The illustrations on this page have been compiled from a variety of sources. If advised that copyright has been infringed I will immediately remove the particular illustration(s).
I: Early Egyptian Constellations
18: The decan stars
Part of Egyptian coffin lid showing two Egyptian astronomer's assistants and hieroglyphic list of decan stars and the star's positions. (See: Ancient Astronomers by Anthony Aveni, 1993 (Page 42).) There were two systems of decanal stars: (1) the original system of rising decans, and (2) the later system of transit decans.
(1) Sources
The Egyptian system of decans are known through 4 main groups of sources: (1) the diagonal star clocks on the inside surface of wooden coffin lids from the 9th Dynasty to the 12th Dynasty, (2) the cenotaph of Seti I, the tomb of Ramses IV, and Papyrus Carlsberg I, (3) the tomb of Senmut and later similar monuments, and (4) Hellenistic-Roman monuments and astrological documents. (The latter group of sources considered the decans simply as thirds of zodiacal signs.) In both the cenotaph of Seti I and the tomb of Ramses IV the decans are represented on the body of the sky goddess Nut. Carlsberg Papyrus Number I is an extensive commentary on the inscriptions on monuments. (Additionally, there is a fragment remaining of a diagonal star clock on a ceiling in the Osireion at Abydos which dates from the XIXth dynasty.)
(2) Decan system
The decans are an Egyptian system of 36 stars/star groups (asterisms). The decans could be groups of stars or single bright (conspicuous) stars. The ancient Egyptians used special constellations (asterisms), the decans, to divide their year into 36 parts. They rose at particular hours of the night during 36 successive periods of 10 days each, constituting the year. A decan indicated the one and same hour during 10 days. (Each specific decan rose above the eastern horizon at dawn for an annual period of 10 days.) As the stars rise 4 minutes later night by night a given decan was replaced after 10 day by its predecessor to mark a given hour. Otto Neugebauer believed the 36 decans formed the old year of 360 days. The 5 additional or epagomenal days were "ignored" but undoubtedly were taken into account during the development of the decan system. (The earliest Egyptian calendars indicate that the 5 epagomenal days were not regarded as belonging to the year. The New Year festival begins on the 1st Thoth, not on the 1st of the epagomenal days.) A more recent view by Anne-Sophie von Bomhard is that the original decan system was designed for a year of 365 days. The Egyptian "star clocks" (i.e., decans) are the earliest detailed astronomical texts known.
According to the accepted interpretation made by Otto Neugebauer in Egyptian Astronomical Texts (Volume 1, 1960), based the Book of Nut texts, the decan stars circled the sky in a zone approximately parallel to and slightly south of the ecliptic. The decans (a Greek term) lay within a wide equatorial belt and began with Sepedet (= Sirius). (Sepedet (= literally, "the excellent" but also "The Great Star") was sometimes called the "Mistress of the Year.") Sirius (Sepedet) is the only one of the decans able to be unambiguously identified. (Neugebauer's identification of the location of the decanal belt is disputed by Kurt Locher "New arguments for the celestial location of the decanal belt and for the origin of the s3h-hieroglyph." (Atti di sesto congresso internazionale di egittologia. (2 Volumes, 1992-1993.)); and Joanne Conman "It's About Time: Ancient Egyptian Cosmology." (Studien zur Altägyptischen Kultur, Band 31, 2003).
The texts relating to the system of decan stars date from 2200 BCE to 1200 BCE. Decanal "star clocks" (also (mistakenly) termed "diagonal calendars") decorated the inside surface of Egyptian (wooden) coffin lids, in both drawings and texts, starting circa 2100 BCE (with the practice ending circa 1800 BCE). (Our principal knowledge of astronomy in the Middle Kingdom period comes from wooden coffin lids, primarily from the 9th and 10th Dynasties. The painted scenes (sometimes carved) on the inside surface of the coffin lids are actually tables of "rising stars.") They are also shown on the tomb ceilings of Seti I (1318-1304 BCE) and on some of the ceilings/walls of royal tombs of the Ramesside period (12th-century BCE). They show that there was a system of 36 named "equatorial" stars rising within 10 days of each other (and were based on the civil calendar year). Pictures of decans comprise most of the celestial representations in Egyptian tombs.
The system of decan stars was used to indicate the hours of the night throughout the year. Lists of decans were prepared to determine the hour of the night if the calendar date was known, or to determine the decan if the hour of the night was known. The use of the decan stars for time measurement during the night likely led to the twelve-division of the period of complete darkness. Of the 18 decans marking the period from sunset to sunrise 3 were assigned to each interval of twilight. This left 12 decans to mark the hours of total darkness. The 12-unit division of the night therefore probably originated in the combining of the decanal stars with the civil calendar decades. The twenty-four division of day and night (i.e., 24 hour system) eventually derived from this. (The original 24 hour division was actually a system of "hours" of uneven length and uneven distribution between daylight and night. As early as circa 2100 BCE the Egyptian priests were using the system of 24 hours. According to one authority this comprised 10 daylight hours, 2 twilight hours, and 12 night hours. This system was obsolete by the time of Seti I. By the Ramesside period (circa 1300/1200 BCE) there was a simpler more even division of 24 hours into 12 hours of night and 12 hours of daylight each. It has been proposed, however, that the division of day and night into 12 hours each may have been initiated by the fact that the year was divided into 12 months.)
The "hours" successively marked by each decan star for an interval of 10 days were, however, actually only an "hour" of approximately 45 minutes duration. (Each decan would rise approximately 45 minutes later each night.) (The division of the hour into 60 minutes was the invention of the Babylonians.)
The decanal system has been traced back as far as the 3rd Dynasty (circa 2800 BCE) and may be older still. The contents of coffin lids establishes that the decanal system, of dividing the night into 12 hours according to the rising of stars or groups of stars, was in place at least by circa 2150 BCE. The contents of the Pyramid Texts show that the system of decans was established by at least the 24th century BCE.
The primary reason for the Egyptians to study the night sky seems to have been to establish the civil calendar (which was apparently initiated with the heliacal rising of Sothis (= Sirius)) on a firm basis. (The civil calendar was the official calendar. It was a simple calculating tool that could be followed automatically. The civil calendar remained unchanged in Egypt from its establishment circa early 3rd millennium BCE until near the end of the 1st millennium BCE.) The Egyptian calendar-year on which the system of decans (star clocks) was originally constructed was the civil or "wandering" year which consisted of 12 months of 3 10-day weeks, divided into 3 seasons of 4 months each, followed by 5 epagomenal days (called "the days upon the year"/"those beyond the year"). The civil calendar had been long established when the decans first appeared on the inside surface of coffin lids of the Middle Kingdom period. Otto Neugebauer (The Exact Sciences in Antiquity, 1957, Page 82) wrote: "In tracing back the history of the Egyptian decans we discover the interaction of the two main components of Egyptian time reckoning: the rising of Sirius as the harbinger of the inundation, and the simple scheme of the civil year of 12 months of three decades each." To assist the establishment of a civil (year) calendar the sky was divided into a scheme of 36 decans, with each decan (characterised by a bright star or distinctive star group) marking 36 ten-day periods, to which was added 5 epagonal days.
There were two systems of decanal stars. The first (and original) system used heliacal risings. The second (and later) system used meridian transits.
(3) Decan lists
The decanal system involved the arrangement of 10-day intervals throughout the year. The decan lists were essentially set out in tables consisting of 36 columns with (usually) 12 rows or divisions. The columns in the tables covered the year in 10-day intervals. The rows in the tables covered the 12 decanal hours of the night. In each of the 36 columns the decans are placed in the order in which they rise above the horizon (or transit the meridian). With each of the successive 36 columns the name of a specific decan is moved one line higher to its place in the preceding column (i.e., the second decan becomes the first and so on). This results in a diagonal structure (diagonal pattern) which is the reason for the early name "diagonal calendars" being given to these texts (but properly "star clocks" or "diagonal star clocks"). A complete diagonal calendar contains 36 transverse columns.
(4) Rising decans
The decanal system consisted of 36 rising stars and used the heliacal risings of stars/asterisms on the eastern horizon as markers. Each period of 10 days was first marked by the heliacal rising of the next decan on the eastern horizon. They rose heliacally 10 days apart and all had the same invisible interval of 70 days prior to their heliacal rising. (At least ideally all the decans had the same duration of invisibility as their leader Sirius. All decans were invisible for 70 days between acronychal setting and heliacal rising - because of being in the light.)
By the time of the New Kingdom period (circa 1550-1100 BCE) the usefulness of the original decan system of hours had ceased. By the 10th Dynasty and 11th Dynasty the original decan system had become completely unusable and in the 12th Dynasty were subjected to a radical revision. Many old decans were dropped out and many new decans were introduced.
(5) Transit decans
From the Book of Nut texts we can identify the introduction of a new decanal system that can be termed transit decanal clocks. This new system, termed the Ramesside star clocks, used the transiting of the meridian by decans (their culminations) to mark the night-time hours. (The time of decan transits involved the time they crossed the meridian i.e., reached the highest point in the sky (culmination).) This new method of indicating the night hours arose by combining only those stars which behave like Sirius with 10-day weeks of the civil calendar. Likewise with the previous system of decans, this attempt to substitute the culmination of stars for their heliacal rising also did not last.
The Ramesside (20th Dynasty) star clocks are star tables which measure hours by means of transits, in half month intervals. (One of the most important documents relating to Egyptian astronomy is the long table of (decan) star transits (culminations) for each hour of the night on every fortnight of the year. This is given with most accuracy in the tomb off Ramses VI) These are different star clocks to the earlier system of decans. Only a few of the stars/asterisms used in the earlier decanal star clocks are the same as, or near to, those used in the Ramesside star clocks. The evidence for these later star clocks comes exclusively from the ceilings of a number of Egyptian royal tombs of the Ramesside period (Ramses VI, Ramses VII, and Ramses IX of the 12th-century BCE). Two sets of star tables appear in the tomb of Ramesses VI, one set of star tables appears in the tomb of Ramesses VII, and one set of star tables appears in the tomb of Ramesses IX. The texts consist of 24 star clock tables for the 24 half-month intervals of one year. These particular ceilings also include other astronomical information: (1) lists of decans and their divinities, (2) constellations, and (3) the days of the lunar month.
(6) Time-keeping corrections
The Egyptian civil calendar was invented in the 3rd Dynasty. The Egyptian civil year contained 365 days whilst the system of 36 decans sufficed only for the old year of 360 days. This was taken into account by the originators of the decanal system. They added an additional set of stars/asterisms to indicate the hours of darkness for the 5 epagomenal days. The decans of the 5 additional (epagomenal) days were treated separately. They are depicted on 4 coffin lids separately and appear after the 36 decans of the 360 day year. (The last 5 (extra) days of the year were the birthday festivals of the 5 principal gods/goddesses: Osiris, Isis, Horus, Seth, and Nephthys.) However, keeping the (diagonal) star clocks adjusted was a continuous problem. The Egyptians did not bother to take into consideration the fact that the 365 days did not accurately measure the return of the sun to the same star. The calendrical system based on the decans was flawed by its failure to take into account the fact that the Egyptian civil year was always approximately 6 hours short and the solar year. The result was a slow progressive change took place in the relation between the heliacal rising of a decan and its date in the civil calendar. Rearrangements of the decanal order were attempted in order to counter the resulting mismatch.
The decanal star clocks were eventually replaced by water clocks. What is believed to be one of the oldest Egyptian water clocks was discovered in the tomb of Amehotep I (who died circa 1500 BCE).
The decans were selected for decades of the civil calendar, 3 for each month (of 3 x 10-day weeks), leaving over 5 epagomenal days at the end of the year. The civil year and the astronomical year were often out of phase because the civil calendar contained exactly 365 days. A calendar of 365 days does not accurately measure the return of the sun to the same star. (The fraction of the day left unrecognised was ·2422.) The civil year was shorter than the year based on the risings of stars. The total of 365 days did not vary. (The Egyptians did not take leap years into account. No intercalary day was inserted in any year. As a result the civil calendar moved further and further away from the actual seasons.) As a consequence there is a relentless slow movement in the relation between the heliacal rising of a decan and its date in the civil calendar. The civil year through the natural (astronomical/solar) year by approximately 1 day every 4 years. The beginning of the schematic civil calendar of 365 days "wandered" in the course of time through all the seasons. Every 4 years the beginning of the civil calendar year (1st of Thoth) was delayed by 1 day.
The civil year and the astronomical/solar year (seasonal year) were usually reckoned to coincide only every (approximately) 1460 years (according to the definition of "a solar year") (the so-called Sothic cycle - from the name of the star observed). Only then did the civil calendar year syncronise with the actual seasons (more or less). The "Sothic cycle" of (approximately) 1460 years was the result of connecting the agricultural year with the yearly recurring astronomical phenomenon, the heliacal rising of the star Sirius, which roughly coincided with (i.e., slightly preceded) the beginning of the inundation of the Nile River. The heliacal rising of the star Sirius only attained its importance by its closeness to the inundation of the Nile River. The Egyptian (seasonal) year was considered to begin on July 19th (Julian calendar date) - the date of the heliacal rising of Sirius.
The 3 seasons, however, corresponded to the cycle of the Nile River and agriculture. By the Middle Kingdom period (circa 2040-1640 BCE) the heliacal rising of the star Sirius was established as the event which marked the beginning of the seasonal year. (There is considerable evidence that from an early date the Egyptians regarded the heliacal rising of the star Sirius as marking the beginning of the year. The Egyptians fixed the beginning of their year, but not their civil calendar, with the heliacal rising of Sirius (our Julian calendar date of July 19th = the "coming out of Sepedet). With the civil calendar the first month of the Inundation always followed the 5th epagomenic day, irrespective of whether Sothis had risen or not. (The heliacal rising of the star Sirius fell on the 1st of Thoth once every 1460 Julian calendar years.) The Sothic year was the lapse of time which passed between 2 heliacal risings of the star Sirius, at the same latitude of reference. (Also, the length of a Sirius cycle is somewhat variable.) The solar year was in official use as early as the 12th Dynasty (circa 1938-1756 BCE) period and was defined as beginning at the heliacal rising of Sirius. New year's day (our Julian calendar date of 19th July) marked the beginning of the first season (i.e., the flooding of the Nile). The New Year day was not determined on astronomical grounds (by a celestial event) but was determined by a calendar of 365¼ days and the method of counting 365¼ days from the previous new year's date.
(7) Hellenised decans
The identification of the decans with the ecliptic is a late development. The later division of the ecliptic into 10 degree sections called decans by the Greeks derives from the Egyptian system of decan stars. In Hellenistic times the Egyptian system of decans was brought into a fixed relation to the Babylonian zodiac. Also in Hellenistic times, after the death of Alexander the Great, the 36 decans eventually were defined as thirds of zodiacal signs, each decan representing segments of the ecliptic of exactly 10 degrees length. Usually these segments were not given special names but were simply counted as 1st, 2nd, or 3rd decan of the zodiacal sign in question. This system of the use of decans continued through into medieval astrology.
Appendix:
The interpretation of the rows of diagonal star tables is controversial. The "star clock" outline explanation given above follows the work of Otto Neugebauer and Richard Parker. In 2007 the Egyptologist Sarah Symons ("A Star's Year: The Annual Cycle in the Ancient Egyptian Sky." In: Steele, John. (Editor). Calendar and Years: Astronomy and Time in the Ancient Near East. (Pages 1-33). ) proposed the more neutral term "diagonal star table." A common past term for diagonal star tables has been "star calendars." The common current term is "diagonal star clocks." However, Sarah Symons points out that just because the rows of diagonal star tables are related to the hours of the night does not necessarily mean that the tables are clocks. As early as 1936 the naturalised American astronomer Alexander Pogo ("Three unpublished calendars from Asyut." (Osiris, Volume I, Pages 500-509).) questioned whether the intended function of the diagonal star tables was as hourly timekeeping devices. In 1998 the Egyptologist Leo Depuydt "Ancient Egyptian star clocks and their theory." (Bibliotheca Orientalis, Volume LV, Number 1/2, January-April, Pages 5-43).) likewise questioned whether the intended function of the diagonal star tables was as hourly timekeeping devices.
Copyright © 2006-2008 by Gary D. Thompson
This Web Page was last updated on: Sunday, August 31, 2008, 6:00 pm.
This Web Page was created using Arachnophilia 4.0 and FrontPage 2003.
You can reach me here by email: 