Astronomy of ancient Egypt, the origin of the calendar

Picture of the Great Sphinx of Giza with the sky in the background

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The observation of the sky led the ancient Egyptians to invent the calendar to govern their religious festivals and control the annual flood of the Nile

The Elephantine calendar
During the reign of Thutmose III, this calendar of festivals was elaborated, in which the Heliacal Ortho of Sirius is mentioned, when the flood begins. 
Louvre Museum, Paris.
The Elephantine calendar
During the reign of Thutmose III, this calendar of festivals was elaborated, in which the Heliacal Ortho of Sirius is mentioned, when the flood begins. 
Louvre Museum, Paris.

Nut, the heavenly goddess
Shu, the god of the air, tries to separate the goddess of the sky Nut and the god of the earth Geb.
Nut, the heavenly goddess
Shu, the god of the air, tries to separate the goddess of the sky Nut and the god of the earth Geb.
 Dendera Zodiac
In a chapel of Osiris in the temple of Dendera, a relief was discovered that represents the cardinal points and the 36 deans (on the left, a reproduction).
Dendera Zodiac
In a chapel of Osiris in the temple of Dendera, a relief was discovered that represents the cardinal points and the 36 deans (on the left, a reproduction).
The job of checking the time
The "observers of the hours" wore a panther skin studded with stars. In the image, the star watcher anen. Diorite statue. Dynasty XVIII. Egyptian Museum, Turin.
The job of checking the time
The “observers of the hours” wore a panther skin studded with stars. 
In the image, the star watcher anen. 
Diorite statue. 
Dynasty XVIII. 
Egyptian Museum, Turin.
The sky seen by the Egyptians
In one of the chambers of the tomb of Queen Hatshepsut is the oldest astronomical ceiling discovered to date in ancient Egypt. This roof is divided into two parts, which correspond to the north and south of the sky, that is, the northern and southern sky. The animals, symbols and divinities that populate this "sky" are a precious key to understanding how the Egyptians of Antiquity interpret the cosmos.
The sky seen by the Egyptians
In one of the chambers of the tomb of Queen Hatshepsut is the oldest astronomical ceiling discovered to date in ancient Egypt. 
This roof is divided into two parts, which correspond to the north and south of the sky, that is, the northern and southern sky. 
The animals, symbols and divinities that populate this “sky” are a precious key to understanding how the Egyptians of Antiquity interpret the cosmos.
Orientation of a temple
Seshat, goddess of writing, together with the king, tightens a rope to determine the axis of the temple based on certain astronomical calculations. Relief of the temple of Edfu.
The orientation of a temple
Seshat, goddess of writing, together with the king, tightens a rope to determine the axis of the temple based on certain astronomical calculations. 
Relief of the temple of Edfu.


Astronomy in ancient Egypt, the origin of the calendar

“Their calendar is, in my opinion, better than that of the Greeks, because […] the cycle of the seasons always appears at the same time for them.” Already the Greek Herodotus, the father of history, marveled in the fifth century BC. for the perfection of the calendar of the ancient Egyptians. Since then the analysis of the calendar has fascinated many scholars. 

This is an exciting issue, which raises a number of important questions such as how many calendars were used in Egypt? What were their origins? How did they evolve? And, finally, can the Egyptian chronology be set from astronomical data related to the dates marked on the calendar? Most of these questions can be answered in a simple and reasonable way in the context of the Pharaonic culture itself. 

The calendar, in turn, is part of the broader interest the Egyptians felt for the world of stars. In the pharaonic culture, the sky became a crucial element of the landscape. Today, we can learn about the astronomical practices of the ancient Egyptians thanks to a large number of hieroglyphic sources, from monumental inscriptions, pyramid texts and astronomical papyri to star clocks or celestial diagrams. 

The archaeological sites offer researchers the keys to the employment that the Egyptians made of their astronomical knowledge. As shown in the layout of their great monuments, such as temples or tombs, including the pyramids.

The beginnings of astronomy in Egypt and its Saharan environment go back beyond the Predynastic period. In fact, in the Neolithic site of Nabta Playa, dated around 4000 BC, alignments of stones have been found in which the first intention of time control seems to be reflected. That could indicate the importance of the summer solstice (21 of June, the longest day of the year) as a temporary marker already at such early dates. However, the interpretation of this site is very controversial among Egyptologists. 

The origin of the calendar 

The first undoubted astronomical observations and the oldest iconography are located in the original phase of the Egyptian civilization proper, in the Predynastic and the Protodynamic, as well as during the reign of the first pharaohs of Dynasty II, Hotepskhemuy or Nebre, when it began to develop solar worship. It is precisely at this time when the genesis and the early evolution of the 365-day civil calendar must have occurred. one of the most momentous discoveries of the ancient Egyptians. Of which there is evidence that it was already in full use during the Old Kingdom. 

Astronomy, or the observation of heaven in a broader sense, was a discipline that in Egypt was never very separate from religion. In fact, the best astronomical “texts” – representations of the night sky – have been found in tombs or temples. Also, the Egyptian astronomers, the imy unut or “observers of the hours”, were mostly priests, some of high-rank, in addition to exercising some other profession. 

Among them, it is worth mentioning Imhotep (in which the inventor of the civil calendar has been wanted to see on numerous occasions). Also, Senenmut, respective architects of the stepped pyramid of Djoser in Saqqara and the majestic temple of Queen Hatshepsut in Deir el-Bahari. So was Anen, brother of Queen Tiyi and uncle of Akhenaten, one of the few “astronomers” of whom a portrait is preserved. 

The civil calendar of ancient Egypt consisted of twelve months of 30 days each (divided into three groups of ten), which made a total of 360 days. To these were added the “Five over the Year“, five additional days, called epagomenes by the Greeks, who, at least since the New Kingdom, were dedicated to five of the most important deities of the ancient Egyptians: Osiris, Isis, Set, Neftis and Haroeris; in fact, they were considered to be the respective days of the birth of each of those gods. These five days, which completed the total of 365, were considered separate and not within the general calculation of the year. 

This instant was supposed to mark, or announce – at least since the Middle Kingdom – the arrival of the flood of the Nile

Sirius appears on the horizon 

One of the peculiarities of this 365-day calendar is that it lacks a leap year. Since the duration of the tropic year (that of the seasons) is practically a quarter of a day, this means that all cyclical events, including astronomical events, are delayed one day every four years.

The Syrian star, called Sopdet by the Egyptians, gives rise to one of these unique events: its first annual appearance at dawn, the so-called heliacal ortho. 

The ancient Egyptians called it peret sopdet. Wich, was one of the most important festivals of its annual cycle since it was supposed that this moment marked, or announced – at least since the Middle Kingdom – the arrival of the flood of the Nile. This natural phenomenon had enormous social and economic significance.

With the 365-day calendar system, however, the date of the Sirian Ortho was delayed one day every four years, which meant that it turned the entire civil calendar over a period of just under fifteen centuries. 

A calendar for everyone 

Since the dawn of modern Egyptology, several hypotheses have been proposed on the origin of the civil calendar: some authors consider it a calendar of solar origin, others stellar, others luniestellar … However, perhaps the most suggestive hypothesis is the one that raises that the Nile had something to do with it

At the origin, before the unification of the country, the Nile Valley societies had to be governed by local lunar calendars determined by the Nile, since the phases of the river’s flood marked life in the territory, as can be deduced from the three seasons in which they divided the year and the name given to each of them: Flood (akhet), Resurgence (peret) and Drought (shemu). 

However, once Egypt was unified, it became necessary to create a calendar that governed the destinations throughout the country. Traditionally it has been thought that the origin of this civil calendar is related to the Heliacal ortho of Sirius, but there is no documentary evidence to prove it. Almost certainly the duration of the civil calendar was determined through solar observations, of which there is varied and incontestable evidence for very early periods. In fact, the duration of the civil calendar thus established is very close to that of the tropical year of 365.2425 days. 

For the realization of some festivals, a computation of time guided by the phases of the moon was maintained as a vestige

It is interesting to ask about the number of independent calendars that were in operation in Egypt. Quite possibly, the civil calendar reigned supremely in ancient Egypt since its invention as the standard form of time calculation for almost all public and private activities, at least until the conquest of the country by the Persians and Alexander the Great. However, for the realization of some festivals, a computation of time guided by the phases of the moon, whose origin may be in the original local lunar calendars ruled by the Nile, was maintained as a vestige; As in the modern Gregorian calendar, Easter is set by the moon (as in the original Jewish calendar), but according to the current calendar dictated by the sun. 

The civil calendar would remain the official one in Egypt until the Roman conquest, at which time it was replaced by the Alexandrian calendar, almost identical but with an additional day or epagomene every four years. In the year 46 BC, Julius Caesar adopted this Egyptian calendar and is the one that was in force in Europe until the Gregorian reform of Pope Gregory XIII, in 1582. 

Star Chronology 

The Egyptian calendar has played a fundamental role in determining the reign dates of the various pharaohs. The chronological study is one of the most controversial and fascinating disciplines of Egyptology, and almost every Egyptologist opts for one or another chronology; At least there are five chronological systems of frequent use, each with its advantages, disadvantages and particularities, including also more recent, controversial and some even revolutionary proposals. In any case, the backbone of all of them is made up of astronomical dates, mainly the “sotíacas” – referred to the dates of the Heliacal ortho of the star Sirius, called Sothis by the Greeks – and the lunar. 

For example, two inscriptions of the reigns of Sesostris III (dynasty XII) and Ramses II (dynasty XIX) allow dating the Middle and New Empires, respectively. The first one says: «I inform you that the exit of Sirius [peret sopdet] will take place there in IV peret 16», while the second one says: «Year 52, second month of peret, day 27 in the House of Ramses Meriamón, or Piramsés, [is] novilunio [psedjentyu] ». 

It can be seen how the dates are recorded by the year of the reign of the pharaoh, the season, the month and the day, as well as the information that that day was the date of the Heliacal ortho of Sirius or the novilunium, respectively. Egyptologists rely on these last references to try to fix the exact year to which reference is made, although the particular criteria mean that there are differences of tens of years, even centuries, between the dates of each student. 

Archeology and astronomy are not as different and distant disciplines as it may seem, since the two study the past: archeology, the past of man; astronomy, that of the universe, with the common objective of understanding our present and trying to improve the future. The ancient Egyptians also used astronomy for the same purpose, making it a generator of some of the key elements of their culture, such as their calendar; elements that undoubtedly helped them find their place in the world. The stability and longevity of their civilization clearly show it. 

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