Copernicus and the calendar

A recent blog post on Yovisto repeats a very widespread myth concerning Copernicus, his De revolutionibus and the calendar reform of 1582. This particular myth is so prevalent that I have no illusions about stamping it out but as a bone fide history of science myth buster I thought I could at least put the record straight in my little corner of the Internet.

Astronomers and mathematicians had been aware that all was not well with the Julian calendar since at least the time of the Venerable Bead Bede in the ninth-century eight-century CE. The average length of the solar year produced by this calendar, with a leap year every four years, was about eleven minutes too long producing a slippage of the calendar against the natural year of one day every one hundred and twenty-eight years. This might not seem like an awful lot but over the centuries it accumulates. As the Gregorian calendar reform was introduced in 1582 the calendar had already slipped ten days against the natural solar year since the fourth-century CE. The Catholic Church was particularly concerned about this slippage because of their desire to celebrate Easter on the appointed day, a moving date dependent on the vernal equinox and the phases of the moon.

Over the centuries the Church made various attempts to start the process of investigating the cause of this slippage and finding a lasting solution but none of them came to fruition. It’s worth pausing to ask why? The answer is to be found in the very strange power structure of the Catholic Church. The papacy is an elected absolute monarchy. This bizarre and probably unique structure means that there is little or no continuity in new policies from one papacy to the next. If one pope sets about wanting to reform the calendar and then dies his successor doesn’t automatically carry on where he left off but everything goes back to zero, with calendar reform somewhere down the to-do list. This factor combined with the fact that most popes only get elected when fairly old, and thus are not very long in office, meant that the many attempts to reform the calendar set in motion over the centuries all stalled before they could make any real headway. Things changed, however in the sixteenth century.

During the Fifth Lateran Council Pope Leo X invited Paul of Middelburg, Bishop of Fossombrone, an astronomer and avid advocate of calendar reform, to advise him on the possibilities of bring the calendar into line with the natural solar year. Paul had already been considering the problem for a couple of decades and in 1514 he persuaded the Pope to send out letters to all the European monarchs requesting them to consult their astronomers on the subject. Enter Nicolaus Copernicus Warmienis. We know that Copernicus was one of the astronomers consulted because Paul tells us so, unfortunately we don’t know the exact nature of his reply because it no longer exists. However the topic does get mentioned by Copernicus himself in De revolutionibus, published, effectively posthumously, in 1543. He writes:

For not so long ago under Leo X the Lateran Council considered the problem of reforming the ecclesiastical calendar. The issue remained undecided then only because the lengths of the year and month and the motions of the sun and moon were regarded as not yet adequately measured. From that time on, at the suggestion of that most distinguished man, Paul, bishop of Fossombrone, who was then in charge of the matter, I have directed my attention to a more precise study of these topics.

Copernicus’ involvement tells us that he was already considered to be an astronomical authority in 1514. His claim, made here, for the failure of this particular calendar reform is, however, not historically accurate. In reality Leo X became involved in an altercation with the French who invaded Italy in 1515 and calendar reform got put onto the back burner once again. Of historical interest is Galileo’s totally erroneous claim that Copernicus actually took part in the calendar reform deliberations at the Lateran Council, he didn’t. This is by no means the only spurious claim made by Galileo concerning Copernicus.

This time however Gregory XIII, who became Pope in 1572, took up the reform where it had been broken off and lived long enough to see it through to its conclusion in 1582. I don’t intend to go into all the gory details in this post, but only to address the question of Copernicus’ supposed involvement in this successful reform of the calendar. As stated at the beginning of this post, Yovisto repeated a common myth about this supposed involvement. In quoting it here I have no desire to put the good folks at Yovisto in the stocks, as one can find this claim made in numerous places including in several Wikipedia articles. They write:

Both Reinhold’s Prutenic Tables and Copernicus’ studies were the foundation for the Calendar Reform by Pope Gregory XIII in 1582.

Here Yovisto exaggerates a little, as the usual claim is that Copernicus’ determination of the length of the tropical year, as addressed by him in the passage quoted above, was used in the new improved Gregorian calendar. This claim is bogus.

Luigi LIlio  artist unknown

Luigi LIlio
artist unknown

The mathematical model on which the Gregorian calendar reform was based was produced by the Italian physician and astronomer, Luigi Lilio, who very definitely used the length of the tropical year, as determined by the Alfonsine Tables and neither Copernicus’ from De revolutionibus nor Reinhold’s from the Prutenic Tables, they differ. The historical confusion exists because in his writings, after the event, to justify and defend the new calendar against its critics, Christoph Clavius does in fact discuss Copernicus determinations of the length of the tropical year showing it to be in agreement with the figure used in the calendar reform. If the three sources under discussion all have differing lengths for the tropical year, and they do, and if Lilio used the Alfonsine length, which he did, how then can Copernicus’ figure be in agreement? If you convert the three differing lengths into days expressed in sexagesimal fractions (that’s base sixty!), as was general astronomical practice at the time, they all begin 356; 14, 33 differing only the third sexagesimal fraction. All three sources therefore agree if rounded off to 365; 14, 33 days as was done for the Gregorian calendar.


This might seem like a trivial point, and in some senses it is, but the supposed superiority of Copernicus’ determination of the length of tropical solar year, shown by its use in the epoch defining Gregorian calendar reform, is quoted as a historical proof of the general superiority of Copernicus’ work and used as an argument against those, ignoramuses, who were too blind to recognise that superiority and immediately adopt his system.


Filed under History of Astronomy, Renaissance Science

10 responses to “Copernicus and the calendar

  1. avi

    Hi and many thanks. Only a grammatical tip: Venerable Beade is, as you know, Venerable Bede.
    Excellent Post and Blog



  2. Baerista

    Nice post, Thony. You are certainly right about the equivalence between the various sources, if the year-lengths are rounded to the second sexagesimal place. Swerdlow pointed this out in 1974. However, the Gregorian calendar also has a lunar component and, as Swerdlow showed in 1986, the mean synodic month of Lilio’s reform agrees best with the Prutenic tables, not with the Alfonsine ones. Also, Bede lived in the eighth century (not ninth) and there is no evidence that he recognized any problem with the Julian year.

    • thonyc

      Bede may not have known that the length of the Julian year was inaccurate but he did know that something was not right with the calendar.

      • Baerista

        Only with regard to the lunar calendar – if at all. What Bede basically says is that there can be discrepancies of up to two days between the observed and the calculated Moon, but that this is explicable on account of temporary shifts brought about by the saltus lunae, embolisms, and the fact that the lunar age changes only once every 24h in a discrete count of days. He then goes on to proclaim that there is nothing at all wrong with this calendar and that everything returns to order after 19 years. The claim that Bede was some kind of would-be calendar reform is often bandied about, but there is very little evidence for it. In fact, I’d like to see it.

  3. Thanks for posting. In addition to Noel Swerdlow’s two articles in the Journal for the History of Astronomy (a note on the Gregorian civil calendar in 1974 and a longer 1986 article on the length of the solar year) readers may also wish to go back to the myth-busting article written by Edward Rosen (“Galileo’s Misstatements about Copernicus,” Isis 49 (1958): 319-330), which was prompted by the publication of Stillman Drake’s translations and commentaries of several essays by Galileo in Discoveries and Opinions of Galileo (1957).

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  7. In his book *God’s Philosophers* James Hannam writes “In the 1570s, Pope Gregory XIII set up a commission to decide on the method and implementation of reform. Because his system made calculations easier, the commission used Copernicus rather than Ptolemy to determine the length of the year”.

    So is this just outright false then? Thanks

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