Most ancient people had calendars based upon the seasons, or the Sun, or the Moon, or the stars. The Ancient Egyptians had calendars based on all four! And they were all in use at the same time!
The calendars were so important to the Ancient Egyptians that Pharaoh had to swear an oath not to change them.
Some books on Ancient Egypt think that because children today only have one calendar it is best not to teach them that the Egyptians had lots of them, so they simplify everything into just one calendar. But I think that if Ancient Egyptian children could cope with lots of calendars then so can today's children. After all, most children at school in England today know about the Christian calendar, the Moslem calendar, the Hindu calendar and the Chinese New Year!
In Britain we have four seasons: spring, summer, autumn and winter. In other regions of the world such as parts of Africa and Asia there is a dry season and a rainy season.
There were three seasons in Ancient Egypt and these depended, like everything else in Egypt, on the River Nile.Akhet (Inundation) was the season when the River Nile was in flood
Using our modern calendar these seasons were about
The Ancient Egyptians wrote in hieroglyph. We can translate hieroglyphs into English, but we can also transliterate them, that is, write them in English letters so we know how to pronounce them. For example, the hieroglyphs the Ancient Egyptians used for the first season of the year we can translate as inundation and transliterate as Akhet. But because we cannot be quite certain how Ancient Egyptian was pronounced you may see it transliterated in different ways. And of course similarly for all Ancient Egyptian words.
Akhet began at the time of the heliacal rising of the brightest star in the sky, the star we call Sirius (or the Dog Star), the Greeks Sothis and the Egyptians Serpet. This is where the stars come in!
To people living in towns in England today the stars are not very important - we can only see a very few of the very brightest stars, and these not every night. But in Ancient Egypt
So the people living in ancient Egypt saw thousands of stars blazing down on them, not just occasionally but all night and every night. To them, as to almost all the ancient people, the stars were very important indeed.
The Earth's atmosphere contains lots of water vapour. As we go higher and it gets colder some of this water vapour condenses to form droplets of water. These are the clouds we can see. Higher still the atmosphere is colder still and contains not droplets of water but very tiny crystals of ice. We cannot see these ice crystals but they scatter the Sun's rays. This makes the day-time sky seem very bright, and blue. (Snow is also made of crystals of ice and so scatters the light in the same way. Snow fields are very bright, and blue if you look through them rather than down on them.) We cannot see the stars in the day time because the sky is too bright, but they are still there: if we go up in a rocket, or even a high-flying jet, above the layer of ice crystals, the sky is black and we can see the stars even by day. We can also see the stars during a total eclipse of the Sun.
The Earth is spinning on its axis - this is why the Sun and Moon rise and set. The point in the sky exactly about the Earth's North Pole is called the Celestial North Pole, and as the Earth rotates all the stars seem to move in a circle round it. Polaris (the Pole Star) is very close to the Celestial North Pole so is always visible every night and always due North. The stars near Polaris, the circumpolar stars (from the Latin for around the pole), circle round Polaris but do not rise and set; the stars further away from Polaris rise in the East and set in the West, just like the Sun and Moon. Many ancient people associated the circumpolar stars, which never set, with the immortal gods. Although the circumpolar stars do not rise and set we can only see them at night. The stars of the constellation called Ursa Major (which English children call the Great Bear and American children call the Dipper) are circumpolar stars.
The time at which a star (except of course a circumpolar star) rises and sets gets about four minutes earlier every day. So if one day a star is rising after sunset and setting before sunrise, a few weeks later it will be rising before sunset, but of course we cannot see it because the sky is too bright. By the time we can see it, after sunset, it will already be high in the sky, and it will set a long time before sunrise. A few weeks later it will rise after sunrise and set before sunset so we shall not see it at all; then a few weeks later it will rise just before sunrise, as the sun begins to light up the sky, and we shall start to see it in the dawn sky. At this time it will only be visible for a short time, until the sky is too bright to see it, but each day it will rise four minutes earlier and so be visible for a little longer. Eventually it will be rising just after sunset and setting before sunrise so we shall see it all night, and then the whole cycle starts all over again.
Every star of course rises and sets at a different time, but every night there are always plenty of different stars to see.
The first time a star reappeared after its period of invisibility was very important to the ancient Egyptians, and to all the ancient people. This first rising of a star in the dawn sky in the East, just before Sunrise, is called its heliacal rising (from the Greek for rising with the Sun). To the Egyptians it represented the end of the time the star had spent in the Underworld.
Almost as soon as the first people began to settle in the valley of the River Nile, before 4200 (remember all dates in Ancient Egypt are BCE so we do not usually say so), they noticed that the heliacal rising of Serpet, after a period of seventy days of invisibility, always came a few days before the start of the annual inundation. This enabled the ancient Egyptians to predict the coming of the inundation. The heliacal rising of Serpet was the start of Akhet, and the new year.
The time at which the River Nile starts to flood depends upon a number of factors, and these all depend on the time taken for the Earth to go round the Sun, which gives us our seasons. This is a solar year, about three hundred and sixty five and a quarter days - the quarter day is the reason why we have to have leap years, when we add an extra day. But a solar year is not exactly three hundred and sixty five and a quarter days so we must not add an extra day every four years. But, for reasons not explained here, the time between two heliacal risings of Serpet, at the latitude of Egypt, is about twelve minutes longer than a solar year.This means that their stellar calendar lost one day in about a hundred years compared to a solar calendar.
By the time the difference between the solar and the stellar calendar was big enough to make the day they had predicted for the start of the inundation to be wrong by more than a few days, the Ancient Egyptians had realised the start of the inundation depended upon a solar year and not the heliacal rising of Serpet, and had measured the length of a solar year so precisely that they were able to make a calendar more accurate than the Julian calendar, which had a leap year every four years, being used in Europe nearly four thousand years later! But they were not allowed to change the old calendar, so Akhet still started on the day of the heliacal rising of Serpet, even though they were using a solar calendar to predict the actual date of the start of the inundation.
The Moon rises and sets because the Earth is spinning on its axis, but the Moon is also going round the Earth and this affects how we see the Moon.
The Moon does not give out any light of its own: we can only see it because the Sun is shining on it, so the only part of the Moon that we can see is the part which is facing both the Earth and the Sun.
The Moon crosses (or more usually passes just over or under) a line between the Earth and the Sun about once every twenty nine and a half days. As it crosses this line no part of it faces both the Earth and the Sun so we cannot see the Moon at all. The first evening after this has happened we shall see the New Moon as a very tiny crescent in the Western sky just after Sunset. Gradually this crescent will get bigger and bigger, until after about fourteen days we shall have a Full Moon. Then it will get smaller again and eventually we shall be able to see just a very tiny crescent in the Eastern sky just before Sunrise. The next night we shall not see the Moon at all and then the whole cycle starts all over again. As we can only see the New Moon in the evening, the time from our first sighting of one New Moon to our first sighting of the next must be a whole number of days, and is either twenty nine or thirty days. This is a lunar month ( from moonth).
Most ancient religions used lunar calendars. All the religious festivals were celebrated on a calendar based on lunar months. Each month started the first evening the New Moon was visible, and lasted for either twenty nine or thirty days, until the first sighting of the next New Moon. The Ancient Egyptian priests also used a lunar calendar for all their religious festivals, but their new month started not the first day the New Moon was visible just after Sunset in the West but the first morning the Old Moon was not visible just before Sunrise in the East.
In Ancient Egypt there were (usually) twelve lunar months in the religious year. The twelfth month was called Wep-Renet. But as twelve lunar months come to only three hundred and fifty four days, slightly less than a solar year, the Priests added an extra month, called Thoth, if the heliacal rising of Serpet occurred during the last eleven days of Wep-Renet, so as to keep the religious calendar in step with the seasonal calendar and the flooding of the Nile. Usually they needed to add the extra month about once every three years.
The Ancient Egyptians were highly organised with a very efficient central government. The administrators needed a calendar simpler than one in which they did not know when the month started until the Priests had looked at the sunrise, and in which all the months had the same number of days and all the years the same number of months. So they introduced a civil calendar containing twelve months each with thirty days, and each month containing three weeks of ten days, and then five days of public holidays to bring the year to three hundred and sixty five days. These five holidays celebrated the birthdays of Osiris, Isis, Horus, Nephthys and Seth. The problem was that this calendar did not have leap years (although the Priests knew about them) because all years had to be the same length, so compared to the seasonal calendar it lost about a day every four years, or nearly a month every hundred years. So sometimes they were celebrating the birthday of Osiris in Akhet, sometimes in Peret and sometimes in Shomu.
A calendar which wanders in this way is called a vague calendar, from the Latin word vagus which means wandering.
Unfortunately in English we have to use the word day to mean both the time between sunrise and sunset and a period of twenty four hours, so we can say There are twenty four hours in a day, but the days are longer in the summer than the winter. Fortunately it is usually clear which we mean!
Most of the ancient people ended one day, and so of course started the next, at sunset, but the Ancient Egyptians started their new day at sunrise.
The Ancient Egyptians divided the day-time into twelve hours, numbered one to twelve, and the night-time into another twelve. The hours were not all the same length: in the summer the hours of the day were longer than the hours of the night, and of course the other way round in the winter.
The skies in Ancient Egypt were always clear so measuring the passing of the hours was easy: during the day they used a sundial and at night they used the stars.
Different stars rise at different times, depending on the seasons, and the Ancient Egyptians measured the hours of darkness by watching for the rising of certain special stars. The special stars they used to measure the hours of darkness were called decans. The stars painted on the ceilings of the pyramids were there so that the dead King could tell the time!
Later on they used water clocks ( clepsydra) to time things more accurately than they could with just a sundial or the stars.
We may think that the use of four different calendars all at the same time would have been very confusing but the Ancient Egyptians seemed to manage very well.
The Ancient Egyptians were the first people to make a calendar which kept in step with the Sun and the seasons. It was a Greek living in Egypt who persuaded Julius Caesar to adopt a calendar based upon the Ancient Egyptian calendar throughout the Roman world. We call this calendar the Julian calendar, after Julius Caesar.
There is more about the Sun, the Moon and the stars, and also all the other things we can see in the sky, in another section of this web site. To link to it please click here
Most of the information on this page comes from Mapping Time by E G Richards. There is another, very long, article about different calendars and their history, not only the Ancient Egyptian calendar, elsewhere in this web site. To link to it please click here
© Barry Gray May 2000