Episode 32: All Along the Watchers of the Hour

August 5, 2023

This month we tour the astronomers of ancient Egypt from the Old Kingdom to the last native Pharaoh of Egypt just prior to Alexander's conquest. We look at how astronomers fulfilled their primary responsibility of keeping track of time during the night so that the priests could perform the appropriate rituals in the temple. Finally, we end our journey through Egypt with their calendar, arguably the most reasonable calendar any civilization has ever used.


Good evening, and welcome to the Song of Urania, a podcast about the history of astronomy from antiquity to the present with new episodes every full moon. My name is Joe Antognini.

Well so far in our journey through Ancient Egyptian astronomy we’ve delved into Egyptian ideas about cosmology, the creation of the world, the end of the world, and some of the astronomical connections of Ancient Egypt’s most famous artifacts, the Pyramids. But the Pyramids date back to very nearly the beginning of Egyptian history, and Egypt lasted for quite a while after that. So this month I’d like to go through what exactly the Egyptian astronomers were up to all those years.

Now as I mentioned in the last episode, the first person whose name survives that could plausibly be claimed to be an astronomer is the great Imhotep, who designed the first of Egypt’s many pyramids, the step pyramid of Djoser. But Imhotep was really first and foremost remembered as an architect and physician, and the claim that he was also an astronomer is a little more tenuous. The first really definitive record we have of an astronomer in ancient Egypt is a man named Tjenti, who lived sometime around 2300 BC, in the middle of the 6th Dynasty, so right when the Pyramid building fad was winding down. Above the entrance to his tomb are listed his most important titles, “Great of the Seers, Grand vizier, superior lector priest, sole companion, scribe of the divine book, superior of the secret of the sky, who sees the secret of the sky.” The last title, “superior of the secret of the sky, who sees the secret of the sky” is clearly astronomical in nature. But the first title, “Great of the Seers” may also have been a title reserved for astronomers, “seer” being taken rather literally as someone who sees the sky and understands it. Another possibility, though, is that the word referred to mining as someone who went out and sought metal. Incidentally, this title, “Great of the Seers” is also given to Imhotep and is one of the circumstantial pieces of evidence we have that he was also involved with astronomy.

Well, not too much more survives about Tjenti, all we can really conclude from him is that very early on in Egyptian history the Egyptian state was interested in the heavens and appointed officials to high positions to study it. After Tjenti we’ll skip all the way across the first intermediate period, the middle kingdom, and the second intermediate period until we get to the next astronomer at the beginning of the New Kingdom. This is not to say that there aren’t any astronomers over all these centuries, but again all we have are some names and titles, and in many cases the titles are only suggestive rather than definitively known to be astronomical. One such title is “watcher on the terrace,” and another is the formidable title “superior of the darkness.”

By the time we get to the New Kingdom the artifacts that survive are somewhat more substantial. The first is a statue built in honor of a priest and astronomer by the name of Aanen in the 18th dynasty. The statue depicts the man clothed in a panther skin decorated with stars. The back of the statue has an inscription that states that he is a “lector-priest who knows the behavior of heaven.” To understand what this position actually entailed it’s worth becoming a bit more familiar with the hierarchy of the ancient Egyptian priesthood.

One of the features of Ancient Egyptian religion that is perhaps somewhat foreign to us in the modern world is the elaborate hierarchy of the priesthood. And really, this was not unique to Ancient Egypt, many of the more complex ancient civilizations like the Babylonians, Jews, Romans, and so on also had more hierarchy in their religious org charts than we are used to today. Most modern religions have a fairly flat organizational structure. In many of the Protestant faiths, for instance, you have your pastor, and that is pretty much it. Or if you are Jewish you will have your rabbi, or if you are Muslim you will have your imam. Your pastor or rabbi or imam might on occasion go to a conference with other pastors or rabbis, and certain sects might recognize some overall leader or group of leaders, but that is essentially where the hierarchy ends. The closest analog to the ancient religious structure that persists in the modern world is perhaps the Roman Catholic Church with its various ranks of deacons, priests, bishops, cardinals, and all headed up by the pope. But even this staid institution abolished many of the more obscure ranks after the Second Vatican Council, like the minor orders of acolyte, exorcist, lector, and porter, and, more recently, the position of monsignor. But if you want to imagine what the structure of Egyptian or Babylonian religion was like in the 2nd millennium BC, think of something like the Catholic Church in the late 19th century.

In the case of Ancient Egypt, at least during the New Kingdom, the head of the religion was the High Priest of Amun. Under him was the Second Priest of Amun, followed by the Third Priest of Amun and then the Fourth Priest of Amun. These roles are also sometimes called the High Prophet of Amun, Second Prophet of Amun, and so on. The Second Priest of Amun seems to have had more logistical responsibilities and to put it in more modern corporate terms, may have effectively been the chief operating officer of the temple, and was responsible for administering the property, managing the staff, and so forth. Despite the name, though, the Second Priest of Amun was not the second ranking priest in the hierarchy under the High Priest. That role fell to a priestess who was given the title God’s Wife of Amun, and she was explicitly ranked above the Second Priest. Initially this role was just ceremonial and was given to one of the female members of the Pharaoh’s family, either his mother, wife, or his oldest daughter. But over time the position of God’s Wife of Amun began to acquire real duties and more substantial power. As the role became more powerful, the pool of women who were eligible for the title became more restricted. The Pharaoh’s mother or wife were no longer eligible and eligibility was limited to his sisters or daughters who, in turn, had to remain celibate in order to prevent a new center of dynastic power from emerging. Incidentally, one of the records we have of the role of God’s Wife of Amun relates to a secondary title she held, called God’s Hand. You may recall from two episodes back when I described the Egyptian cosmogony, that Amun “put his hand into his fist and made orgasm with it.” It seems that this creation event was ritually recreated with God’s Wife of Amun playing the role of God’s Hand, though just how literally the temple ritual reproduced this act we cannot say.

Well, below these highest ranks of priests and priestesses was the lector priest, which is what Aanen’s statue says he was. Though, as it happens, this statue may have been cast earlier in his career, because he apparently later rose to the rank of Second Priest of Amun, at least according to his mother’s tomb. The lector priest was responsible for, as the name suggests, reading, in particular reading the spells and hymns of the temple rituals. The spells and hymns that the lector recited were not simply for the benefit of those listening. In the ancient Egyptian religion, the recitation of these words by the lector priest had what is variously transcribed as “heka,” or “hu,” and referred to an authoritative utterance or “creative command.” In other words, by reading these spells, the lector priest was not merely wishing for their effect to take place, or beseeching the gods for their effect to take place, but by the act of his reading them this caused their effect to take place. Incidentally, the lector priest was the only role in the priesthood which was apparently reserved for men. There are records of women serving in all other roles, including Second Priest and High Priest, though in that case the position was merged with God’s Wife. But there are no records of a woman ever serving as lector priest.

Below the lector priest there were a variety of more specialized roles. The sem priests performed funeral services, and were generally well respected because, after all, they held the power of the afterlife in their hands. Only by their saying the spells correctly after a person’s death could the deceased enter into the afterlife, so you would want to be on their good side. That said, sem priests were also responsible for preparing the body for mummification, and after handling the dead body they would become ritually unclean for a period, during which the priest who did the dirty deed was ritually insulted. There were also doctor priests who were responsible for healing, and ka-priests who would perform offerings to the dead at their tomb. Of special interest to us are the hour priests, who were responsible for keeping the time and ensuring that the various rituals were conducted at the appropriate hours, and I’ll have more to say about them later. At the bottom of the hierarchy were the “wab” priests, who were more or less there to do odd jobs. But the hierarchy does not end there. Below the wab priests was a role that has been called the “pastophoroi.” The pastophoroi were not priests, and were definitely not allowed to call themselves priests. Their official role was as doorkeeper and they worked in the courtyard area of the temple that was open to the general public. They would stand guard at the threshold of the temple and raise and lower the door curtains for those entering or leaving. But the pastophoroi had other, unofficial roles in Egyptian religion, and had a particular reputation for having the ability to interpret dreams. They probably also had duties in tending to the needs of the general public. Another feature of ancient Egyptian religion, and many other religions of the ancient world, that might be somewhat surprising to us in the modern era is that the rituals that the priests conducted involved the general public very little. Today, almost by definition, we associate a religious ceremony with the presence of a congregation of some kind, the people coming together and collectively worshiping their god. But in the ancient world, not just among the ancient Egyptians, but among the ancient Greeks, Jews, Babylonians, too, in general the priests performed their rituals in private, and this was especially true for the most important rituals, where even ordinary priests were not permitted to take part, they were conducted only by the high priest.

Of all these different kinds of priests, only the high priests were full-time jobs. The rest performed their duties on a part-time basis. Every four months they were obliged to serve in their priestly role for one month. During this month they lived at the temple and had to maintain ritual cleanliness. The other three months they went back to their ordinary life.

One of the better descriptions of the array of priestly roles comes to us from the Hellenistic author Clement of Alexandria who described a ritual called the Procession of Osiris in the late 1st century or early 2nd century AD. The procession was lead by a singer, followed by the horoskopos. Later comes the horologion, or the hour-priest who holds what is called the “astronomical palm leaf,” which are probably the two instruments associated with Egyptian astronomers, the bay and the merkhet which I’ll describe in more detail later on. Clement writes, “He must have the astrological books of Hermes, which are four in number, always in his mouth. Of these, one is about the order of the fixed stars that are visible, and another about the conjunctions and luminous appearances of the sun and moon; and the rest respecting their risings.” Further along in the procession is the Sacred Scribe, who wears wings on his head and carries a book, ink, and a reed. Knowledge of astronomy was also a requirement for the Sacred Scribe, who had to be fluent in hieroglyphics and be able to notate the position of the sun, moon, and five planets.

Well, even after the end of the New Kingdom there are a number of surviving references to various Egyptian astronomers. A man named Amun Ankhefenkhounsu carved his titles into the doorway of his house, and these included “who explains the ‘snt’ of the two gates of heaven.” What is the ‘snt’ you ask? Lots of people have asked that and no one has an answer. The precise definition is unclear, but the word is definitely associated with astronomy and may have referred to astronomical records that the priests kept. Later on a monument records that when the Kushite king Tanutamun visited the temple complex of Karnak, he was received by a “priest of the great snt” and the “astronomers of the temple of Amun-Re, who know the secrets of Nut.” Others get titles like “inspector of the astronomers, the one who knows the heliacal rising of Sirius,” “Chief of the astronomers,” “superior of the secret in the sky, the earth and the Duat,” “he who opens the two gates of heaven in Karnak.” Not to belabor the point, but we can see that there was a strong connection between astronomy and the temple priesthood in ancient Egypt.

Probably the astronomer with the most extensive surviving description is maybe unsurprisingly one of the later ones, a man named Horkhebi who lived in the 30th dynasty, right before Alexander’s conquest. He had a statue made of himself which has a description of his skills running down the back:

He is the one who announces the risings and settings at their time, with the gods who predict the future, for which he has purified himself on their heliacal risings, when the decan is next to Benu above them, so that he can satisfy them with his predictions; the one who observes the culmination of every star in the sky …, the one who announces the heliacal rising of Sirius at the beginning of the year and observes her on her first day of the festival, having calculated her course …, the one who observes what she does every day, so that everything she has planned is in his charge; the one who knows what goes to the north and south of the solar disc, making known all its omens and predictions, so that he can say what they cause when they arrive at their time; the one who divides the hours of the day and the night without making mistakes during the night …, one who is wise in every phenomenon observed in the sky, who has waited for them, instructed in their winds and their omens.

The initial line about him announcing the risings and settings of the stars with the gods who predict the future has a clear implication that by this time the Egyptians were observing the heavens for astrological purposes. This is actually something that seems to have come rather late in the history of Egyptian astronomy.

I mentioned a few episodes back that in the 20th century there were some debates about whether or not astrology first developed in Egypt or Mesopotamia. The astrology of the two cultures bears a lot of similarities to each other and there was quite a lot of cross-cultural contact during the first millennium BC, so it’s rather difficult to disentangle who influenced whom. We know, for instance, that there were Egyptian scribes in Assyria in the 8th century BC. But did those scribes tell the Assyrians about astrological practices from their homeland in Egypt, or did they bring Assyrian astrological practices back with them when they returned home? The general consensus today is that Mesopotamia had priority and that Egypt gradually adopted Mesopotamian astrological practices over the course of the first millennium BC. One of the more compelling pieces of evidence that Egypt imported its astrology from Mesopotamia is that although the Egyptian numbering system was base-10, when it came to their astrological calculations they always used a base-60 number system, just like the Babylonians. But, to give the Egyptians their due, there is also evidence that they in turn influenced the Mesopotamians. Documents in the Assyrian city of Nineveh written around 650 BC indicate that the Assyrians had adopted the Egyptian standard for their hours.

Well, just as the Romans went bananas for astrology when they started to learn about it from the Greeks, the Egyptians gradually became more enamored with the subject over the centuries. By the 5th century, Herodotus wrote,

The Egyptians assign each month and each day to some god; they can tell what fortune and what end and what disposition a man shall have according to the day of birth. … They have made themselves more omens than all the other nations put together; when an ominous thing happens they take note of the outcome and write it down; and if something of a like kind happens again they think it will have a like result.

Nearly half a millennium later Diodorus wrote,

The positions and arrangements of the stars as well as their motions have always been the subject of careful observation among the Egyptians, if anywhere in the world; they have preserved to this day the records concerning each of these stars over an incredible number of years, this subject of study having been zealously preserved among them from ancient times, and they have also observed with the utmost avidity the motions and orbits and stoppings of the planets, as well as the influences of each one on the generation of all living things — the good or the evil effects, namely, of which they are the cause. And while they are often successful in predicting to men the events which are going to befall them in the course of their lives, not infrequently they foretell destructions of the crops, or on the other hand, abundant yields, and pestilences that are to attack men or beasts, and as a result of their long observations they have prior knowledge of earthquakes and floods, of the risings of the comets, and of all things which the ordinary man looks upon as beyond all finding out.

During the Roman Era we have on the order of 60 Egyptian horoscopes that survive along with numerous coffins that were decorated with astrological symbolism. Around this time it was common for a wealthy person’s coffin to depict the goddess Nut surrounded by the zodiac along with a description of the individual’s natal horoscope. Some of these coffins even come with a kind of receipt and record just how much it cost the astrologer for his services. If you’re curious about getting your horoscope written in Egypt in the first century BC, it cost 10 obols. How much is 10 obols? As always directly translating ancient currency to modern money is a little iffy, but 10 obols was worth of order $1000 today. For comparison, some of the other references we have are that 1 obol would get you 4 bottles of wine in ancient Egypt, and the regulated price of a prostitute in Egypt was 3 obols per night.

Now, although Egypt appears to have originally imported its astrology from Mesopotamia the Egyptians did add their own unique flavor. The heliacal rising of Sirius, for instance, played a big role in Egyptian astronomy for millennia and naturally was incorporated into their astrology as well as a significant event that needed to be interpreted.

By the Hellenistic Era Egypt was probably the most important place for astrology in the classical world and the ancient temples got in on it by becoming increasingly involved in astrology themselves. Now unfortunately many of the specifics about Egyptian astrology don’t survive because the Egyptian priests, like many of the ancient cults, were notoriously secretive.

Strabo wrote of his travels to Egypt in the second part of the 1st century BC that

At Heliopolis the houses of the priests and schools of Plato and Eudoxus were pointed out to us; for Eudoxus went up to that place with Plato, and they both passed thirteen years with the priests, as is stated by some writers; for since these priests excelled in their knowledge of the heavenly bodies, albeit secretive and slow to impart it, Plato and Eudoxus prevailed of them in time and by courting their favor to let them learn some of the principles of their doctrines; but the barbarians concealed most things.

So after 13 years of persistence Plato and Eudoxus were able to glean a few scraps from the Egyptian priests, but no more. The author Vettius Valens speculated in the 2nd century BC that the Egyptian priests might have been so secretive because the actually didn’t know what they were talking about, and wrote

I cannot tell whether the Ancients, although knowing the efficacy of forecasting, were driven by envy to hide this art because of their vainglory and because the human mind finds this art difficult; or whether they spoke in such riddles even though they had not, in fact, grasped what Nature had created, had prescribed, and had bestowed abundantly on mankind after sealing it with Fate. Of all the lovely elements of the numerous great creations in the world, none seems to me to have been begrudged by God for man’s daily use. God would not have revealed it if He had not wished to provide it for use. In contrast, men have revealed this art only as they wished or as they were able. As a result, when I read over their work on astrology, I wonder at the crookedness and the obscurity of their thought. But I reveal whatever I have discovered by my experience, and in addition, I do not wish to conceal whatever I have gone on to discover because of the remarkable quality of many forecasts, both good and bad.

Now, while astrology really blew up in the first millennium BC after the Late Bronze Age Collapse and the end of Egyptian self-rule, if we look back to the New Kingdom there is some evidence for astrological practices back then, too. The tomb of Ramesses the Great, for instance, has the inscription, “The star of every decad will come forth for you to lengthen all your years…. You will sail like Orion on the heaven, your lifetime is its lifetime.” And a monument of his successor, King Merneptah, says “He is the one who will bring the end of his enemies, say those who scrutinize their stars and those who know all their formulas by watching the winds.” This, incidentally, is another point of similarity between Egyptian and Mesopotamian astrology because in Babylonia it was believed that one of the factors in determining the meaning of a lunar eclipse was the direction that the winds were blowing during the eclipse.

Well, I’ve taken something of a detour in our journey through the astronomers of ancient Egypt, but it’s also worth mentioning that the last native king of Egypt, Nectanebo II, who reigned from 358–340 BC had a deep interest in astrology and was often depicted with an astrology board. You may remember from back in Episode 26 on Roman astrology that astrologers of the ancient world often used a board and gemstones to visualize their predictions, the board representing the zodiac and the gemstones representing the positions of the planets.

I would also be remiss if I did not mention two other figures that show up in ancient Egypt, Petosiris and Nekauba. Nekauba was supposedly a king during the 26th dynasty and Petosiris was a priest, but whether these two figures were actual historical figures is unknown. If they did exist, we know virtually nothing for certain about them. The names Petosiris and Nekauba, however, came to be legendary by the first millennium and they were said to be sages and astronomers of unparalleled ability. They were supposed to have calculated the size of the universe out to the distance of Saturn and a document that purports to be a letter from Petosiris to Nekauba details a new numerological technique. But these legends about these ancient figures probably arose later on with the ulterior purpose of proving the superiority of Egyptian astronomy over Babylonian and Greek astronomy.

Well okay, I’ve gone through and listed a number of astronomers whose names have survived down to us, but I still haven’t said much about what they specifically did. Their focus on astrology came later, largely after the Late Bronze Age collapse, so what did they spend their time doing during the two millennia of the Old Kingdom, Middle Kingdom, and New Kingdom? In this period the main responsibility of the astronomers was timekeeping. During the night, the priests in the temple had to perform various rituals that corresponded to the journey of the Sun-God Ra through the underworld of the Duat to ensure that he survived the journey and rose safely the next morning. In this journey the Sun passed through twelve gates or twelve regions at the twelve hours of the night, and each region had its own character. The nature of this journey survives to us in the Amduat which tells us that Ra, riding in a boat, begins his journey at the western horizon and then in the second hour of night enters a realm called Wernes, or the Waters of Osiris. By the fourth hour he encounters a sandy land ruled by a hawk-God called Seker and must navigate a serpentine path. At the fifth hour he reaches the tomb of Osiris, the god of fertility and the afterlife. Osiris’s tomb is marked by a pyramid, which recalls the creation of the world from the Nun by Atum. You may recall that when Atum created himself out of the formless Nun, he emerged as a mound of Earth from the water below. Underneath Osiris’s tomb is a lake of fire. The most important part of Ra’s nocturnal journey comes at midnight, the sixth hour. Ra’s ka, or soul, had left him at sunset, but reunites with him at this crucial time, marking the point of resurrection. This was the most sacred moment during the night and was thus the most important for the temple priests. Then in the seventh hour Ra must battle against the monster Sep, a serpent who represents chaos and darkness. At the eighth hour, Ra opens the doors of Osiris’s tomb, and by the ninth Ra leaves the realm of Seker, and then in the 10th and 11th hour heals in the waters of Wernes. By the 12th hour he has reached the eastern horizon and is ready to rise at dawn for the new day.

It was the duty of the priests in the temple to ritually mark each of these events through the night to protect Ra on his journey, so they naturally needed to know what time it was in the middle of the night and it was the astronomers’ job to tell them this. In fact, one of the main words in Ancient Egyptian that gets translated as astronomer is etymologically connected to the word for “hour,” and literally means to “watcher of the hour.”

The division of the night into twelve hours is definitely known during the New Kingdom but is probably much older than that. We know that the Egyptians had the concept of hours during the Old Kingdom but the surviving texts don’t say exactly how many there were, so we can’t say for sure that there were always twelve. Interestingly, it seems that from the start, the Egyptians were especially interested in keeping track of the hours during the night rather than the day, because the hieroglyph used for the word hour consists of three stars.

To track the course of the hours through the night the astronomers stood on the terrace of the temple and observed the motions of the stars. What they were looking for evolved somewhat over the course of the millennia, but the general principle is what is called a star clock. The astronomer would watch for a particular star to rise or transit the meridian. When it did, this would mark the start of a particular hour. Now, the stars don’t rise or transit at the same time every night, they rise about 4 minutes earlier day, so you can’t look for the same stars all through the year, you have to regularly change them up.

The earliest method that the Egyptians used is what is called a “diagonal star clock.” They chose a set of 36 stars called “decanal stars” that are about 10 degrees apart from each other in right ascension. On any particular day, they would watch for the rising of 12 of these stars, every third one in the overall set of 36. The rising of each of these 12 stars corresponded to a particular hour of the night. Then, to account for this 4 minute daily drift, after 10 days they would switch to the next set of 12 stars. If you write out the stars in a table with each row being one of the hours and each column being one of the 36 sets of 12 stars, you find that as you move to the right the same star shifts up one row and so it appears to form a diagonal line, hence the term “diagonal star clock.”

This technique is quite old and dates back to the First Intermediate Period. Being so old we don’t have as much detail about the technique as we would like and there has actually been some debate as to whether or not these so-called diagonal star clocks were really clocks at all or if they were just observations of the stars. One of the issues is that all the surviving records come from funeral texts — the tables of decanal stars were written onto coffins or the walls of tombs. Because the surviving star clocks are primarily funerary in nature, they probably don’t represent the pinnacle of precision that Egyptian astronomy was capable of at the time. The versions of the star clocks that the Egyptians drew onto their coffins were probably more schematic since their primary function in this context was symbolic. One of the things we’d like to know, for instance, but don’t, is whether or not the rising of the star corresponded to the beginning or end of the particular hour it’s listed under.

Well, later on, during the Middle Kingdom the diagonal star clocks evolved somewhat into a new form, termed the decanal transit clocks. The idea was pretty much identical to their predecessor, except instead of looking for a star as it rose, they watched for when the star transited the meridian. This has the advantage that it’s generally a lot easier to see a transit since the star is overhead compared to the rising when the star is low on the horizon. First of all, even in the best of conditions, there can be a bit of ambiguity in spotting it as it rises and in practice you might have to wait for it to get a bit above the horizon to really be sure you’re seeing the right star. But the bigger problem is that if the atmospheric conditions aren’t perfect — if there’s dust or haze or clouds, this will generally be a more severe problem on the horizon. So you are just more likely to see stars that are right above you. Incidentally, the way the Egyptians referred to a star transiting the meridian was “a star which meets the lake.” You may remember that as I was detailing the journey of Ra through the Duat during the 12 hours of the night, when he gets to midnight and is at the nadir under the Earth, there is a lake beneath the tomb of Osiris. Well the Egyptians believed that a lake also existed symmetrically on the opposite side of the celestial sphere at the zenith as well, hence the “star meeting the lake.”

Well, the next major development in the star clocks has to wait until the New Kingdom, and as the Pharaoh Ramesses the Great is the big name of the New Kingdom, this new generation of star clocks gets referred to as the Ramesside star clocks. The older transit clocks still existed, but from this point forward they were only used for decorative purposes. It becomes clear that the Ramesside star clocks were the official way that time was kept during the night.

The Ramesside star clocks had a few differences from the diagonal star clocks and transit clocks that came before. The first is that the number of tables decreased. Instead of 36 tables of 12 stars there were now only 24 tables, so instead of switching to a new table every 10 days, you only had to switch tables twice a month. Interestingly, there is almost no overlap in the stars chosen for the Ramesside star clocks compared to the earlier transit clocks.

The other major change was that the astronomers were no longer looking only for transits across the meridian. In addition to the meridian, they tracked six other lines, three to the east of the meridian and three to the west. The way that this was depicted was with a grid of 7 vertical lines, representing the 7 lines of transit, and 12 or 13 horizontal lines, representing the various hours of the night. Superimposed on this grid they would draw a seated man whose center was aligned with the meridian line. They then described the other lines of transit by saying something like “when the star is on the left ear,” or “when the star is on the right shoulder.” How exactly the observations worked in practice is not known for certain, but one way it has been envisioned is that probably two astronomers would be facing each other on the roof of the temple, one on the north side, and one on the south side. The one on the south side would be holding a rod and the astronomer opposite him would watch for when the star crossed the rod. If the astronomer was looking for a transit across the meridian, the rod would be held vertically, if he was looking for a transit, “at the ear,” the rod would be tilted so that it went by the ear, and so forth. Now, since it relied on people holding their instruments, a technique like this could not have been very accurate, and this is true for any of the plausible interpretations of how these observations were made. But it’s worth bearing in mind once again that the evidence we have all comes from the funerary context, so the precise technical details were probably omitted.

One thing we do know for sure, though, is that he astronomers of Ancient Egypt used two instruments to make their alignments, called the “bay,” and the “merkhet.” Virtually every depiction of an Egyptian astronomer shows them holding a bay, and sometimes a merkhet as well, and a few of these instruments have actually survived down to us.

The bay was a tool used to make alignments. It was a palm rib with a groove in it. You can imagine a shape sort of like a flat stick that widens out at one end, and then has a deep notch in the middle of the wide end. Based on the drawings, it seems that the astronomer would hold the bay in his outstretched arm and then observe when the star fell into the groove. Now, if you’re just holding it in your arm, it’s hard to get a lot of precision since even if you hold your arm as still as possible it’s still going to wobble around a fair amount, so it’s possible that in practice they put it on something else to stabilize it, but if they did it doesn’t show up in the representations that we have. The bay seems to have been a kind of symbol of the Egyptian astronomer and was a prized possession for any astronomer. They weren’t plain, either, they were decorated and engraved with hieroglyphs.

One of the bays that survives is engraved with the astronomer’s name in a rather elaborate way, “Attend to the monitoring of the celebration and that all the men are placed in their hours, for the ka of the astronomer Horu, justified, son of the son of the King of the Two Lands, Horuduja, justified, son of his mother, Asetakhbit.”

The other main tool of the Egyptian astronomer was the merkhet, which literally translates from the ancient Egyptian word for “device.” The merkhet is a simple shape, it’s just a beam with a nub at one end. Presumably a plumb line was hung from the nub. Together these two instruments allowed a pair of astronomers to make alignments. One would hold a plumb line from the merkhet, and the other some distance away would observe the alignment between the plumb line and a background star using his bay. The merkhets that survive are also engraved with hieroglyphs. One says, “I know the movement of the two discs and of all the stars towards their corresponding place, for the ka of the astronomer Horu, son of Horudja.” The “two discs” here presumably refer to the Sun and the Moon.

Well, the Egyptians were primarily interested in keeping track of the hours during the night, but it’s a useful thing to be able to do this during the day as well. However, the star clocks weren’t so useful then, so for this, the Egyptians had a variety of sundials. In fact, the merkhet could do double duty as a shadow clock. The idea was that you would put it down flat with the little nub sticking up. Then in the morning, you’d have the merkhet going east-west with the nub on the eastern side. Then the Sun would cast a shadow on the long part of the merkhet. In the afternoon you’d flip it so that the nub was on the western end. Then the long part of the merkhet would then be marked with the various hours. By seeing which mark the shadow fell on you could tell what time it was. At least approximately. The merkhet seems to have been only designed to roughly tell the time during the day, maybe just as a kind of bonus function. The length of the shadows at particular times of the day changes over the course of the year, but the markings of the merkhet didn’t take this into account. They just use one set of markings all year long, so around the winter solstice it would be off by more than an hour. But the Egyptians nevertheless had purpose-built sundials which did have different markings for the different times of year.

Now these two methods of keeping time, the star clocks and the sundials, suffer from one big shortcoming. The weather has to cooperate for them to be of any use. If it’s cloudy out they’re no good. So to deal with this problem the Egyptians became quite adept at building another set of timekeeping devices called clepsydra, or water clocks. Now, whenever possible the Egyptians preferred to use the star clocks. Those were considered authoritative and the water clocks were only there as a backup. An inscription on one water clock explicitly states that it was only to be used if the stars could not be seen. But they were nevertheless quite good at their task.

Incidentally, this is not our first encounter with water clocks. The ancient Mesopotamians also seem to have developed water clocks as there are a number of references to them in the Babylonian texts, including the Enuma Anu Enlil and Mul Apin which I discussed way back in the first few episodes. But unfortunately no examples of Babylonian water clocks survive, so we are really just left to speculate about how they worked.

Fortunately this is not the case with ancient Egypt. First of all, we have many more detailed textual sources about their water clocks, and more importantly, hundreds of examples survive, so we have much more detail about how they worked, though, to be fair, most of the surviving examples are quite late, from the Ptolemaic or Roman eras.

But the oldest reference, however, comes from the beginning of the New Kingdom, right at the start of the 18th dynasty. There is an epitaph on the tomb of a man named Amenemaht. This man was a “bearer of the royal seal,” which means that he was a high official. The epitaph has 16 columns, although the top part has been destroyed. But, from what survived, we see that one of the things that the tomb claims is that he invented the clepsydra. Part of the epitaph reads, “I made a merkhyt calculated for the year. It was beautiful for the king of Upper and Lower Egypt, justified of voice… It was counted in view of each of its seasons. Never before had something like it been made since the primeval time of the land. I made this august measuring vessel in honor of the king of Upper and Lower Egypt, Djeserkare, justified of voice, divided in half… When entering [the season of] Shemu, … embracing the moon at his specific time, every hour at its time. The water flows off through one spout.”

Well, the oldest kind of Egyptian water clock is what is called an outflow clepsydra. It’s a pretty simple thing, it’s basically a cylindrical vessel that you fill with water that has a spout at the bottom to let the water run out. When the vessel is full, the pressure at the bottom is higher, so the water will run out faster than when the vessel is near empty. To correct for this, the vessel was narrower at the bottom than the top so that the change in height of the water was approximately constant. Then on the inside of the vessel they made a vertical line of 12 indentations to mark the different hours. When the level of the water fell to the next indentation that meant the arrival of the next hour.

But to improve accuracy, the clepsydra did not just have a single set of these indentations. Since the Egyptians divided their night evenly into 12 hours regardless of the season, an hour took longer in winter than it was in summer. But because the time that it took the water to run out of the vessel was the same regardless, they had to make an accommodation for this. So the clepsydra was actually marked with 13 sets of indentations, each for the different months with the spacings wider on some months and closer on others.

Now, in the oldest surviving examples only the vessel itself survives. Unfortunately the spout itself doesn’t survive which means it’s hard to tell what the outflow rate from the clepsydra actually was, which means we can’t tell the overall accuracy of the device. But we can tell from the angle of the walls how accurate the device was in relative terms. For an ideal water clock to have its height drop at a constant rate, the wall should be angled at 103° from horizontal. One of the older water clocks was constructed with an angle of 110°, so not too far off from the ideal inclination.

We can also tell from the spacing between the markings in the line corresponding to a given month how long the Egyptians believed the night to be during that month. Again, without a surviving spout we can’t determine this accuracy in absolute terms, but we can see what it was in relative terms. The ratio of the spacings between the marks for the months corresponding to the winter and summer solstices is 7:6. In other words, if the device measured the length of the night to be 12 hours long during the equinox, then the markings on the clepsydra implied that the length of the night during the winter solstice was 12 hours and 55 minutes. Now, this ratio is not quite right, it would have been more accurate if it was 7:5 instead of 7:6, which would put the length of night during the winter solstice to be closer to 14 hours long. But compared to the Babylonians this was far better. The Babylonians, you might recall from way back when, used a ratio of 2:1 for the length of night during the winter solstice vs the summer solstice. In other words, they effectively assumed that during the winter solstice the night was 16 hours long which is way, way off. You have to get quite far north before night is that long.

Well during the Ptolemaic period a new and improved kind of clepsydra was developed, called an inflow clepsydra. As its name implies, rather than watching the level drop as water flows out of the vessel, in the case of an inflow clepsydra you pour water into the vessel and watch the water level rise. The technique they used to ensure a constant flow rate is rather ingenious. The clepsydra consisted of three separate vessels stacked on top of each other. The topmost vessel acted as a kind of reservoir. It stored the bulk of the water and the water would flow out of it into the middle vessel, and this flow rate from this reservoir did need to be especially regular. The middle vessel acted as a kind of regulator. It was kept filled right to the brim and had an overflow gutter around its rim to drain the excess water away. Then, in the bottom of this middle vessel there was a small hole so that water then drained into the bottommost vessel. This vessel was the gauge and would have marks at various heights to represent the different hours. Because the water level of the second vessel was constant, the flow rate from its bottom would be constant as well, and so the height of the water in the bottom vessel would grow at a constant rate. This way, with an inflow clepsydra, the Egyptians did not need to worry about the top part being wider than the bottom part to account for the different flow rates as they did with the outflow clepsydras.

Well, if you look at a bunch of surviving Egyptian clepsydras you’ll notice a common feature. Almost all of them are decorated with baboons. In the case of an outflow clepsydra the baboon would typically be strategically placed so that as the water flowed out of the vessel it would appear as though the baboon was urinating. The reason for all these baboons is that the Egyptians associated baboons with the god Thoth and often depicted Thoth as a man with the head of a baboon. I haven’t said much about Thoth in the pantheon of Egyptian gods, but he was a moon-god, and probably due to this connection to the moon, he also developed a connection to time. So, it was only natural to put a representation of the time god on your clocks.

Well, I’ve tried to make clear in this episode that throughout most of Egyptian history, the principal job of astronomers was in keeping time, particularly keeping time during the night to keep the temple rituals on track. In fact this job of timekeeping was so important that it wasn’t limited to the temples. The Pharaoh himself also employed a staff of astronomers to keep track of time for him and these astronomers were not priests. But so far I’ve focused just on shorter time periods, the hours through the night. Of course in every society it’s generally been even more important to keep track of time on a longer scale, where you are in the year, and the astronomers were also employed for this job. But one of the nice things about the Egyptian calendar is that it is so logical and elegant that there is not actually a great deal to say about it. Otto Neugebauer called it “the only intelligent calendar which ever existed in human history,” though fans of the French Revolutionary calendar might quibble with that claim.

It’s maybe a statement of some kind about human nature that this calendar, though being incredibly logical and simple and one of the earliest to be devised, was never widely adopted outside of the culture that originally developed it. And after Egypt was folded into the Roman Empire this unique calendar fell into disuse.

The idea was very simple. The Egyptians took the length of the year to be exactly 365 days. They had 12 months of 30 days apiece. This left five days left over, which they called “days upon the year,” or which also get called “epagomenal days.” They treated these days as existing outside of any particular year. As you might expect, they were a period of rest in the calendar, but maybe surprisingly, it wasn’t considered an especially jubilant or celebratory time. To the contrary, the epagomenal days had something of a nefarious, sinister character in Egyptian culture. They were a time of especially somber religious rituals that the Pharaoh himself would take part in to ensure a safe passage into the new year.

Now despite its simplicity, the calendar took some time to develop. Like nearly every other culture the Egyptians seem to have started out with a lunar calendar. In fact these lunar origins were preserved in the hieroglyph for “month,” which was represented by a crescent moon on top of a star. Somewhat more uniquely originally the Egyptians started their new month on the last day that a crescent moon was visible before sunrise rather than the first day it is visible after sunset. This is a little trickier to get right since you can’t really know for sure that you’ve seen the last crescent moon until you’ve waited until the next day and not seen it, but presumably with practice the Egyptian astronomers could tell based on how thin the crescent was whether or not it was the last day of visibility. Since this original calendar was lunar, they had to deal with the problem of intercalations. The length of 12 lunar months is about 355 days, so your calendar shifts by about 10 days every year and periodically you need to stick an extra month in. For this, they relied on the heliacal risings of Sirius.

Now I haven’t said as much about the role of Sirius in Egyptian astronomy as it perhaps deserves, because Sirius, known by the Egyptians as “Sothis,” really played a central role. The reason is that Sirius is the brightest star in the night sky, and its heliacal rising was roughly around the summer solstice, which corresponded to the time of the year when the Nile began to flood. As I tried to emphasize two episodes back in the first episode on Egyptian astronomy, the flooding of the Nile was the central event in Egyptian life. Everything revolved around it. Consequently, the rising of the Nile marked the start of the new agricultural year, and because this corresponded with the heliacal rising of Sirius, that was an ideal point to formally begin the new year. In the Temple of Hathor, for instance, we see the inscription, “Years are reckoned from the shining-forth of Sothis.”

The annual journey of Sothis from heliacal rising to heliacal rising was taken as model of a person’s life cycle. After rising, the star spent 80 days on the eastern sky. This time corresponded to childhood. Then, after 80 days, around sunset it would be at the meridian. At this point, the star was said to “work” for the next 120 days because it was possible to observe the star transiting the meridian. This meant that during this time of the year, the star could be used in a star clock and provide some value to the astronomers. After this 120 period of working, the star entered into a time of old age, where for 90 days it would rise sometime during the night, but wouldn’t quite make it to the meridian before sunrise. Then Sirius would have its heliacal setting and it would spend 70 days too close to the Sun to be visible at any point during the night. This 70 days that Sirius spent below the horizon, or in the Duat, was associated with death and for this reason, the ritual embalming process to produce the famed Egyptian mummies lasted 70 days.

So, in the very earliest calendar, the Egyptians used the heliacal rising of Sirius to regulate intercalations. If the heliacal rising occurred in the last 11 days of the month, an extra month would be added to the calendar.

Now throughout this series we’ve come across various intercalation schemes in different cultures and a main impetus that drove a lot of astronomical research was to be able to predict in advance when a new intercalary month would be needed. If you’re an administrator, it’s just useful to be able to have a calendar far in advance rather than winging it month by month. So very early on, certainly by the 5th dynasty, and probably during the 4th, the Egyptians developed the so-called “secular calendar” which is the precise, regular calendar of 12 months of 30 days, with the 5 extra epagomenal days tacked on at the end.

Now, this calendar has an issue, which is that the true length of the year is about 365.24 days. Since they didn’t have any leap days, each and every year was exactly 365 days, the calendar drifted by about a quarter of a day every year. If the Egyptians noticed this as a bug, they labelled it “won’t fix.” There is no evidence that the Egyptians really saw this drift as a problem at all. And, to be fair, this drift, while measurable, really isn’t all that great. If you imagine a healthy person’s working life in ancient Egypt to be about 40 years, the calendar drifted by about 10 days over the course of that time span. Given that the time that the Nile floods can vary by about a month and a half, and that’s what everyone really cared about, this gradual drift of a quarter of a day per year wasn’t such a big deal. During the New Kingdom and the Ptolemaic Eras there were a handful of abortive efforts to modify the calendar and introduce a leap day, but these changes faced so much pushback that they were abandoned and it wasn’t until the Roman Emperor Augustus could exert his iron will upon the Egyptian people, and more importantly the Egyptian priesthood, that they used a leap day.

Now the development of the 365 day secular calendar still leaves unanswered the question of how the Egyptians figured out that 365 was a good number of days to have in your year. And this is actually an open question. There are four possibilities. Probably the one with the most evidence is the so-called Sothic hypothesis. The heliacal rising of Sirius was an important event in the Egyptian year and was closely watched for. So, just by counting the number of days between successive heliacal risings, they would have gotten about 365 days to the year. Incidentally, one of the curious features of the heliacal rising of Sirius is that although the length of the year is about 365.24 days, or just shy of 365 and a quarter, Sirius has a rather high proper motion, and this motion on the sky somewhat adds to the time between successive risings, so that the time between heliacal risings of Sirius is almost exactly 365 days and a quarter.

Another hypothesis is the solar hypothesis, that the Egyptians measured the time between successive solstices or equinoxes. The main issue with this hypothesis is that there’s not a lot evidence that the Egyptians were all that interested in the solstices and equinoxes. It’s true that we can see some alignments with some of their structures towards the solstices, and of course the Pyramids are oriented towards the cardinal directions which would include the equinoxes, but it doesn’t seem that they really cared about these events per se, and it’s not clear that they went to the effort to measure them accurately enough to get to 365 days in a year.

Another plausible hypothesis is the lunar hypothesis, that they used the average length of a lunar year. After all, they had evidently been using a lunar calendar before, so the data probably would have been available to them. The last hypothesis, favored by Otto Neugebauer, but probably few others, is the so-called Nilotic hypothesis, that they used the average time between risings of the Nile to figure out that the year had 365 days. The main thing that this theory has to recommend it is that the Egyptians cared very deeply about the flooding of the Nile and definitely kept track of it. But the main problem with this idea is that as I mentioned earlier, there’s a lot of variation in the time that the Nile floods, up to a month and a half, so the only way this could have worked would be to have taken records collected from many years and averaged them all out to get to 365 days. Now, this could certainly have been done in a person’s lifetime, but the idea of averaging many years of very noisy records like that over many years is a rather modern one, and there’s just not much evidence that people thought that way in the ancient world, or at least in ancient Egypt in the 4th or 5th dynasty.

Regardless, the Egyptians ultimately ended up on a 365 day year and so as the centuries passed by the calendar drifted through the year with respect to the seasons. This actually makes understanding what the Egyptian seasons were a little bit challenging. One of the unusual features of the Egyptian calendar is that they only recognized three seasons. In most places the seasons are identified based on the weather patterns. But in Egypt the weather didn’t change all that much through the year and more importantly, it was not really relevant to agriculture. Instead, the Nile ruled everything. So the ancient Egyptians had three seasons each one four months long, based on what the Nile was doing. The first was called Akhet, which meant “inundation,” or “flood.” This was around late June or early July when the waters from the monsoons in the Ethiopian highlands had run off and finally reached the first cataract, causing a sudden rise in the river’s levels. The following season was called “peret,” and meant “emergence,” or “planting.” At this point the waters subsided but left the soil in the river valley damp, so this is when planting took place, and throughout the season the plants would grow. Finally, the third season was “Shemu,” which meant “low water.” The soil had dried out by this point and the crops were grown, so this was the time of the harvest. Originally the months were just numbered based on how far into the season they were, so one month would be called, “first month of the flood,” the next would be “second month of the flood,” and so on. It wasn’t until sometime during the New Kingdom the months acquired unique names.

Now, the translations I just listed for the three seasons is the standard way that akhet, peret, and shemu are understood. But there has actually been a little bit of debate as to whether or not those translations are correct or if they should be shifted over by one. After all, because the Egyptian calendar had 365 days, as the centuries passed the four months that corresponded to akhet just drifted through the real seasons. So it’s a little hard to tell which months originally correspond to which actual seasons.

Well, there are more details about Egyptian astronomy that I could go into but the world is large and time is short, so I am afraid we will have to move on from Ancient Egypt. In the next month we will go east and begin to look at the astronomy of another great civilization that developed in a river valley, ancient India. I hope you’ll join me then. Until the next full moon, good night and clear skies.

Additional references

  • Belmonte, Astronomy of Ancient Egypt
  • Clagett Ancient Egyptian Science Vol. 2
  • Magli, Archaeoastronomy
  • Kelly, Exploring Ancient Skies
  • Ruggles, Handbook of Archaeoastronomy