Classic Computer Magazine Archive ANTIC VOL. 8, NO. 3 / JULY 1989



By Chris Carrier

Feel like a Pharoah! Convert any date from the Julian or Gregorian calendars back to the calendars used in ancient Egypt. This BASIC program works on Atari 8-bit computers with at least 48K memory.

Your Atari can make you an Egyptologist. Egypt Calendar takes any date in the Julian or the modern Gregorian calendars, and gives the equivalent dates in both of the civil calendars used in ancient Egypt, one of which survives even to this day.

Just enter the date, and a screen full of information about that day and year appears. Not only does the program give the original Egyptian calendar date, but the year is given as determined by the reigns of several ancient kings, or by the Alexandrian, Augustan and Coptic calendars. The program can even tell you the day of the week on which that date fell, or the Julian Day number, useful for astronomers.


Type in Listing 1, EGYPT.BAS, check it with TYPO.II, and SAVE a copy to disk before you RUN it.

The program will then ask you to input a date. If the date is between 1582-1923, you will be prompted to specify whether the date uses the Julian or Gregorian calendar. The Gregorian calendar began in 1582 A.D., but the last nation to switch from Julian to Gregorian (Greece) did not do so until 1923.

Then press [RETURN] to see how your date translates. At the top of your screen the date you entered will be displayed, followed by the Julian Day Number. Used by chronologists and astronomers, this number simply tells you how many days have passed since January 1, 4713 B.C. (JD #0).

The original Egyptian calendar date follows, with the year as determined by eight different eras. At the very bottom of the screen is the date of the Sothic Rising, the astrological phenomenon used by the ancient Egyptians to track the actual, as opposed to calendar, year.

During the first couple of millenia of the calendar's existence, the months had no names, but were simply referred to as the first, second, third.. .month of their season.


The ancient Egyptians, among their other accomplishments, were probably the first people in the history of the world to discover the number of days in a year down to the nearest integer.

The original version of the Egyptian calendar had a week of 10 days, a month of 3 weeks or 30 days, a season defined as 4 months or 120 days, three seasons equaling 360 days, which were followed by five unnamed epagomenal, or "outside the calendar," days to total 365 days in a year.

The year began with the season of Akhit (Flood, as in Nile River), followed by Perit (Winter) and Shemu (Summer). Egypt Calendar gives this date thus: Shemu 2-14 for the 14th day of the second month in the summer (Shemu) season. The five epagomenal days are treated by the program as a five-day, fifth month of Shemu. Because a tradition developed that any work done on the epagomenal days was unlucky, the ancient Egyptians ended their year with a five-day festival.

The great advantage of the Egyptian calendar is that it was easy to use-it survived in daily use for more than 3,000 years. Astronomers and historians used the calendar for convenience of chronology as late as the 16th century. France tried a version of it shortly after the French Revolution, in the late 18th century. This calendar has even drawn praise from a 20th century astronomer (Neugebeuer, "A History of Ancient Mathematical Astronomy") for being the -"most sensible of all calendars used. by mankind", with its easy-to-use 10-day weeks, 30-day months, and 365-day years.


The great disadvantage of Egyptian calendars is that the number of days in the year is not 365, but rather 365.2422 days in a tropical year (the cycle of the seasons) or 365.2564 days in a sidereal year (one orbit of Earth with respect to the stars).

Therefore, the Egyptian calendar ran about 1 day fast every 4 years, so if an annual event occurred on, say, Akhit 1-1 in a given year, it would occur on Akhit 1-2 after four years, Akhit 1-3 after eight years, during the second month of Akhit after about 120 years, and eventually, the event would go around the entire year. It would have been obvious within, say, 100 and certainly before 200 years that the calendar needed to be corrected for the true length of the year.

However, the calendar wasn't corrected for thousands of years, but the Egyptians invented a second new year. The "true" year, as opposed to the calendar year, began when Sirius, the brightest star in the sky (except for our own sun) appeared for the first time in the predawn sky after having been behind the sun and invisible. Because the Egyptian name for Sirius is "Sothis," this annual event is called the Sothic Rising.

On July 20, 139 A.D., a Roman living in Egypt by the name of Censorinus observed a Sothic Rising on the first day of the Egyptian year. He furthermore stated that as the Julian calendar contained 365.25 days, and the Egyptian 365, that 1460 Julian years equaled 1461 Egyptian years, and he labeled this the "Sothic Cycle," or the "Great Year" of the Old Egyptian calendar, now known as the "Julian Sothic Cycle."

Since Censorinus' time the exact length of the sidereal year has become known, and the true Sothic Cycle is 1422 Old Egyptian years long. The next time a Sothic Rising coincides with Akhit 1-1 will be on August 27, 2985 A.D.

Also of note is the fact that the Sothic Rising occurs in the Julian calendar about 1 day later every 150 years, and in our Gregorian calendar, 1 day later every 72 years. This is because of the precession of the equinoxes, the same phenomenon which makes Polaris the North Star in our lifetimes and Thuban the North star during the building of the Great Pyramid.

Although any estimates as to exactly when the Egyptian calendar began to function are only educated guesses, Sothic Risings on Akhit 1-1 occurred in 1282-85 B.C., 2706-09 B.C., and 4130-33 B C. The program designates the last mentioned date as Cycle #0, and it is believed that the calendar may have started regular use about the beginning of Cycle #1.


There are two surviving accounts of ancient Egyptians observing a Sothic Rising-during the reigns of Senworset III and Amentohep I. Because of this, we can calculate the exact year their reigns began, and this date is given in the program; in ancient Egypt, and in fact in most of the world's monarchies, the year used is the number of years since the accession of the current monarch.

In the second century A.D. the Greek astronomer Ptolemy compiled a "Canon of Kings," a reference book that attempted to standardize the dates of the reigns of the monarchs of various empires. As his reference, he used the 365-day Egyptian year because of its convenience and established a year 1 at the crowning of the Babylonian king Nabonassar.

The Nabonassarian Era is the most common one used in connection with the Old Egyptian calendar, although it was never used by either the Egyptian government or its subjects. (In fact, the World Almanac 1989 gives in "Chronological Cycles" the opening of the Nabonassarian Year 2738 on April 26, 1989-a fact that can be checked using Egypt Calendar.

When Augustus Caesar conquered -Egypt, shortly after the Roman Empire adopted a 365.25 day year, he decreed that a 366th day be added to the Egyptian calendar in every fourth year, thereby setting up a new 365.25 day calendar with an Augustan Era (from Augustus' accession) and an Alexandrian Era (from Rome's conquest of Egypt).

After Augustus' death, however the new calendar went ignored for almost 300 years, until the Coptic Christian Church in Egypt decided to start using it, with an era beginning with the reign of the then-current Roman emperor, Diolectian. Since Diolectian is primarily remembered for persecution of and atrocities against Christians, the Coptics refer to their era, which they use even today, as the "Era of Martyrs." The year 1706 of the Coptic Era of Martyrs begins on Sept. 11, 1989.

Egypt Calendar gives the date in the revised 365.25-day Egyptian calendar, and as names for the months were in use by the time of the reform, these month names are used instead of the season and month number. The year in the New Egyptian calendar begins on August 29 or 30 in the Julian calendar, and between 1900 and 2099 A.D. in the Gregorian calendar, on September 11 or 12. The Alexandrian, Augustan, and Coptic eras are all given by the program.

This program was inspired by the chapter on the Egyptian calendar in O. L. Harvey's "Calendar Conversions by way of the Julian Day Number."


CAL $-the calendar in which the date is being inputted, eitherJulian or Gregorian. Anyone entering a date between 1582-1923 will be prompted for the calendar they want.

ENSEASON$-The season of the year, in the original Egyptian calendar

EMO-The number of the month in the season in the original Egyptian calendar.

EBM-The day of the month in the original Egyptian calendar.

D$-The day of the (seven day) week.

M$-The month of the year in the Julian or Gregorian calendar.

DATE-The day of the month, Julian or Gregorian.

1YEAR-The number of the year, Julian or Gregorian.

ERA$-A.D. or B.C.

JDAY-Julian Day Number, used by chronologists and astronomers, and is a linear count without end. JD #0 was January 1, 4713 B.C.-in the late 20th century A.D., the number is between 2440000 and 2450000.

JSC-Julian Sothic Cycle of Censorinus. 1461 original Egyptian years long.

TSC-True Sothic Cycle 1424 original Egyptian years long.

EY-Number of the original Egyptian year in the current Julian Sothic Cycle.

TEY-Number of the original Egyptian year in the current True Sothic Cycle.

SENWORSET3-Number of original Egyptian years since the coronation of Senworset III.

AMENHOTEP1-Number of original Egyptian years since the coronation of Amentohep I.

NABONASSAR-Number of original Egyptian years since the coronation of Nabonassar.

CY-Year of the Coptic Era of Martyrs, expressed in the New Egyptian calendar with a 366th day every 4th year.

ENMONTH$-The month of the Egyptian year, expressed in the 365.25 day New Egyptian calendar.

CBM-The day of the month in the New Egyptian calendar.

ALEXANDRIAN-The day of the month in the New Egyptian calendar, counting from Augustus' conquest of Alexandria and his attempt to institute a 365.25 day calendar in Egypt.

AUGUSTAN-The day of the month in the New Egyptian calendar, counting from year 1 of Augustus' reign as Emperor of Rome.

SRMONTHS-The Julian or Gregorian month in which Sirius makes its first appearance in the early morning (the Sothic Rising) just be-fore dawn, after having been invisible for a month or so because it was above the horizon during daylight hours only.

SR-The day of the month on which the Sothic Rising occurs.

Chris Carrier lives in Sacramento, California. His interests include astronomy. chronology and games. His articles have appeared in USA Today and the Barrow Sun, the northernmost newspaper in North America. This is his first appearance in Antic

Listing 1: EGYPT.BAS Download