013. History
of Science to 17th century |
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Many thanks to the pedagogical model developed in Mythology and Folklore and other online courses by Laura Gibbs, which have been an inspiration for this course.
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This course is currently undergoing major reconstruction to bring it into alignment with the new version of the course at Janux |
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