Regiomontanus and the "True" Easter
With Easter right around the corner, we thought we’d highlight a topic that always seems so mystifying: the date of Easter, and specifically how this was addressed by the influential, pioneering astronomer Johannes Müller von Königsberg (1436 – 1476), better known as Regiomontanus.
The complications surrounding the dating of Easter stem from the historical prominence of lunar calendars, which measure time in cycles of approximately 30 days, and the current prevalence of solar calendars, which measure a day according to the earth’s revolution around its North-South axis, and a year according to one revolution of the earth around the sun. Jesus was crucified on a Friday, and the resurrection is said to have occurred the following Sunday, that is, after the first day of Passover. The date of Passover, which celebrates the exodus of the Jewish people from slavery in Egypt, is decided according to the Hebrew lunisolar liturgical calendar and is observed on the 15th day of Nisan, the first month of spring, with 15 Nisan being the first full moon.
In 325 AD, the Council of Nicaea decided that Easter Sunday would fall on the first Sunday after the Paschal Full Moon – that is, the first full moon on or after the vernal or spring equinox. This is not a true full moon, however, but an ecclesiastical full moon, calculated specifically for liturgical purposes. Prior to this date, Easter could be celebrated at different times in different churches. Sunday wasn’t even regularized as the day of celebration: because Passover could occur any day of the week, some Christians felt that so too could Easter. In an effort to create uniformity in the religion, Sunday was an important part of the Council’s decision.
Further, based on the Church of Alexandria, the date of the equinox came to be defined as 21 March. Alexandria was an important centre for scientific scholarship and the local practice was to celebrate Easter the first Sunday following the fourteenth day of a lunar month occurring on or after 21 March. This coincided with the first date of the vernal equinox in Alexandria, which was 20 March in 325 AD, when the Council met.
However, this, combined with the solar-calendar then in use, meant that the dating of Easter became particularly confusing by the fifteenth century. This is wonderfully demonstrated in Regiomontanus’s famous, and famously important, Kalendarium, presented here in an extraordinary illuminated copy of the Latin edition published in Venice in 1476. (See the complete description of this copy here.)
Regiomontanus, Johannes (1436-1476). Kalendarium. Venice, Bernhard Maler, Peter Loeslein and Erhard Ratdolt, 1476. See the complete description of this copy here.
Regiomontanus’s Calendar represents the first application of modern scientific methods of astronomical calculation and observation in addressing problems arising from the lunar calendar, like that of Easter; at the end of the Latin edition, he included a section on the errors of ecclesiastical calculations of Easter, which, he explains, developed out of erroneous assumptions about the length of the year and the lunar cycle:
“…since the rule for leap years is not quite accurate, and because the new moon moves forward about six hours every 76 years, and also nowadays the vernal equinox falls on March 11, it is the case that whenever the full moon falls between March 11 and March 21, then this full moon is now the true Easter full moon – albeit in contradiction with Abbot Dionysius Exiguus’s computus. Hence, those who know that March 11 is the true vernal equinox (a seemingly well-known fact), those who determine the full moon either by observation or by table, and those who know how the divine law reads, all of these people will wonder why it is that whenever there is a full moon in this interval, Easter does not fall on the following Sunday, but rather four or five weeks later, by church decree.” (Translated by E. Zinner, Regiomontanus: His Life and Work, p. 353)
The reference to Dionysius Exiguus is significant as it was this monk who, in creating a table of Easter dates based on the Alexandrian-standardized equinox, determined that Jesus was born in the Roman year of 753 and consequently restarted the count by inventing the Anno Domini (AD) system.
Strikingly, as Regiomontanus notes, the vernal equinox was, in his time, 11 March. This was a consequence of the Julian calendar, which, as the name suggests, was implemented by Julius Caesar on 1 January BCE and later adapted by his successor, Augustus. Based on a solar year, it had an average length of 365.25 days—just slightly off from the actual length of a year which is 365.24219 days. The Julius calendar included leap years to make up for this, but they were inaccurately calculated, such that the calendar year began to shift from the solar year. As a result, every 128 years the seasons would begin a day earlier. By the fifteenth century, this meant that the Julian calendar had fallen behind the solar calendar by about a week, and the vernal equinox was, for Regiomontanus’s time, 11 March.
By looking at the phases of the moon and the spring equinox, Regiomontanus demonstrated how the dates for Easter Sunday differed dramatically between 1477 and 1532 in comparison to the dates determined according to the ecclesiastical Easter calculation established by Dionysius, that is, between Easter determined according to the observed reality of the equinox versus the canonical date of the equinox. Regiomontanus also included a table indicating Easter’s occurrence for each year up to 1531.
Such discrepancy, he averred, was always going to result in embarrassment and he notes that “for this reason it is presumably worth taking the trouble to determine, for a few years in advance, just how the correct rule for Easter has deviated from the old Dionysian Rule, so as to be able to fight off such unanticipated arrows of ridicule.” (Translation by E. Zinner, p. 353)
Indeed, this was a big problem, and it was partially because of how well Regiomontanus was able to support his critique of the dating of Easter that he was summoned to Rome by Pope Sixtus IV in 1475 to help with a long-planned calendar reform. The subject of reforming the calendar had been discussed for some 200 years, largely because of the issue presented by Easter—as the most sacred observance of the Christian church, it was important that it was being observed on the right day!
Unfortunately, however, the changes would have to wait, as Regiomontanus died in the summer of 1476. It wasn’t until 1582 that adequate updates were implemented, by Pope Gregory XIII, in order to realign Easter with the vernal equinox. The two calendars are almost the same, but in the Gregorian calendar only centenary years divisible by 400 are leap years, making the average year 365.2425 days (which is still not exactly 365.24219 days, but the difference is one of only a single day in 3226 years, so it seems pretty good). In order to make up for the shift, the year 1582 lost 10 days and 5 October became 15 October.
When Pope Gregory XIII implemented his new calendar, he also changed the way Easter was determined, first by un-fixing the date of the vernal equinox—which could now be either 20 March or 21 March, depending on more accurate timing of the instant of the equinox—and through a more precise determination of the full moon. However, Eastern and Western Churches have variously adopted the Gregorian calendar and a reformed Julian calendar and thus adhere to different equinox dates, hence why holiday dates largely continue to vary between Eastern and Western churches.
Interestingly, in 1476, probably after this Latin edition, Regiomontanus’s Calendar was also issued in Italian, also by Ratdolt, who was responsible for publishing this Latin edition. The Italian edition omits the disquisition on the true date of Easter and the table of its incidence from 1488 to 1531, and thus has thirty leaves instead of thirty-two. As Zinner points out, this was only included in Latin, that is, the “only in the calendar understandable by scholars,” since “he was obviously trying to avoid being accused of stirring up large segments of the population against the Church by referring to errors in the Church’s calculations.” (Zinner, Regiomontanus, p. 126) Scholars had long been well aware that the calendar was drifting, but one still had to be careful where the Church was concerned, especially regarding such an important day for the Christian faith.
The use of charts was part of the wonderful appeal of Regiomontanus’s work. He was one of the first to realize the impact printing would have in disseminating scientific knowledge and in 1472 he established his own private press in Nuremberg for the production of the Calendar and other mathematical and astronomical works. The German astronomer thus “incorporated in his productions the first solutions to a host of typographical problems: tabula data [...]; pioneering printed geometrical diagrams, illustrations of eclipses and planetary models (some systematically coloured by hand under the supervision of the press); the first volvelles and sundials with built-in brass arms in a printed book” (M. H. Shank, “The Geometrical Diagrams”, p. 27).
In Venice, Ratdolt replicated Regiomontanus' pioneering results and simultaneously produced a Latin and an Italian edition of the Calendar for the years 1475-1530, a veritable instrument-book for calculating moon phases, eclipses, and other astronomical events. In addition to the section devoted to Easter, the publication includes charts for daylight hours and seasonal locations of the sun in the sky, phases of the moon, and conversions of planetary hours to equal hours.
Regiomontanus’s Kalendarium is also the first book to include a title page.
As a typographical work, the Calendar represents other important firsts as well. In addition to being the first Italian book to feature extensive use of woodcut initials, this Venetian publication is rightly famous for bearing the earliest known example of an ornamental title-page in the history of printing: even if in verse, it provides the date, place and the names of the printers responsible for the publication:
Aureus hic liber est: non est preciosior ulla / Gema kalendario: quod docet istud opus./ [...] Hoc Ioannes opus Regio de Monte probatum / Composuit: tota notus in Italia. / Quod Veneta impressum fuit in tellure per illos / Inferius quorum nomina picta loco. 1476. Bernardus Pictor de Augusta, Petrus Loslein de Langencen, Erhardus Ratdolt de Augusta.
The border framing the title-page is designed in the purest Renaissance style. As Goldsmith states, the floral and foliate motifs recall the ornaments carved in relief by Lombardi in the marble pilasters of the Venetian church Santa Maria dei Miracoli. In the copy presented here, the border is illuminated. In this Ratdolt's Venetian edition “one recognizes an undeniable Italian Renaissance influence in both the borders and initials [...] Here, a new harmony is achieved by Ratdolt's congruous design in both initials and borders, which seem to have been executed by the same cutter, resulting in some of the most beautiful borders ever included in a printed book” (D. Laube, The Stylistic Development of German Book Illustration, p. 54).
How to cite this information
Julia Stimac, "Regiomontanus and the 'True' Easter," 31 March 2021, www.prphbooks.com/blog/easter. Accessed [date].This post is licensed under CC BY-SA 4.0.