Ever wonder about terms like "Gregorian Calendar", "Julian Calendar," "Julian Date," "Leap year, "Sidereal year, " "Sothic Cycle," "Synodic," "Tropical year," that get used when people talk about time and the calendar? Here is a brief history of the development of our modern calendar that gives us a interesting perspective on the subject.
As far back as the First Dynasty of Babylon, circa 1900 B.C., in the reign of King Hammurabi, people realized that there was some kind of a periodicity to the year, and especially that it apparently had something to do with the phases of the moon. The result was that they came up with an annual calendar of twelve lunar (or "synodic") months to make up their "lunar year."
But they soon realized that a year of twelve lunar months - actually 354.37 days - began to slip as far as getting the same day of the month occurring at the same time of the year - the synodic year was not exactly the same length as the the year based on the sun. This was a very serious problem for a simple agricultural society when the planting season turned up in the middle of winter. A serious problem for their society was that they regarded the moon as a god and whatever the gods do, must be absolutely accepted and not tampered with. But then they tried to patch it up by adding an extra month to the year every so often. What they came up with was an "intercalary month" that was added to the year every 3rd, 6th, 8th, 11th, 14th, 17th, and 19th years. Thus over a long cycle of 19 years, their calendar was within some 20 days of solar time.
About this time, they decided on a work cycle for people where every so often, they would rest and then repeat the cycle. They selected this period to be seven days, because there were five planets plus the Sun plus the Moon.
This Babylonian calendar was very adequate for their times and was considered so successful, that it was still being used later on in Hebrew, Greek and early Roman times.
Meanwhile, over in Egypt, the early Egyptians, as far back as 2700 B.C., realized that with the length of the year some 365 days, and with the very important feature of their lives being the annual Nile flooding, they needed as accurate a calendar as possible.
That led them to develop a solar-based calendar with 365 days. Since the year is actually about 365 and 1/4 days, the days in this calendar still began to slip over the span of several years. Over 40 years, this calendar lost one day.
One concept they came up with was the realization that over the interval of 1461 of these "Egyptian years," there would be 1460 solar years. Which is where the "Sothic Cycle" that one still hears about in esoteric circles came from. This led to the Pharaoh Ptolemy III trying to fix things up in 238 B.C. by adding a day every four years, a change that was rejected at the time, but finally accepted some 200 years later.
Over in Greece about by 380 B.C., the Greeks also realized that the year is actually about 365 and 1/4 days and tried to make adjustments the lunar-based calendar they had inherited from the Babylonians. But that was too radical a suggestion for the times and was not accepted by the conservative religious authorities of the time.
By Roman times, a lunar-based calendar was being used throughout most of the Roman empire, in which an intercalary month was added every so often. Exactly when this intercalary month was added appears to have been more at political and/or religious whim than any hard agreement - the party in power usually wanted the extra month so they could stay in power longer...
In 46 B.C., Julius Caesar had his fill of the bickering and, having returned from his campaign in Egypt and seeing the simplicity of their solar-based calendar, decreed that a solar-based calendar would be adopted by the Roman Empire. He even went so far as to rely on an Egyptian (remember the Romans and Eqyptians were bitter enemies at the time) astronomer and mathematician named Sosigenes who was part of Cleopatra's entourage when she appeared At Caesar's palace in Rome and effectively said "here is your baby and where is my apartment?" To phase in this new calendar, Caesar let the year 46 B.C. continue on for 445 (!) days and this period went down in history as the "Year of Confusion." The modified Egyptian calendar went into effect in the year 45 B.C. as the "Julian Calendar" and continues in much the same form in a few places around the world even to this day.
In the Julian calendar, the extra five days left over from the simple 12 months every year approach were distributed throughout the year - thus we get months with different number of days in the month. The Romans regarded February as an unlucky month so they made it a short month so people would have as few "unlucky" days a year as possible.
To take care of the extra 1/4 day, Caesar and/or Sosigenes decided every fourth year will have 366 days. Thus the "Julian year" with its 365.25 days a year came into being.
In 325 A.D., the Council of Nicaea officially adopted this Julian calendar and made it the standard for the Christian faith.
Meanwhile, the Moslem empire was becoming established and it put great stress on learning. By the late 700's, the city of Baghdad was a world center of science and the arts - to the extent of bringing in scholars and texts from India. This "brain drain" from India introduced the concept of zero - which it turned out was vitally necessary for further development of the calendar.
Several advances in mathematics and understanding the calendar were made over the next several centuries by various Arabic scholars. About 950 A.D. they made the mathematical leap of understanding that the positional notation can be used to write down decimal fractions.
One of the more notable discoveries was by a Umar ibn Ibrahim al-Kayyami (1048 - 1131) who is known to the West only for his poetry under the name of Omar Khayyam. Actually, Omar Khayyam was very prolific in several fields, such as astronomy and mathematics, in addition to poetry. But his real claim to fame was measuring the length of the year too accurately! His length - 365.242198581 days - was an accuracy which exceeds the very gradual slowing of the rotation of the earth.
In 1263, Roger Bacon, the English scholar, pointed out that the the Julian calendar made the year too long; a point that was again merely observed by the conservative religious authorities of the day.
In 1472, a Prussian astronomer, Regiomontaus, was summoned to the Vatican to help Pope Sixtus IV revise the Julian calendar; as many authorities had been complaining for a long time that it was outmoded. But again, nothing was done for the next hundred years.
A bit later, the emerging science of astronomy made a reform of the calendar absolutely necessary - the extra several minutes a year were just too much for astronomers such as Tycho Brahe, who made a plea to the Pope for a calendar reform. In 1582, Pope Gregory XIII, finally declared a calendar reform or readjustment that dropped the ten days from October 5, 1582 to October 15, 1582, that had accumulated since Julius Caesar's time.
At this time, the leap year calculation of adding a day every four years was kept, but the calculation was extended so a cycle of every four hundred years would see only 388 365-day years. This was obtained by mandating that the even century years, such as 1700, 1800, and 1900, even though divisible by four, are not to be leap years unless they are also divisible by 400. Which brings us to the modern Gregorian calendar which closely tracks the actual year of 365.24220 days. Or, to put it into another perspective, the Gregorian calendar gains about 2.88 hours on the sun every 400 years.
Thus per the Gregorian calendar, the year 2000 is a leap year and February has 29 days; the year 2100 is not a leap year and February has only 28 days.
Another change that was added when the Gregorian calendar was phased in was the adoption of the first of January as definitely the beginning of the year, instead of March 25, or sometimes a date local to this or that town.
But the rest of the world took awhile to buy into this new-fangled way of counting time. Great Britain and the American colonies held out until 1752. To compensate for the slippage between the Julian calendar ant the Gregorian calendar, they made up for it by dropping eleven days; changing September 2, 1752 to September 13, 1752. Even now, we still see historical records about events in the American Colonies where dates are expressed as "O.S." - which means "Old Style" or Julian Calendar. To convert from "O.S." dates to the Gregorian calendar, add 11 days.
Now, we realize that the actual length of the (tropical) year is 365 days, 5 hours, 48 minutes, 46 seconds or 365.24220 days long.
The Julian year accounted for a year of 365.25 days; which meant it was 11 minutes, 14 seconds a year too long, or it gained 3.12 days every 400 years.
The Gregorian year accounts for a year of 365d 5h 48m 20s, which means it is 26 seconds a year too long, or it gains 0.12 days every 400 years.
The odd term "leap year" came about as follows - The 365-day year is 52 weeks and one day long. So as long as we stay with 365-day years, a some given date, such as April 12, 1969, which is a Sunday, will fall on a Monday the next year, and on a Tuesday the next year. But when we include an extra day for the leap year, the day of the week appears to "leap" an extra one day, such as from a Tuesday to a Thursday.
The following calendar year lengths from Reference (1) have been calculated and some were actually used at various times throughout history by various cultures -
|Present||Atomic||365d 5h 48m 46s||None|
|141-127B.C.||Hipparchus||365d 5h 55m||+6m 14s|
|45 B.C.||Julius Caesar||365d 6h||+11m 14s|
|A.D. 139||Ptolemy||365d 5h 55m 13s||+6m 27s|
|499 A.D.||Arayabhata||365d 8h 36m 30s||+2h 47m 44s|
|882 A.D.||al-Battani||365d 5h 48m 24s||-22s|
|1100 A.D.||Omar Khayyam||365d 5h 49m 12s||+26s|
|1252 A.D.||Alfonsine||365d 5h 49m 16s||+30s|
|1440 A.D.||Ulugh Beg||365d 5h 49m 15s||+29s|
|1543 A.D.||Copernicus||365d 5h 49m 29s||+43s|
|1574 A.D.||Danti||365d 5h 48m||-46s|
|1582 A.D.||Gregorian||365d 5h 48m 20s||-26s|
("Error" - a negative value ->means that the calendar year is shorter that the actual tropical year.)
- David Ewing Duncan, "Calendar." Avon Books, 1998, ISBN 0-380-97528-9.
- Isaac Asimov, "Of Time, Space, and Other Things." Avon Books, 1969, ISBN 0-380-00325-2.
- Isaac Asimov, "Asimov's Biographical Encyclopedia of Science and Technology." Doubleday & Company, 1964, LC #64-16199.
- Isaac Asimov, "Asimov's Guide to Science." Basic Books, Inc., 1972, SBN 465-00472-5.
- William Langer, "An Encylopedia of World History." Houghton, Mifflin Company, 1952, LC #52-9589.
Copyright © 1999, E. Stiltner
Another, much more technical, perspective on the problem of calculating calendar dates is "Julian and Gregorian Day Numbers"