Decoding the Calendar
from the Très Riches Heures of Duc de Berry
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Introduction
One of the most famous and beautiful medieval calendars begins the Très Riches Heures of Duke de Berry. Each month of the calendar is represented by a full-page illumination depicting a detailed scene of medieval life during that month, along with astrological data along the concentric circles at the top of the page. The facing page is a text calendar.
The top circular section of the illuminated calendar pages is not filled in for the months of January, April, May, and August. This indicates that the illumination and calligraphy were done by different artisans. In order to fill in the top section of the missing months, I examined these calendar pages in more detail to determine the purpose of each section.
First, the facing text page of each month is complete and can be used as a reference. Here are the first five days of the text page for January:
Each text page starts with an illuminated KL which stands for the Latin kalendas — the name of the first day of each month. Dates were not specified using the same system we use in modern times. Instead, the complex ancient Roman system was used, based on lunar cycles. Days are counted down backwards from the nones (the fifth or seventh day of the month), then from the ides (either the thirteenth or fifteenth day), and again from the first day of the following month (the kalends). These three special days were also feast days in the month — as in the familiar "Ides of March."
You can see in the example above that the nonas falls on the fifth day of January, and the days preceding are numbered two, three, four going backwards from the nonas. This day number is shown in red in the third column. The longer text in the middle of the page indicates the names of saints for each day. To the right of the saints is the length of the day between sunrise and sunset in hours (blue) and minutes (red). To the far right is the "New Golden Number," which I will describe in detail later. It is related to the normal "Golden Number" shown in gold in the first column. Finally, the second column (in black) is the dominical letter indicating the day of the week. This perpetual calendar starts with the letter A for January 1; for a given year, you can compute the dominical letter, which indicates the letter on which Sunday falls for that year.
Here is a closeup view of the semi-circular region above the illuminated calendar page for the month of February:
At the center is a monochrome blue illustration of a chariot holding the sun being drawn by horses. This same picture appears on all twelve calendar pages. The innermost ring indicates the day of the month in arabic numbers starting at one — for February it goes from one to twenty-eight. Each number alternates in gold and red. Notice that they are not showing the Roman day as described above on the text page, but rather the more modern absolute day number.
The next two rings are used to show the dates on which new moons occur and are related to the Golden Number, discussed below. The next ring has the text "primacones Lune mensis novembus dies xxx." primacones Lune roughly translates to "beginning phase of the moon" or "new moon"; mensis is Latin for month, followed by the name of the month in Latin; dies is Latin for days, followed by the number of days in the month in Roman numerals.
The next ring of blue illumination depicts the falling and rising sign of the zodiac for the month. In this case, Aquarius is falling (ending) and Pisces is rising (beginning). Gold stars illuminate the blue background. Outside is the text "finis graduum aquari initium pisaum gradus xix" — roughly "the final steps of Aquarius. The initial steps of Pisces" with the xix indicating the day of the rising sign that falls at the end of the month. Here, the end of the month is the nineteenth day of Pisces. The very outside ring shows the days within the solar calendar of the zodiac numbered sequentially from one to thirty in arabic numerals.
Golden Numbers
Calendars were often included as part of a Book of Hours because of the religious significance placed on many dates. In particular, determining what day of the year Easter would fall upon was of major importance to the Church. Based upon the Bible, the Church defined Easter as:
"the Sunday after the first full moon of Spring. The full moon may be on the first day of Spring (March 21st), but Easter will never be, and if the full moon is on a Sunday, Easter is on the following Sunday."
In 325 AD, the Council of Nicaea hammered out a set of compromises to deal with the potential problems of determining the exact moment of the full moon and problems with variance depending upon longitude. Their compromise states that the Easter full moon happens on a date, not a particular time of day, and that it is on the same date everywhere in the world. It also states that the date of the Easter full moon is determined by the motions of a "mathematical moon" which only approximates the real moon. For clarity, the calculated date of the Easter full moon is called the Paschal full moon.
So, in order to calculate when Easter would occur, one needed to know when the Paschal full moon after the first day of spring (March 21st) would occur. In developing the perpetual calendar of the Très Riches Heures, the dates of all such computed full moons are shown. But how were these full moon dates computed, and how does one decode their number and letter representations on the calendar?
In medieval times, they used the Julian Calendar, divided into the twelve months we use today, with a leap year every four years by adding a day to February. Astronomers as early as Babylonian times noticed that 235 lunar months (29.53 days each) was almost exactly 19 years of 365.25 days (235 × 29.53 = 6939.55, 19 × 365.25 = 6939.75). In other words, if a full moon occurred on a specific date, it would occur on that same date nineteen years later. Since the lunar cycle repeated every 19 years, they assigned a "Golden Number" to each year indicating its position in this 19-year cycle. To compute the Golden Number for a particular year, divide the year by 19, take the remainder and add one. For example, the Golden Number for 1492 is 11. Easter occurs on only 19 specific dates, and the date is given by the Golden Number for that year.
Since the 19-year cycle is true for any full moon, not just the Easter full moon, you can construct a perpetual calendar that tells you when each full moon will occur. The date is the same for any two years with the same Golden Number. The same is true for any particular phase of the moon. While Easter is defined by a full moon, it is easier to observe the new moon by looking for the first sliver. These new moons were observed and tabulated as the Metonic cycle producing the following table:
| Golden Number | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| January | 23 | 12 | 1/31 | 20 | 9 | 28 | 17 | 6 | 25 | 14 | 3 | 22 | 11 | 30 | 19 | 8 | 27 | 16 | 5 |
| February | 21 | 10 | — | 18 | 7 | 26 | 15 | 4 | 23 | 12 | 2 | 20 | 9 | 28 | 17 | 6 | 25 | 14 | 3 |
| March | 23 | 12 | 1/31 | 20 | 9 | 28 | 17 | 6 | 25 | 14 | 3 | 22 | 11 | 30 | 19 | 8 | 27 | 16 | 5 |
| April | 21 | 10 | 29 | 18 | 7 | 26 | 15 | 5 | 23 | 12 | 2 | 20 | 9 | 28 | 17 | 6 | 25 | 14 | 4 |
| May | 21 | 10 | 29 | 18 | 7 | 26 | 15 | 4 | 23 | 12 | 1/31 | 20 | 9 | 28 | 17 | 6 | 25 | 14 | 3 |
| June | 19 | 8 | 27 | 16 | 5 | 24 | 13 | 3 | 21 | 10 | 29 | 18 | 7 | 26 | 15 | 4 | 23 | 12 | 2 |
| July | 19 | 8 | 27 | 16 | 5 | 24 | 13 | 2 | 21 | 10 | 29 | 18 | 7 | 26 | 15 | 4 | 23 | 12 | 1/31 |
| August | 17 | 6 | 25 | 14 | 3 | 22 | 11 | 1/30 | 19 | 8 | 27 | 16 | 5 | 24 | 13 | 2 | 21 | 10 | 29 |
| September | 16 | 5 | 24 | 13 | 2 | 21 | 10 | 29 | 18 | 7 | 26 | 15 | 4 | 23 | 12 | 1 | 20 | 9 | 28 |
| October | 15 | 4 | 23 | 12 | 2/31 | 20 | 9 | 28 | 17 | 6 | 25 | 14 | 3 | 22 | 11 | 1/30 | 19 | 8 | 27 |
| November | 14 | 3 | 22 | 11 | 30 | 19 | 8 | 27 | 16 | 5 | 24 | 13 | 2 | 21 | 10 | 29 | 18 | 7 | 25 |
| December | 13 | 2 | 21 | 10 | 29 | 18 | 7 | 26 | 15 | 4 | 23 | 12 | 2/31 | 20 | 9 | 28 | 17 | 6 | 24 |
To read this table, compute the Golden Number for a particular year, then read down the column to determine what day(s) of the month will have a new moon. For example, if the Golden Number is 2, there will be a new moon on January 1st and 31st, then on March 1st and 31st, then on April 29th, and so on. The full moon is simply thirteen days after the new moon (the 14th day of the lunar cycle).
For the purposes of creating a perpetual calendar, one looks at the row for a particular month. For example, in January there will be a new moon on January 23rd when the Golden Number is one, or a new moon on January 12th if the Golden Number is two, and so on.
Go back up and look at the first five days of the text calendar page for January. Look at the gold numbers in the first column: an iii next to January 1, a xi (a bit hard to read in this smaller version, but clear in the original) next to January 3, and a xix next to January 5. Now look at the new-moon table: a new moon occurs on January 1 when the Golden Number is 3, on January 3 when it is 11, and on January 5 when it is 19. So this first column of gold numbers indicates the Golden Number for which a new moon occurs on that specific date. Each text page exactly matches the table above.
However, this calendar is not exact. 235 lunar months isn't quite exactly 19 years, and this table was derived in 325 AD. Over time, the discrepancy between the computed new moon and the observed new moon grew — about one day every 310 years. So by the year 1416 or so, when the Très Riches Heures was created, this discrepancy was about three days. That was quite an obvious difference and was causing much friction between scholars and the Church.
Eventually, in 1582, the Church would commission Christoph Clavius to come up with an improved system, resulting in the Gregorian Calendar that we use today. (Many non-papal countries delayed the adoption of this new calendar until the 18th century.) However, even before the official change, many scholars of the late Middle Ages were tackling the problem and devising more accurate systems. The Duke of Berry was clearly aware of this — an inventory of his library lists several maps and scientific works, including an astrological treatise on the seven planets. Such works provided tables of the observed new-moon cycle, so in his calendar he has his artisans list the "New Golden Number," shown in gold on the right-most column of the text calendar.
For example, January 1 is labeled with xix, January 3 with viii and so on. Looking carefully at this "New Golden Number," you'll see that it follows the same pattern as the original Golden Number, but is shifted by approximately three days. But this isn't consistent — sometimes the shift is by two days and sometimes by four. For example, on a year with Golden Number of 19, the original computation would have a new moon on January 5, but the new list shows a new moon on January 1, which is four days earlier. Clearly these new Golden Numbers were taken from some astronomical work of the time, but the exact source is not known. Several scholars in Oxford had published works within the previous hundred years discussing the problem and proposing corrections.
This brings us, finally, to the decoding of the two inner rings of the illuminated calendar pages. The determination of Easter was still based on the original Golden Numbers at this time. But the Duke of Berry decided to show the actual dates of the new moon on his perpetual calendar's illuminated pages based upon the New Golden Number, rather than the new moons computed by the Church. Each crescent moon symbol represents a date upon which there can be a new moon. The New Golden Number for those years that have a new moon on that date are shown inside the moon symbols by a letter rather than a number — 'a' represents a year with New Golden Number of one, 'b' for two, and so on. Two things to keep in mind: first, there was no letter 'j' in this period, so while 'i' was used for 9, 'k' was used for 10 — and 't' represents 19 even though it's the 20th letter in the modern alphabet. Second, they used the long form of 's', so the symbol for 18 is sometimes confused with an 'l' or 'f'.
The Scroll
For the scroll done based upon these calendar pages (Scroll 21), I needed to show the full calendar arches for the months of May and January. Both of these months were left blank in the original document. Instead of learning how to decode these calendar pages, I could have simply taken the New Golden Numbers from the facing text pages and used that information to fill in the arches. However, once I learned the purpose of the New Golden Number and understood that the Duke had purposely displayed the actual new-moon cycle rather than the Church-computed new-moon cycle, I realized I had another alternative. Rather than filling in the pages on the scroll with the new-moon cycle from the fifteenth century, I further updated the New Golden Number to the present day. So, the new-moon cycles shown on the scroll are computed for the year 2004 AD, resulting in an additional correction of nearly two days as compared to the New Golden Number used in the original manuscript.
Conclusion
Jean, Duke of Berry, was obviously an extraordinary noble in his time. During the tumultuous fifteenth century, nearly one hundred years before the Reformation, and one hundred and sixty years before the development of the Gregorian calendar, the Duke was reading scholarly works from Oxford and publishing in his own Book of Hours the fact that the Church was wrong about the date of Easter. Perhaps only a Noble could get away with such heresy. The fact that the right-most column of the text pages matches the encoded letters within the illuminated arch pages clearly shows that this column was not added at a later date, but was done at the time the original manuscript was created. This blend of art and science makes the Très Riches Heures an even more extraordinary manuscript.
Bibliography
- Cazelles, Raymond and Rathofer, Johannes. Illuminations of Heaven and Earth (The Glories of the Très Riches Heures du Duc de Berry). Harry N. Abrams, Inc., New York, 1988.
- A very high-quality reproduction and study of many pages from the Très Riches Heures, including a basic description of the calendar pages. This source has very clear and detailed enlargements of the illuminated calendar pages.
- ortelius.de/kalender/east_en.php — The Calculation of Easter
- Excellent background on the history of computing Easter. The Golden Number table comes from this reference and has been verified with other calculations. Many sources on the Internet do not compute this table correctly for the old Julian calendar.
- davros.org/misc/easter.html — Whence Easter
- Another good source for the computation of Easter from the old calendar.
- iclasses.org new and full moon calculator
- An online calculator for determining the new-moon cycle for any modern year.