Episodic Survey of the History of the Constellations


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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 2 systems of decanal stars: (1) the original system of rising decans, and (2) the later system of transit decans. Also, (3) the later Tanis system (whose application is uncertain). The decan system is uniquely Egyptian in origin. The Egyptian decans (star tables) appear to be the oldest attested astronomical writings anywhere.

 

(1) Introduction

The oldest literary evidence for Egyptian astronomical beliefs/ideas are the Pyramid Texts. These comprise a group of magical spells and ritual utterances inscribed on the walls of the burial chambers of late Fifth Dynasty onwards (circa 2400 BCE) of Egyptian kings. The constellation Orion was associated with the god Osiris as early as the Pyramid Texts. The next major astronomical development in Egypt was the so-called "star clocks" of the Old Kingdom period (circa 2200 BCE). (However, the star clocks/star tables are usually dated much earlier than the coffins they appear on.) It has been proposed by Gay Robins that the system of star clocks/star tables may have originated in the temple ritual system - connected with the waking of the god (gods/goddesses) at dawn. The depiction of information on the star clocks/star tables on coffin lids is perhaps more artistic/conceptual than accurate. They helped to delineate the lid of the coffin as the night sky. The concept being presented was universal harmony and order (ma’at).

The star clocks/star table in the Middle Kingdom period appear exclusively in coffins. In the New kingdom period they are relocated to the ceilings of tombs.

(2) Sources

Our knowledge of early Egyptian astronomy is mostly derived from inscriptions and representations on the ceilings of tombs and on coffin lids of the Middle Kingdom and the New Kingdom. No Egyptian record of astronomical observation has been discovered to date. The only Egyptian eclipse record known documents a solar eclipse for 601 CE.

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.)

The majority of diagonal star tables (also termed 'diagonal star clocks' or 'diagonal star calendars' are mostly found on particular ancient Egyptian coffin lids (the underside of the innermost coffin lids) from Asyut. One has been found in Aswan and also one has been found in the Osireion. Most of the tables (hieroglyphic texts) are found on coffin lids dating to the First Intermediate Period and early Middle Kingdom (around 2150 BC). Presently (2013) 25 examples are known.

The decans travelled over the body of the sky goddess Nut. When the Demotic document known as Carlsberg Papyrus Number I was discovered it contained the picture of the sky goddess Nut with all the decans and their dates of rising and setting. However, this picture was gone when the document reached Copenhagen.

It is generally accepted that the diagonal star clock tables record the rising of stars/asterisms called decans throughout the course of the night. A list of usually 12 decans make up a column entry for one 10-day period. A complete diagonal star clock includes 36 of these columns plus a few others which list all the decans used.

(3) Dating the information

Sometimes the astronomical information carried by a temple ceiling or a tomb ceiling is earlier than the date of construction of the structure or date of placement of the information on the ceiling. As examples: (1) The decanal transit tables found on the ceiling of the Cenotaph of Seti 1 (circa 1306-1290 BCE) date from at least as early as the reign of Sesostris III in the 12th dynasty (circa 19th century BCE. (2) The earliest copies of the Ramesside Star Clock (star table system measuring meridian transits) are found in the Tomb of Ramesses VI (circa 1151-1143 BCE in the 20th dynasty), but the content implies a date that is some time between circa 1500 BCE and 1470 BCE in the 18th dynasty.

(4) Decan system

The decans are an Egyptian system of 36 stars/star groups (asterisms). In the earliest system decans are certain selected stars/asterisms that rise at 12 intervals during the night, and at 10-day intervals through the year. A later system was based on decans transiting the meridian. (The term decan is from the Greek meaning "10 days apart.") 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. Because the original decan ("star clock") was organised for the civil year of 365 days (not the Sothic year of approximately 365¼ days) they would inevitably soon lose they usefulness. The decan system would require some place shifts and the addition of new decans. 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.); Joanne Conman "It's About Time: Ancient Egyptian Cosmology." (Studien zur Altägyptischen Kultur, Band 31, 2003). See also the unpublished doctoral thesis by Sarah Symons (1999) Ancient Egyptian Astronomy: Timekeeping and Cosmography in the New Kingdom.)

Some tie during the 3rd-milennium BCE Egyptians star watchers divided the night into 12 hours determined by the heliacal risings of stars or small groups of stars called decans. The texts relating to the system of decan stars date from 2200 BCE to 1200 BCE. According to the Egyptologist Richard Parker we only know for certain that it was by the 24th-century BCE that the stars were first used to tell the time at night. Also, it is only by 2150 BCE that it is known for certain that the night hours totalled 12. (It is presently thought that "star clocks" are likely to be an Old Kingdom invention.) 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). The coffin lid "star clocks" largely had a funerary purpose and therefore were largely symbolic rather than intending to be accurate. 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. However, as the length of night varied with the time of the year, these decans did not measure equal lengths of time. at the latitude of Egypt the night in midwinter is almost 50 percent longer than the night at midsummer. How much the Egyptians were aware of this difference is not known. 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 or near meridian transits.

(5) Decan lists

Many Egyptian monuments incorporate lists of decans.

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). Every 10 days the 12 hours of the night are defined/marked by a different combination of 12 successive stars. 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 perhaps properly "star clocks" or "diagonal star clocks"). However, not all are arranged in a manner that would enable them to function as 'star clocks.' Regarding "diagonal calendars." A complete diagonal calendar contains 36 transverse columns.

Basically 3 lists of decans were constructed. The comparison of all the variations in the decan lists enables a grouping into 5 families. Three of rising decans, one of decans in transit, and one that cannot be assigned with certainty to either. The 5 families of decans are named from the first example of each. The 5 families are the Senmut, Seti I A, and Seti I C families of rising decans; the Seti I B family of transiting decans; and the Tanis family of uncertain application. It is suggested that it is probably 'a mixed and artificial list'. The Tanis family of decans are late in origin, their application is uncertain, they appear to be an artificial construct, and they are unusable as a true 'star clock.' Further reduction is possible to (1) the Seti I B family and (2) the Tanis family.

(6) Identification of decans

 The identification of the various decans is has proven to be extremely difficult. All of the extant so-called star clocks are corrupt textually to some degree. Even when but even when there is certainty with the decanal names and their translations, their proper astronomical identity has mostly remained unknown (due to divergences between records).

(7) Rising decans

The earliest example of a list of rising decans appears on a panel in the tomb of Senmut (Senemut).

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.

(8) Transit decans

By the time of the New Kingdom (circa 1530 BCE) the Egyptians changed from using 18 stars rising at night to using 13 stars crossing the meridian; with one of the 13 stars marking the beginning of the night. The new system of using the transiting of the meridian by decans. i.e., their culminations, to mark the night time hours, is identified from later texts which may be collectively called the Book of Nut or the Cosmology of Seti I and Ramesses IV. Due to observer difficulties this particular method was not associated with any astronomical accuracy - it was an approximate system.

The change from rising decans to transit decans resulted in changes in the decanal list. They differed in all but 3 of the stars/asterisms they used.

The Seti I B family of transit decans (examples: Book of Nut texts (of which we have only a few extant examples; 9 different copies of various dates - 3 of which are monumental sources); the ceiling depiction of the goddess of the sky, Nut, bending over the earth, Geb, and supported by the air god, Shu.) were undoubtedly established through observation. The modern title of the Book of Nut is, 'The Fundamentals of the Course of the Stars.'

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 new system of meridian transits of stars for time-keeping purposes used stars belonging to both constellations and asterisms. (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 (i.e., 15-day cycle/"week"). (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 of 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). The term 'Ramesside star clocks' denotes they were painted for the benefit of the deceased Ramesside period Pharoah. 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 (panels) 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.

There was no provision for the 5 epagomenal days of the year.

(9) Tanis family of decans

The Tanis family of decans, is found in examples from the 26th Dynasty down to the end of the 1st century CE. The Esna ceiling has the decans of the Seti I B family in a strip next to those of the Tanis family in a strip.

(10) Time-keeping corrections

The Egyptian civil calendar was invented in the 3rd Dynasty. The original decan system was organised for the civil year. 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 of the solar year. (The Sothic year was 365¼ days.) The lack of a leap year in the Egyptian civil calendar resulted in the risings of decans becoming out of phase with it. 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. Marshall Clagett (Ancient Egyptian Science II, 1995, Page 56) writes: "Evidence for a revision toward the end of the twelfth dynasty exists." However, by the time of the 12th dynasty a new system based on decans transiting the meridian was developed.

The decanal star clocks were eventually replaced by water clocks (clepsydra). Water clocks were probably used for indicating the passage of hours of the night. In use, the water clock was filled with water, which leaked out slowly from a small hole near the bottom. The time is indicated by the level of the water in the vessel, which is shaped so that it falls at a uniform rate. The oldest existing water clock was found by Georges Legrain in 1904 in a "cachette" he had discovered a year earlier in a courtyard of the Temple of Amun at Karnak. It was buried under the court of the Eighth Pylon with a large variety of sculptural pieces (approximately 20,000). All the items were buried at the same time during or immediately after the Ptolemaic Period. The buried items variously date from the Old Kingdom through to the Ptolemaic Period. The water clock, which is made of alabaster, is inscribed with a dedication to Amenhotep III who ruled 1390/1-1352/3 BCE. This water clock is dated to circa 1417-1379 BC, during the reign of Amenhotep III where it was used in the Temple of Amun at Karnak  It is apparently a later copy of an earlier water clock developed by the Egyptian court official (astronomer) Amenemhet for Amenhotep I. This earlier water clock is known from a hieroglyphic inscription only appearing on the wall of the tomb of Amenemhet. The hieroglyphs somewhat describe the water clock and dedicate it to Amenhotep I. The autobiographical tomb inscription states that Amenemhet invented the (first) water clock for Amenhotep I so he could tell the time when it was cloudy. This tomb inscription is the oldest documentation of the water clock in Egypt. The development of the water clock by Amenemhet is dated to circa 1550 BCE. Amehotep I died circa 1500 BCE. The actual water clock has not been found. The tomb of Amenemhet was discovered circa 1895 by the Italian archaeologist Ernesto Schiaparelli. (The oldest known example of an Egyptian sundial dates to 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.

(11) Hellenised decans

After the 12th Dynasty there is no contemporary evidence for use of any decanal 'star clocks.' The single example from the time of Pharaoh Merneptah (reigned 1223 – 1211 BCE) whilst late (the latest we have), has only a funerary purpose (and also could suit only a time some 6 centuries earlier). There are numerous lists of decans from later monuments, extending well into the Roman period, but these are never in the form of a 'star clock.'

However long the practice of time-keeping by the observation of decans lasted, the title or "hour-watcher" (imy wnwt or wnwty) persisted until the Ptolemaic period. However, perhaps by this period with the general meaning of astronomer once water clocks, shadow clocks, and sundials came into common use. (Water clocks were used almost exclusively for telling the telling time at night.)

The identification of the decans with the ecliptic is a late development. After the transmission of the Greek zodiac to Egypt in the Ptolemaic period, the system of Egyptian decans was adjusted to the new system.

The Egyptian system of 36 decan-stars was based on an equatorial system of measurement. The Greeks, however, always measured their stellar coordinated relative to the ecliptic. This had consequences when the Greeks adopted and adapted the Egyptian system of decans to their own ecliptical system. In Hellenistic times the Egyptian system of decans was brought into a fixed relation to the Babylonian-Greek zodiac. Simply, the astronomical and time-keeping significance of the 36 decan-stars became redundant. 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, 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.

In the Hellenistic period the decans for the most part seemed to be associated with or represented divisions of the areas assigned to the zodiacal figures, each sign being divided into 3 decans (except in a few cases of 4 decans), which, when absorbed by Greek and Roman zodiacs, were merely the names of the 3 10-degree divisions of each of the zodiacal signs. The decans came to represent one-third of a zodiacal sign. 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.

What remained - but only for a short period - were the myths/lore the Egyptian had associated with each decan. The 36 decans, and their particular divinities, became an important part both of Late Egyptian religion and also of the theoretical apparatus of astrology. The Greeks very quickly replaced all the native Egyptian myths/lore with their own astronomical myths. Hence, in this new form the decans continued to play a part in the later astrology of Greece, Rome. India, Islam, and medieval and Renaissance Europe.

The use in Greek astronomy of Egyptian decan names had little conformity with the stars/asterisms used by the ancient Egyptians for time-keeping during the night. (Later Greek use of the Egyptian decans amalgamated/combined 2 different series of decans - Seti I B and Tanis.) By the time of their use by the Greeks the Egyptian decans were simply used to mark the 10-degree divisions of the zodiacal signs. There is no reason to believe their Greek use remain tied to the old Egyptian decan names and old Egyptian decan stars/asterisms.

Appendix 1: The Nature of the Decans: Star Clocks, or Star Calendars, or Diagonal Star Tables?

The interpretation of the rows of diagonal star tables is controversial. The operation of the diagonal clocks is not suitably established and neither is the exact length of the decanal hours. 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. See also: Depuydt, Leo. (2010). "Ancient Egyptian star tables: A reinterpretation of their fundamental structure." In: Imhausen, Annette. and Pommerening, Tanja. (Editors). Writings of Early Scholars in the Ancient Near East, Egypt, Rome, and Greece. (Pages 241-276). Depuydt writes (Page 251): "The Earlier and Later star tables represent the yearly motion of the star sky basically in the same way, in spite of the different modes of representation. The focus is on selected nights and what happens in them from evening to morning. The nights in question occur at fixed intervals, either 10 or 15 days, and span the entire year. I am personally convinced that he star tables never served as clocks .... ... If star tables are just records of celestial observation and not clocks, the hours can easily be interpreted as an integral and necessary part of the act of observation. ... The star tables are only one mode of evoking the yearly motion of the stars."

Sarah Symons, McMaster University, Abstract of Presentation: "Comparison of Near-horizon Astronomical Events Recorded in Ancient Egyptian Diagonal Star Tables" (Biennial History of Astronomy Workshop – University of Notre Dame X, July 6-10, 2011): Diagonal star tables are Egyptian hieroglyphic texts listing the configuration of certain stars during the night at ten-day intervals. Most of the tables are found on coffin lids dating to the First Intermediate Period and early Middle Kingdom (around 2150 BC). Twenty-two examples are known, of which one will be presented here for the first time. Each column of a diagonal star table typically contains twelve star names. Stars which occur in these tables are known as "decans." The complete, ordered set of stars in a table (spanning up to forty such columns and more than forty stars) is called a "decan list." Previous attempts have been made to classify the tables into groups or taxonomies by considering details of layout. However, with further sources identified, only one classification criterion emerges as significant: the decan list. This criterion divides the surviving tables into two "families," each containing a characteristic decan list. The star tables record that decans performed a certain action, in order, throughout the night. The action in question is commonly thought to be a star's rising, but since the method of observation is never explicitly stated in or near the star tables, this is an assumption. Leitz hypothesised that the action could instead be the setting of stars. This presentation describes a current research project investigating the observational methods which may have been used to construct the original decan lists. Some star names occur in both decan lists, others are unique to one. By comparing occurrences of two decans which are present in both families, it can be shown that the spacing between two such decans is often different in each family. This appears to be in conflict with the motion of real stars in the sky: if two stars rise with a certain time separation, that separation remains constant night after night unless the observer changes location significantly or a large number of years passes. However, if the events being observed are different in one family, the time separation between such decans can and does change. For example, stars that rise together do not necessarily set together. The study investigates my recent proposal that one family of diagonal star tables records rising stars while the other, perhaps intended to be a complementary twin, records setting stars."

More recently: "Stars of the Dead." by Sarah Symons and Elizabeth Tasker (Scientific American, Volume 313, Number 4, October 1, 2015, Pages 70-75). An important article. "Though long thought to serve as a kind of clock for the proper timing of religious rituals at night, these star tables may, recent research suggests, actually have acted more as a map for directing the [newly] dead to new realms of existence in the afterlife among the stars."

Appendix 2: The Location of the Decans

The identification of the various decan names and their proper astronomical identity is extremely difficult. All of the extant star clocks are corrupt textually. Neugebauer and Parker systematically worked through the decan material. Their results were a meticulous analysis of the evidence. Otto Neugebauer, based on the Book of Nut texts, proposed the decan stars circled the sky in a zone approximately parallel to, and slightly south of, the ecliptic. Conman follows William Petrie in identifying the decans as ecliptic stars (i.e., actually marked out along the ecliptic line). Conman finds agreement with the New Kingdom decanal texts. Conman's proposed solution is also based on the Book of Nut texts and the interpretation of those texts given in the Carlsberg Papyri. Decan models usually propose the decan stars were a belt (1) south of, and parallel to, the ecliptic; (2) a belt roughly following the line of the ecliptic; and (3) a belt near the ecliptic (proximity of the ecliptic). The decans are areas of 10 degrees. The late identification of  decans as areas of 10 degrees along the ecliptic is not sufficient reason to assume that this was also the case for the original system of decan stars established during the Old Kingdom period. The original set of rising decan stars were likely south of the ecliptic. The later transit decan stars were perhaps located differently (i.e., roughly followed the line of the ecliptic). See: Belmonte, Juan. (2002) "The Decans and the Ancient Egyptian Skylore: An Astronomical Approach." (Memorie della Societa Astronomica Italiana, Volume 73 (Special Volume 1), Pages 43-57); and, Lull, José. and Belmonte, Antonio. (2009). "The constellations of ancient Egypt." In: Belmonte, Juan. and Shaltout, Mosalam. (Editors). In Search of Cosmic Order: Selected Essays on Egyptian Archaeoastronomy. It presently seems agreed that all the decans do not have the same latitude.

Appendix 3: The Different Decan Lists in the Book of Nut

Within the Book of Nut ('Fundamentals of the Course of the Stars') there exist 2 different decan lists that cannot be reconciled with each other. Alexandra von Lieven believes it likely that the list of stars on the body of Nut originates from the Old Kingdom (the astronomical data indicate the 12th-century circa 1850 BCE - linked to an ideal rising of the decan Sothis on the New Year's day of the 1st month of the Akhet season) and the data list from the Middle Kingdom is a secondary addition of data from this period (linked to a random point in time). See: von Lieven, Alexandra. (2010). "Translating the Fundamentals of the Course of the Stars." In: Imhausen, Annette. and Pommerening, Tanja. (Editors). Writings of Early Scholars in the Ancient Near East, Egypt, and Greece. (Pages 139-151).

Appendix 4: The Myth of an Advanced Egyptian Astronomy

There are no technical records or writings dealing with Egyptian astronomy until the 1st-millennium BCE, after Egypt's conquest by Persia. Prior to this time the Egyptians used simple astronomical methods to measure time and to develop accurate calendars, as well as to directionally align their buildings. Egyptian astronomy began to flourish after Egypt was conquered by Alexander the Great (in the 4th-century BCE). Many of the late Egyptian astronomers, were in fact, of Greek heritage and Egyptian astronomy during this period was, in actuality, Hellenistic in character.

Rene Grognard wrote (Historia Matematica, 10 February, 2000):

"Another comment on possible sources of Greek astronomy in Egypt. I don't think that even Herodotus could have made such a claim. Again a long association with Egyptologists (and study of the classical Middle Kingdom language -- for fun) at the nearby Macquarie University gave me no ground to doubt the assessment of Otto Neugebauer and Richard Parker ("Egyptian Astronomical Texts"). Egyptian astronomy was far too primitive to serve as basis to Greek astronomy. For instance the most elaborate "relics" of Egyptian astronomy: the Egyptian Star clock systems (using decans in rising as on the early coffin lids or in transit as in Ramesside period) were unworkable because simply based on the civil year of 36 x 10 days + 5 epagomenoi (in Egyptian: "the ones [left] over"). Indeed there were endlessly corrected with the result of a bewildering confusion of surviving evidence painstakingly sifted by Neugebauer & Parker. In fact it might very well be argued that water clocks were finally adopted simply as a more practical device to measure time. There is no text indicating any Egyptian interest in the complexity of celestial motions, exception made for the 70 day period of invisibility between the acronychal (= at dusk) setting and the heliacal rising of Sirius (and the decanal stars) : period "spent" in the Underworld ("dwt") between their "death" and "rising." This period was adopted for the canonical duration of embalmment."

At least outwardly, there are no surviving inscriptions or documents otherwise indicating that Egyptian astronomical knowledge was not more than tomb decoration, and not very carefully managed over time as a body of knowledge. Notwithstanding a number of temple and pyramid alignments, and several papyri codices suggesting a sophisticated knowledge of trigonometry and even algebra, no similar astronomical documents have survived, or records of astronomical observations. All of the early Egyptian astronomical texts are simply crude observational schemes, lacking mathematical elements. The earliest known copies of an almanac date from 1220 BCE at the time of Ramses the Great. Mathematical-astronomical papyri only appear at the beginning of the 2nd-century BCE. This is also the period when astrological papyri appear. Horoscopes and planetary texts, written in Greek or in Demotic (or both), based on computations, appear later. This indicates that Egyptian astrological lore originated in the Ptolemaic period and is a Hellenistic creation. The Vienna papyrus which described lunar and solar eclipses and their portends (omens), was likely copied by a scribe in the late 2nd-century CE, and presents astronomical knowledge that is essentially Babylonian in origin.

Regarding the usual activities of an ancient Egyptian astronomer: "... in an inscription on his statue (ca. 2nd century, B.C.) an astronomer and snake charmer named Harkhebi enumerated his astronomical, calendrical, and time-telling activities, including the observation of the stars and announcements of their risings and settings, his purification on the days when the decan Akh rose heliacally beside Venus, his observations of other heliacal risings, and particularly his foretelling of the heliacal rising of Sirius at the beginning of the [civil] year, and so on." (Ancient Egyptian Science, Volume II by Marshall Clagett, 1995, Page 128.)

Regarding astronomical books in the Edfu temple library: "... the list of books in the library room of the temple of Edfu (built by Ptolemy VIII, Euergetes II, 170-63, 145-116 B.C.). This catalogue ... had books on the Knowledge of the Periodic Returns of the Two Celestial Spirits: the Sun and the Moon, and on The Governing of the Periodic returns of the Stars. As Otto Neugebauer has shown, Clement of Alexandria (2nd. century A.D.) appears to have read that list as he describes the four Hermetic books on astronomy studied by the Egyptian Horoscopist in order that he might know them by heart: books on the arrangement of the fixed stars, on the position of the sun and the moon and the five planets, on the syzygies and phases of the sun and the moon, and on the risings." (Ancient Egyptian Science, Volume II by Marshall Clagett, 1995, Page 491.)

At least at the end of the Hellenistic period the Egyptians had an advanced knowledge of astronomy. The Denderah zodiac shows the inauguration of the Osiris chapels (decorated over the 3 years  from 50 to 48 BCE) "took place on December 28, 47 BC (the 26th day of Khoiak) the day of a zenithal full moon, a conjunction that takes place only once every 1,480 years." (Encyclopedia of the Archaeology of Ancient Egypt edited by Kathryn Bard (1999), "Denderah" by Sylvie Cauville, Page 300.)

See also: Waziry, Ayman. (2016). "Probability Hypothesis and Evidence of Astronomical Observatories in Ancient Egypt." (Journal of Social Sciences and Humanities, Volume 2, Number 2, Pages 31-50).

Appendix 5: Biennial History of Astronomy Workshop - Notre Dame X, July 6-10, 2011

Sarah Symons, McMaster University (Presentation): "Comparison of Near-horizon Astronomical Events Recorded in Ancient Egyptian Diagonal Star Tables."

Abstract: "Diagonal star tables are Egyptian hieroglyphic texts listing the configuration of certain stars during the night at ten-day intervals. Most of the tables are found on coffin lids dating to the First Intermediate Period and early Middle Kingdom (around 2150 BC). Twenty-two examples are known, of which one will be presented here for the first time.

Each column of a diagonal star table typically contains twelve star names. Stars which occur in these tables are known as "decans." The complete, ordered set of stars in a table (spanning up to forty such columns and more than forty stars) is called a "decan list." Previous attempts have been made to classify the tables into groups or taxonomies by considering details of layout. However, with further sources identified, only one classification criterion emerges as significant: the decan list. This criterion divides the surviving tables into two "families," each containing a characteristic decan list.

The star tables record that decans performed a certain action, in order, throughout the night. The action in question is commonly thought to be a star's rising, but since the method of observation is never explicitly stated in or near the star tables, this is an assumption. Leitz hypothesised that the action could instead be the setting of stars.

This presentation describes a current research project investigating the observational methods which may have been used to construct the original decan lists. Some star names occur in both decan lists, others are unique to one. By comparing occurrences of two decans which are present in both families, it can be shown that the spacing between two such decans is often different in each family. This appears to be in conflict with the motion of real stars in the sky: if two stars rise with a certain time separation, that separation remains constant night after night unless the observer changes location significantly or a large number of years passes. However, if the events being observed are different in one family, the time separation between such decans can and does change. For example, stars that rise together do not necessarily set together.

The study investigates my recent proposal that one family of diagonal star tables records rising stars while the other, perhaps intended to be a complementary twin, records setting stars."

Appendix 6: The identification of Sothis with Sirius

We do not know for certain with which star or constellation Sothis should be identified for all periods of Egyptian history. Whilst it is generally accepted that Sothis is the star Sirius, none of the sources give any evidence for this from before classical times. According to some Egyptologists Egyptian astronomical names did not always remain attached to the same celestial object. Osiris was first associated with Venus; later Osiris was associated with Jupiter. The planet Venus, which was first identified with Osiris, was later identified with Isis. Similar shifts and uncertainties apply to the identification of ancient astronomical names in general, for example, the constellations in the Old Testament book of Job.

Appendix 15: Egyptian Decan System Unrelated to Babylonian system of the "Three Stars Each"

The system of Egyptian decans did not originate in the Babylonian astral system. There were two systems of decanal stars. The first (and original) system (Old Kingdom period, circa 2800 BCE) used heliacal risings. The second (and later) system used meridian transits (New Kingdom period, circa 1500 BCE). The original system of Egyptian decan stars independently established during the Old Kingdom period were a set of 36 rising stars/asterisms along a single linear star path likely south of the ecliptic. The Babylonian Enuma Elish/Astrolabe/Mul.Apin system of 3 x 12 stars dates to the late 2nd-millennium BCE. The later Babylonian system comprised what may be termed an 'equatorial' system of 3 bands with 12 stars in each band. It is indicated that in the developing Babylonian uranography constellations came and went before the scheme eventually became fixed.


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[I am indebted to the horologist (timepieces expert) Gordon Uber for corrective information on Egyptian water clocks and to the UK linguist Mrs Gill Zukovskis for some corrections and questions that prompted the inclusion of additional details to aid clarity.]

Copyright © 2006-2016 by Gary D. Thompson


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