• Seb Falk

New Moon - No Astrolabe!

Last week I wrote about how to read an astronomical calendar: what information they contained and what it was used for. This post is about those calendars as living objects. How and why did their users adapt and add to them? And what can we learn by looking closely at the details?

Take a look at this manuscript page. Isn't it stunning?

This is a page of the Coldingham Breviary at the British Library. But did you even notice the writing underneath? This post is about that writing – it's not what you might think...

According to the British Library website, the writing is "an instruction, written in Old French, about reading a sundial or astrolabe." This is what it says:

Si le numbre est apres deus u trois u quatre solum ce la secunde ure u la tierce u la quarte devant midi est la coniunctioun. Si la pointe est apres le numbre le ure solum cel numbre apres midi est la coniunctioun. Si vus seiez en le secunde cicle prenez le numbre escrit plus bas en lo senestre angle. E dunk ausi cum primes et ensi des autres cicle e quant les quatre cicles sunt passes fest dirichef comencer al primer et fet asan qe le jur comence a midi e termine a midi apres. dunc les ures qe sunt point apres midi sunt a cunter cumenciaunt a midi du jur present ne pas du jur suaunt et sachez utre a plus tost ke vus auet la coniunctioun auerez menue cel ure lende[moyn] prime.

I stared and stared at this, and try as I might, I couldn't see how it described a sundial or astrolabe. So I asked for help. Historians are very helpful people – and they like puzzles too! With assistance from a few people online and offline, I managed to figure out not only what it meant, but where it comes from.

First, here's a rough translation:

If the number is after two or three or four, the second, third or fourth hour before noon is the conjunction. If the dot is after the number, that number hour after noon is the conjunction. If you are in the second cycle, take the number written below at the left corner. And it is the same with the new moons of the other cycles. And when the four cycles have passed, start again from the first. And note that the day begins and ends at noon, so the hours after noon should be counted starting from noon of the current day, not the following one; and also note that you should subtract the conjunction hours earlier [than noon] from the following day.

Now what does all that mean?

It has nothing to do with a sundial or astrolabe. Instead, it describes how to decipher a narrow column of dots and numbers, and use it to predict the new moon.

The numbers and their dots have been added to each page of a calendar. Here's the page for January.

There's only one leaf between this page and the page with the instruction. In fact, that intervening leaf may have been added later – so the numbers and instructions were probably once on opposite pages.

If you look closely at this calendar page, you'll see that the red and blue numbers on the left are in Roman numerals. This attractive calendar was produced, along with the full-page picture of the Virgin and Child, in the late 13th century. But the numbers on the right were added a hundred years later, in the late 14th century.

Here are the top few rows in close-up.

The first thing you notice is that these are not Roman numerals. Although some may look a little unfamiliar, they are basically the same Hindu-Arabic numerals we use today. These numerals gradually came into common use in Europe between the 12th and 15th centuries. Astronomers were some of the earliest people to adopt them, though even in the 14th century there were still some who preferred the traditional Roman numerals.

How does it work?

If you read my post on astronomical calendars, you'll remember that a column of numbers 1 to 19, with gaps in some rows, is likely to be the golden numbers. You'll remember, too, that those golden numbers tell you when the new moon will be.

And so it is here. The new moon is caused by the conjunction of the Sun and Moon – that is, they come together in the sky. The red numbers tell you that the conjunction will take place on that day in the given year of the 19-year "Metonic" cycle. So, for example, in the final year of the cycle, the new moon will be on 1st January.

As the instruction tells us, the four black numbers surrounding each red number tell us the time that day (or the following day) when the conjunction will take place. Each number is valid for one 19-year cycle, making a total of 76 years. The dots on either side of the numbers tell us whether that hour is a.m. or p.m.

But are the numbers right? You may remember from my last post that the golden numbers were "official" – that is, they were used to calculate key Christian feasts on a traditional basis, but they didn't necessarily tell you when the new or full moon could be seen in the sky. You'll also remember that astronomers in the 13th and 14th centuries became increasingly impatient with the official data, and computed their own more accurate predictions of syzygies and eclipses. That's why the new moons in John Somer's tables were out of sync with the official golden numbers that sat right opposite them in the calendar. And it's why golden number 19 is next to Nones – 5th January – on the left of this calendar, but next to 1st January here on the right.

What does this tell us?

Looking closely at the precise layout of the numbers allows us to identify their source. It turns out that they were originally computed for a 76-year cycle starting in 1289. The same numbers crop up in at least two other manuscripts. One of them even has the same set of instructions written in Anglo-Norman.

Now, the handwriting in this manuscript is definitely not from the late 13th century. In fact, I think the hand may belong to John Westwyk, the monk who wrote the equatorium manuscript I spent years studying, and who is the star of my book The Light Ages. He might have had an opportunity to get his hands on this manuscript when he was at the clifftop priory of Tynemouth in the early 1380s. Coldingham was 75 miles up the coast, just across the Scottish border, and John may have annotated the breviary there; or perhaps he did so when it was at Durham, not far south of Tynemouth. Monks and their books were more mobile than you might expect.

It's clear that these calendars weren't just drawn up and decorated to be admired; they were practical tools. And we can see here that they weren't fixed objects either; they were adapted and added to with new materials as they became available. But those materials weren't always the ideal ones. If a 14th-century English monk went to the trouble of copying out a 13th-century astronomical table, and a set of instructions in Anglo-Norman French, it must be because he thought they were useful. But he surely wouldn't have done so if he had had more up-to-date, accessible alternatives available. Monks were often keen astronomers, but sometimes they just had to make do with whatever scientific data they could lay their hands on.

Particular thanks to Peter Jones, Richard Kremer, Philipp Nothaft and Kari Anne Rand for their assistance with this manuscript.

Further Reading

You can read a bit more about such calendars, and lots of other subjects in medieval science, in my forthcoming book, The Light Ages.


  • The Coldingham Breviary is British Library Harley MS 4664. It is fully digitised and available to view online at

  • The other two manuscripts containing the same materials are Cambridge, Gonville & Caius College MS 512 (ff. 179vb-179Cr), and British Library Arundel MS 220 (ff. 308r-313v). The instructions appear in Latin in Oxford, Bodleian Library MS Digby 149 (f. 125v).

For for more on these calendars and the astronomical calculations underpinning them, I recommend C. Philipp E. Nothaft, Scandalous Error: Calendar Reform and Calendrical Astronomy in Medieval Europe (Oxford: Oxford University Press, 2018).



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