How to read a medieval astronomical calendar
Updated: May 17
They're everywhere. Calendars with some – or lots of – astronomical content appear in medical almanacs, in Christian prayer books, psalters and books of hours. And, of course, they are a common feature of astronomy books.
But what do all those columns of letters and numbers mean? This post explains how to make sense of all the information. Sometimes it's complex, sometimes surprisingly simple. We'll work through a particularly detailed example, seeing briefly what each part was for. At the end I'll give you some tips to help you figure out any calendar you might be struggling with. Do please comment with any questions!
(We're focusing on the medieval western Christian calendar here, but we must be aware that Latin astronomy was built on pre-Christian roots, and was influenced by developments by Jewish and Muslim astronomers. Calendars took different forms in different cultures.)
Here's our example - not much space wasted!
The core of the calendar
The contents of calendars varied widely, as each astronomer copied or computed the particular information that would be most useful. However, all calendars in Latin Christendom were built around a necessary core.
The core of the calendar above are the five columns under the word Aprilis (April). Here they are in close-up. From left to right, they are:
The Golden Number (there are always gaps in this column).
The ferial letter (a.k.a. dominical letter)
The day number. This counts down to...
Nones (or Ides, or Kalends)
The name of a saint or other feast celebrated on a certain day.
But wait a moment! Probably the most noticeable thing about this close-up picture is the big space in the middle. What should be there is a giant KL. It stands for Kalends, the first day of the month. Don't be surprised that it's missing! One important thing to note about calendars is that they were living objects, put together over time by users with changing needs. Sometimes bits got left out by scribes/illuminators in a hurry. Sometimes later users added to them, corrected them or customised them in some other way.
Here's another example. Our eye is immediately drawn to the striking gold KL marking the first day of April. Underneath it you have the same five columns, but this calendar, from the 1270s, uses Roman numerals. (There are Arabic numerals on the left - I explain them in this post. But they are a later addition - more customisation!)
What you won't see here, and don't find very often in medieval calendars, is a column of days numbered 1 to 30. Instead, the month was split into three unequal chunks. (The system was inherited from the Romans, based on the calendar reformed by Julius Caesar, which is why we call it the Julian Calendar.)
In the middle of the month was the Ides, on either the 13th or 15th day. (You can find a month-by-month list in this post, which explains the cultural context of these calendars.) The days after Ides count down to the next Kalends. The last day of April was 2 Kalends May, i.e. the second day – counting inclusively – before the Kalends. The day before Ides was 2 Ides, the day before that was 3 Ides... and so on up to 9 Ides, which was called Nones, from the Latin word for "nine". Since that's 9 days before Ides, it was always on the 5th or 7th of the month. From there back to Kalends, the first day of the month, it's just a few more days to count backwards.
That accounts for the countdown of numbers, and the fat capital N (for Nones), Id' (Ides) and Kl (Kalends) just to the right.
To the left of that we have a column of letters A to G known as the ferial letters. In calendars designed to last for many years, these are a clever way of keeping track of the day of the week. In any year you would only have to know what letter Sunday was (the dominical letter). If you know that all the Sundays are C that year, then all the Mondays will be D, all the Tuesdays E, and so on. (You just have remember to shift them if it's a leap year.)
The column to the right of the repeating N, Id' and Kl is usually the widest: it's the feast days. The list in the Coldingham Breviary above is fairly sparse. In this April page, from the celebrated Très Riches Heures (c.1412-16) of Jean, Duke of Berry, there's something for almost every day. In fact the only reason there are any gaps at all is because the scribe didn't quite get around to filling in the REALLY important feasts (St George and Mark the Evangelist) in gold. That's why the letter A's in the column of ferial letters are also missing.
Now, in the calendar at the top of this post we have a narrow, and curious, column of feast days. It goes like this, with one syllable for each day:
Jun-gi-tur Am-bro-si-us A-pri-li Guth-si-bi Ti-bur Ac-ci-pit Al-ple-gum-que Ge-or Mar-cum-que Vi-ta-lem.
Thirty syllables for the 30 days of April. This is a mnemonic – part of a popular scheme named Cisio Janus (which are the first two words of the verse for January). It was a popular way of learning the order of the major feast days, from Ambrose on the 4th to Vitalis on the 28th. It was easily adapted to locally specific feasts – you'll notice for example, that Guthlac, who lived in the kingdom of Mercia around 700, does not appear on the Très Riches Heures calendar above, since he was not venerated outside England.
The final part of the core is the Golden Number. This is a column with numbers 1 to 19 written next to certain letters. They may appear to be randomly distributed, but look closely and you'll see that 16 is eleven days before 15, which is eleven days before 14. And 5 is eleven days before 4, which is eleven days before 3. There are some irregularities, but this is the basic pattern. It tells you the official date of the new moon in any year. It works because 12 lunar months of 29½ days add up to 354, which is 11 days shorter than a normal solar year. Thus April's new moon next year will be (at least officially) 11 days earlier than it was this year. The cycle repeats itself after 19 years, because 19 solar years = 235 lunar months.
All you have to know is the Golden Number for this year. That was vitally important because the date of Easter, and all the movable feasts preceding and following it, depend on the date of the late March/early April full moon, on the 15th day after the new moon. To work out the Golden Number, just divide the year by 19, and add 1 to the remainder. For example, 1392 was golden number 6, because 1392 ÷ 19 = 73 remainder 5, and 5 + 1 = 6.
Of course, Easter itself has to fall on a Sunday, and that's where it helps to know your dominical letter.
The system wasn't perfect, and attempts were made to improve it throughout the Middle Ages. We can see one attempted update in the Très Riches Heures calendar above: a column on the far right headed "new golden number", with its numbers three or four days earlier than the standard list on the far left of the page. This proposal was drawn up by two French astronomers in 1345. Such reform proposals appeared regularly from the creative astronomers of the later Middle Ages; they culminated, of course, in the sixteenth-century Gregorian calendar reform.
Astronomy and geometry
All the above was very standard. Here's where it gets a bit more complicated, and more variable. Let's take a closer look at this calendar. Here it is again (and click here to see it online in high resolution):
Medieval astronomers had their own opinions about what they thought would be useful in a calendar. They also liked to tweak and improve earlier calendars, so the format rarely stayed static for long. This calendar contains a version of the Kalendarium drawn up by a Franciscan friar named John Somer in 1380. It was valid for four 19-year cycles, from 1387 to 1462. But there is also some bonus information Somer didn't include – not least that Cisio Janus mnemonic.
Here is what each column does:
6. Ascendens media nocte. The ascendant at midnight. This tells us what degree of the ecliptic is rising above the horizon at midnight. (The ecliptic is the Sun's yearly path through the stars, passing the well-known zodiac constellations.) So let's say it's 2 April – 4 Nones in the calendar. If the Sun, which is then at the 22nd degree of Aries, is directly underneath us at midnight, the sign of Sagittarius – 19° of Sagittarius, to be precise – will be rising above the horizon.
The data is given as degrees within each zodiac sign. The sign is noted at the top of the column, as well as where it changes – you'll see it switches from Sagittarius to Capricorn halfway through the month.
Note that this depends on the angle between your horizon and the axis of celestial rotation – or the Earth's axis, if you want to be all heliocentric about it. That angle is equivalent to your latitude, which is why over above column 19 the scribe has noted that we are at the latitude of the University of Oxford (assumed to be 51° 50' in the Middle Ages).
7. Medietatis [sic] noctis. Half the length of the night. Since the time from midnight to sunrise is one half the night, this gives you the time of sunrise on any day. It is presented in a column of minutes, with each new hour marked in red. (This was a very common way to save space.) So you can see that from 3 Ides April to 2 Id' – that is, from the 11th to the 12th – the time of sunrise goes from 5h 0m to 4h 58m.
Yes, that Y-shaped number is a 5 (not a 4!), the X with a closed top is a 4, and the upside-down V is a 7. You can see in the left-hand column here that the numbers go 24, 25, 25, 26, 27.
8. Quantitas lure planetarum nocturne. Length of a planetary hour (a.k.a. "unequal hour") at night. (The word ure here, rather than the Latin hora, comes from Norman French.) According to the system of unequal hours, there were always 12 hours between sunrise and sunset, and another 12 between sunset and the next sunrise. Thus in the summer each daytime hour was long and each nighttime hour was short, and vice versa in winter. I don't have space in this post to consider the history of this system, but suffice it to say that it made a lot of sense when lives were shaped by the changing seasons and availability of daylight.
Here the hour-lengths are given in degrees and minutes, rather than hours and minutes. One equal hour corresponds to 15 degrees (since 360 ÷ 24 = 15), and in the same way if you add one daytime and one nighttime planetary hour you will always get 30 degrees, because they always balance each other out.
9. Medium celi ad instans ortus solis. Midheaven at the moment of sunrise. This gives the degree of the ecliptic culminating [at its highest point, in the south] when the Sun rises. Conceptually it's very similar to column 6. Just like column 6, the data is given as degrees within a zodiac sign. This one is in Capricorn for the whole month.
10. Verus locus solis. True place of the Sun. This gives the Sun's longitude on the ecliptic. Do you know your star sign? Your star sign is the 30-degree segment of longitude that the Sun was in on the day of your birth. The Sun travels round the entire zodiac in the course of a year – 360 degrees in 365 days, so a little less than a degree a day. Solar longitude was an absolutely fundamental piece of astronomical information, which is why almost all astrolabes have a calendar on the back to find the Sun's position for any day of the year. It's also fundamental to timekeeping. Again, the data is given in degrees within each zodiac sign, and here we see the Sun move from Aries into Taurus on the 12th of April.
11. (2 columns) Ascendens signorum in circulo directo cuiusmodi est meridianus in omni regione. The ascension of signs on the direct circle, for any latitude or longitude. This is the right ascension: an arc of the celestial equator. It's measured from the equinox to another point on the equator corresponding to a certain longitude. In this case it's the Sun's longitude, in the previous column. It's laid out with separate columns of degrees and minutes.
This is really useful for all kinds of astronomical calculations, which is why a standalone table of right ascensions is a common feature of almanacs. In fact it's unusual to find it in a calendar, as its values aren't really tied to days of the year – John Somer didn't include it in the calendar that this one is based on. But it was so important that it's not surprising another astronomer chose to include it here.
Now we come to a set of columns with rather different information, and with writing in three different directions! At the top it says "mas. d'. igneum orientale. Sol. Saturnus. Jupiter." At the bottom we have corresponding information: "Venus. Mars. Luna. Fe. Nm. terreum. meridionale." What these abbreviated notes tell us are the astrological qualities of the triplicities for the two signs this month. As we've seen, at the start of April the Sun is in Aries, and by the end it is in Taurus. The triplicities were groups of three signs, which share certain qualities. Aries, like Leo and Sagittarius, was thought to be masculine, diurnal, fiery and eastern. Its lords by day were the Sun, Saturn and Jupiter. Taurus (and Virgo and Capricorn) were feminine, nocturnal, earthy and southern. Its lords were Venus, Mars and the Moon.
These theories have ancient roots. In late medieval Europe, astronomers were hugely influenced by the encyclopedic astrological works of Muslim thinkers. Perhaps the most significant was Abu Ma‘shar, whom Latin astronomers knew as Albumasar. Another – and the probable source for the information in this calendar – was al-Qabisi, known as Alcabitius.
Columns 12, 13, 14, 15 and 16 have astrological data based on al-Qabisi's theories:
12. The domus – house or domicile – was the planet strongest in each zodiac sign. This was Mars in Aries, and Venus in Taurus (as we see halfway through the month). But this column also shows the exaltations and dejections (or fallings): individual degrees where certain planets were particularly strong or weak. Here we learn, for instance, that Saturn has a dejection at Aries 21°, and the Moon has an exaltation at Taurus 3°. We also see that Taurus is the odium – that is, the detriment of Mars. As one adaptation of al-Qabisi, written in English at the end of the 14th century, explained:
And there is difference bitwixe fallinge and descendinge and bitwixe detriment or harmynge; for detriment is in the opposite of the hous, fallinge sothly in opposite of exaltation. (Cambridge, Trinity College MS O.5.26, f. 4r)
Thus, because Scorpio was a house of Mars, so Taurus, opposite it, was its detriment.
13. Each sign had five termini – terms. These were segments of anything between 2 and 10 degrees, each dedicated to a particular planet.
14. A facies – face – was a ten-degree segment, based on the decans of Egyptian astrology. Like the planetary hours, these cycled in a strict order, inwards from Saturn to the Moon, before beginning again at Saturn, the outermost planet. Thus, as we see here, the three faces for the end of Aries and the beginning of Taurus were Venus, Mercury, the Moon.
15. Each sign was also divided into masculine and feminine segments. So, as we see here, the sex of the last 8 degrees of Aries and first 8 of Taurus was masculine, followed by 7 feminine degrees, and so on...
16. Finally, as the English translator of al-Qabisi explained, there are the qualities:
In evereche of these signes there be degrees that beth seide [are called] lucidi or bright, and degrees that beth seide tenebrosi or darke, and degrees that beth seide fumosi or fumous and degrees that beth cleped [called] vacui or voide. (MS O.5.26, f. 7v)
17. There's one more column that goes with the five we've just mentioned. We find them in the half-empty column next to the narrow mnemonic of saints' days. Here some degrees are labelled as degrees of diminished fortune or "pits" (puteus), degrees of chronic illness (azamana or azemena), and degrees of increasing fortune (augmentans fortunam).
For all of these different features of particular segments or signs or individual degrees, al-Qabisi drew up a table, making them easy to locate. Here's what it looks like in the Middle English version produced around the same time as our calendar.
John Somer's Kalendarium doesn't give us any of this information, and his contemporary Nicholas of Lynn only included some of it. The maker of this copy managed to cram in an awesome quantity of astrological data.
Some more astronomy
We're going to skip across a little now, and look at the seven columns on the left-hand side of the right-hand page (the recto). You already know most of what you need to know to understand these.
18. Altitudo solis meridiana. The Sun's meridian altitude – its height above the horizon at midday each day. Notice that degrees (black) and minutes (red) are given in the same column.
19. (2 columns) Ascensiones signorum ad latitudinem universitatis Oxonie. Ascensions of signs at the latitude of the University of Oxford. These oblique ascensions, as they're known, are the counterpart to the right ascensions in column 11. In the diagram above, the oblique ascension is ET – that is, the arc of the equator rising above the horizon with a given arc of the ecliptic. Since the turning of the celestial equator, 360 degrees in 24 hours, is how we measure time, the oblique ascensions tell us the time it takes for a particular sign – or the Sun itself on the ecliptic – to rise. This is very useful information, but it is specific to each latitude. Like the right ascensions, it wasn't usually included in calendars, and was more commonly laid out in a separate table.
20. Ascendens in media die ibidem. The ascendant at midday. Just as column 6 tells us the ascendant at midnight, this tells us what degree of the ecliptic will be rising above the horizon at local noon each day.
21. Medietatis diei. Half the length of the day. Just as column 7 gave us the time of sunrise, this gives us the time of sunset. And of course the two columns always add up to 12 hours – in the first line of the table it's 5h18m + 6h42m.
22. Quantitas lure planetarum diurne. Length of a daytime unequal hour, in degrees and minutes. This is the exact counterpart of column 8. And, as I mentioned above, if you add what's in the two columns for any line, you always get 30 degrees: 13° 14' + 16° 46' in the first line.
23. Medium celi ad instans occasus solis. Just as column 9 gave us midheaven at the moment of sunrise, this is midheaven at the moment of sunset.
Conjunctions and eclipses
Nearly there now! The rest of the calendar is taken up with what is the most famous component of astronomical calendars: the dates and times of conjunctions, or new moons, and details of the eclipses that sometimes occurred at conjunction or opposition (full moon). This is the result of the increasing emphasis on the Moon in 13th and 14th-century medicine, as well as weather forecasting and agriculture.
By the way, the single word that means conjunction or opposition is "syzygy". You don't need to know that, but it might come in useful in Scrabble one day.
Whereas all the information we've covered so far is basically permanent – that is, it only changes very slowly, or not at all, the information in columns 24 to 27 is only good for the specific years the calendar was made for. This one covers four 19-year cycles, from 1387 to 1462. The Franciscan Friar John Somer drew it up in 1380, at the request of Joan, mother of king Richard II. Why did he pick 1387 to start it from? Because that was year 1 in a cycle of golden numbers. So, in the four columns, the first covers 1387 to 1405, the second is 1406-24, the third is 1425-43, and the fourth is 1444-62.
At the top, under the heading Coniunctiones equate super meridiem Oxonie (conjunctions calculated on the meridian of Oxford), we have cycles 1 to 4. Each one has three columns: ciclus (which here, confusingly, means the year within that cycle), hora (hour) and minuta (minute). These are true syzygies – using the true rather than mean longitudes of the Sun and Moon – an important development in the 14th century.
Let that sink in for a moment. John Somer has given us every new moon for 76 years, calculated to the nearest minute.
To take one example, we know that 1387 was year 1 of the golden number cycle. So to find the day and time of the new moon in April 1387, we scan down the first column until we find the "1" – there it is on 13 KL May, the 19th of April. The time of new moon was 04:03. Read across and you'll see that 19 years later it was at 04:40 on the same day.
One effect of such precision is to show off just how out-of-date the original golden numbers had become. You only have to look back across the calendar to see that golden number 1, which is supposed to predict the date of the new moon, is not on the 19th at all - it's two days later. It's easy to see how such precise calendars drew attention to the deficiencies of the ecclesiastical calendar. John Somer himself wrote a tract, now alas lost, entitled Castigations of the Calendar.
The final component of the calendar was even more advanced. Columns 28 and 29 give the precise timings and magnitudes of each solar and lunar eclipse. Solar on the left, lunar on the right. As an example, let's take the solar eclipse in 1436, in the bottom right-hand corner of the left-hand (verso) page. Here it is close-up.
What it tells us is that in year 12 of the cycle, on the 16th day of the month, the eclipse would begin at 5:33:44 (yes, that's to the nearest second). The time of greatest eclipse would be at 5:48:09, and the end of the eclipse would be at 6:02:33. Over in the far-left margin we see a little drawing with the numbers 0;16,56. Those represent the proportion of the Sun that would be eclipsed, as a fraction of 12. The number above is a bit less than 17 sixtieths of one-twelfth - that is, only a tiny sliver of the Sun would be eclipsed.
In other calendars we sometimes find this information displayed more graphically. Here it is in a 15th-century copy of John Somer's calendar, in a physician's folding almanac.
The caption above, in English, gives us the date and time of the eclipse. The numbers around the picture, starting from the upper-left quadrant, denote:
the magnitude of the eclipse (notice that the copyist has written 9;16,56 instead of 0;16,56 – we all make mistakes);
the time elapsed from the start to the moment of greatest eclipse;
half the duration of total eclipse – in this case, zero;
the total duration from start to finish – here 0:28:49, which you will see is the difference between 5:33:44 and 6:02:33.
And that's it! Pretty impressive, eh?
Tips and tricks
If all that seems a bit overwhelming, there are some ways to make it easier to figure out a calendar. First of all, it's pretty rare to have so much information on two pages! The core information I started with is the most common. So:
If you have a column with numbers 1 to 19 and some gaps, those will be the golden numbers.
A column of letters A to G will be the ferial/dominical letters.
pictures of circles with segments cut out of them often indicate eclipses, and the numbers next to them represent the magnitude of those eclipses.
Be careful! The noun meridies and adjective meridionalis can refer to a geographical longitude, to the direction south, or to something crossing the meridian (when it is in the south).
Likewise, try not to get confused between media nocte (midnight), medietas noctis (half the length of the night), and medium celi (midheaven, i.e. the meridian).
An astrolabe can be useful to check the calendar data, if you know how to use one (there are a few simulators online). But don't forget that compilers and copyists sometimes made mistakes, just as we do!
Good luck! Don't forget to comment if you have any questions, or things you'd like to see in future posts.
You can read a bit more about such calendars, and lots of other subjects in medieval science, in my forthcoming book, The Light Ages.
All the manuscripts pictured above are available to view online. See if you can figure them out!
Cambridge, Trinity College MS O.5.26: Middle English al-Qabisi
London, British Library Harley MS 321: calendar by John Somer and friend(s)
London, British Library Harley MS 937: physician's folding almanac
London, British Library Harley MS 4664: Coldingham Breviary
Some important books and articles on these calendars, especially in the context of 14th-century England:
Sigmund Eisner (ed.), The Kalendarium of Nicholas of Lynn (Athens, GA: University of Georgia Press, 1980)
Linne R. Mooney (ed.), The Kalendarium of John Somer (Athens, GA: University of Georgia Press, 1998)
John D. North, Chaucer's Universe (Oxford: Clarendon Press, 1988)
C. Philipp E. Nothaft, Scandalous Error: Calendar Reform and Calendrical Astronomy in Medieval Europe (Oxford: Oxford University Press, 2018)
C. Philipp E. Nothaft, “The Astronomical Data in the Très Riches Heures and Their Fourteenth-Century Source,” Journal for the History of Astronomy 46 (2015): 113–29.