Abstract
Easter being defined by the Church as the first Sunday after the 14th lunar day on or next after the spring equinox on 21 March, it became steadily more obvious in the later Middle Ages that the excessive length of the Julian mean solar year was causing the equinox to fall well before its notional date and that the Church’s lunar calendar was inferior to those used by Jews and Muslims. Successive writers from the twelfth to the sixteenth centuries proposed corrections for the Christian calendar, work studied by several recent writers and magisterially surveyed recently by Philipp Nothaft (Scandalous Error, Oxford, 2018). Practical reform, however, was prevented by the Church’s false belief that its Easter reckoning had been immutably decreed by the Council of Nicaea in 325.
In the early fifteenth century, this obstacle seems to have been removed by the conciliar movement, for what one oecumenical council could do another could undo (though Jean Gerson thought the pope could reform the calendar by himself). This apparent opportunity elicited the proposals examined and edited by Solan. None came to fruition; the Council summoned to Rome in 1412 by John XXIII (since 1947 officially an antipope) broke up without achieving anything; its successor at Constance (1414–1418) was preoccupied with ending the papal schism; the council that began sitting at Basel in 1431, though it twice contemplated decreeing a reform, failed to do so and lost a power struggle with Pope Eugene IV, who drew most of its members away to Ferrara and Florence for his project of reunifying Catholics and Orthodox. In 1444, the rump council sadly instructed the clergy of Cologne to keep the Church’s Easter even though it was a week too late.
Solan’s presentation is divided into three parts: “D’un concile à l’autre,” “Typologie des propositions de réforme,” and “Édition et traduction des textes.” The first presents the would-be reformers and their writings: Pierre d’Ailly, bishop of Cambrai, who addressed an Exhortatio super kalendarii correctione to John XXIII and submitted to him a draft decree he might like to sign; Hermann Zoest, monk of Marienfeld, who in the course of 13 years thought up three different schemes; and Johannes Keck, later monk of Tegernsee, who drew up a calendar based on the last of them as if it had been adopted. Most attention goes to Zoest, an expert on the Jewish as well as Christian calendars, author of a Tractatus Phase (i.e. Hebrew pāsēh, “Passover”); a Phaselexis in two versions, the first (edited by Philipp Nothaft, Cahiers de l’Institut du Moyen-Âge grec et latin, 84, 2015, 236–291) to justify the draft decree of 1435, the second, heavily revised, to do the same for that of 1437; and a Compendium paschale still vainly calling for reform.
In the second part, Solan examines the problems and solutions from an astronomical point of view. The spring equinox no longer fell on 21 March, but could the date be lawfully changed and if so should it be determined astronomically or by reference to an idealized version of the Jewish month Nisan? To keep it in place, leap-days would have to be suppressed, every 134 years according to the Alphonsine Tables, commonly adjusted to 136 as a multiple of 4, but other intervals were also canvassed. Not until 1437 was the one-off omission of several days proposed. There was far more agreement that over 304 years the real new moons advanced by about a day on those predicted by the Church’s 19-year cycle, but again questions of principle arose (ought the 14th lunar age be counted from the conjunction or the day after it, or equated with 14 Nisan?) and different means of correction were proposed: recalculation, reduction of their cyclical places by three, or advancement en bloc by 3 days.
This is only a bald summary of Solan’s discussion, which is followed by the texts mentioned, and some others, with French translations, and a glossary. Very occasionally a corner is cut (e.g. p. 673: “C’est l’année tropique (just stated as 365d 5h 48m 45s) qui est utilisé dans le calendrier julien (which assumes 365d 6h)”); but his work has left us all in his debt.