Between Crucifixion and Calendar Reform:
Medieval Christian Perceptions of the Jewish Lunisolar Calendar
Carl Philipp Emanuel Nothaft
The moon would have played a marginal role in Christian religion, had it not been for
the fact that Jesus Christ is reported by the evangelists to have been crucified on a Friday
at the time of Passover. According to the Synoptic Gospels (Matthew 26:17–19; Mark
14:12–16; Luke 22:7–15), the Last Supper was conducted like a regular Passover meal,
which took place on the evening that marked the transition from the 14th to the 15th day
of the month of Nisan in the Jewish calendar. The Gospel of John (19:14, 31), by contrast,
puts the Passion of Jesus on the ‘day of preparation’ for Passover and hence one day earlier,
on 14 Nisan—a scenario that is seemingly confirmed by the Jews’ refusal to enter the Roman praetorium on the day of the crucifixion, ‘lest they should be defiled; but that they
might eat the Passover’ ( John 18:28), implying that the meal lay still ahead. This striking
discrepancy aside, the basic idea of a temporal connection between the Passion and Resurrection of Christ and the Jewish Passover emerges clearly from all four accounts.1
For pagan converts to Christianity who were eager to celebrate the anniversary of these
events at the right time, this created a problem. After the introduction of the Julian calendar in 45 BCE, most inhabitants of the Roman empire had become used to reckoning
with a solar year of 365.25 days, which did not take account of the phases of the moon,
unlike the Jewish calendar, which, in Jerusalem at the time of Jesus, was in all likelihood
still regulated by empirical principles such as the first sighting of the new moon crescent in
the evening sky. The most straightforward way of solving the chronological problem thus
posed was to make the date of Easter dependent on the Jewish calendar, either by always
letting it coincide with the Passover on 14/15 Nisan (a custom later decried as the heretical doctrine of ‘quartodecimanism’) or by celebrating on the Sunday that followed it. This
practice of ‘following the Jews’, common as it may have been during the first three centuries
of Christianity, was eventually abandoned in favor of Easter cycles that projected lunar
dates onto the grid of the Julian calendar, making it thereby possible to calculate the movable feast days long in advance and independently of the Jewish calendar. A leading role in
this development was played by the church of Alexandria, which was the first to employ
the 19-year lunisolar cycle that eventually became the sole basis for medieval Easter reckoning, also known as the science of computus.2
The adoption of Easter cycles fostered an increasing separation of the Christian from
the Jewish pascha, which, owing to diverging standards of calculation and intercalation,
were frequently celebrated a month apart. This is particularly palpable in the case of the
Alexandrian style of Easter reckoning, which stressed the ‘rule of the equinox’, according to
which the Easter full moon (the Christian version of 14 Nisan) could not fall earlier than
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21 March, the assumed date of the vernal equinox, making 22 March the earliest permissible date for Easter Sunday. Scholars have occasionally come to regard this rule of the equinox, which had no direct counterpart in the Jewish calendar, as having been motivated by
a deliberate attempt of the early church to sever its ties to the Synagogue, i.e. to emancipate
Easter from Passover not just theologically, but also calendrically.3
While this interpretation is far from wrong, it only provides part of the picture. When
Peter, who served as the patriarch of Alexandria during the first decade of the fourth century, defended his church’s method of Easter reckoning, he appealed to the divine Law
that had been revealed to Moses and laid down in the book of Exodus. In Peter’s view,
the Alexandrian rule of the equinox closely corresponded to the practice of the ancient
Hebrews. If the Jews of his own age regularly violated the limit of the equinox, celebrating
Passover a month earlier than the Christians, this was only a sign that they had abandoned
the customs of their fathers—a clear sign of the degeneration that Judaism had supposedly
gone through in the wake of the destruction of the Second Temple. According to Peter, one
had to discern two ‘Jewish’ calendars: that of the Old Testament and that of contemporary
Judaism. In this perspective, the Christian Easter calculation was not developed in order to
abandon its roots, but it actually surpassed the contemporary Jewish calendar when it came
to implementing Mosaic Law. The Christians, or so it seemed, were the better Hebrews.4
Most of the context of Peter’s fragmentarily preserved statements is lost, but traces of
his rhetoric can be detected in a number of other late antique sources. It was still present
in the fifth century, when the episcopal sees of Rome and Alexandria repeatedly quarreled
over the correct dating of Easter Sunday. In 444, Paschasinus, bishop of Lilybaeum (today
Marsala, Sicily), wrote a letter to Pope Leo I, in which he advertised the Alexandrian 19year Easter cycle as the ‘calculation of the Hebrews, that is the calculation according to the
Law’ (Hebraeorum, hoc est legalem supputationem).5 As is well known, the Roman pontiff
eventually bowed to this kind of pressure from Egypt, which resulted in the Alexandrian
19-year cycle being adopted as the standard of reckoning in the medieval Latin church.
The subtleties of the Alexandrian construal of the Hebrew vs. Jewish calendar, however,
got evidently lost in transmission from Greek to Latin. A general lack of available sources
on the Jewish calendar other than the Old Testament soon led to a situation in which
early medieval scholars ceased to properly differentiate between the biblical calendar of
the Hebrews and the calendar of the Jews that they may or may not have encountered in
their own time. Instead, they went on to by and large identify Jewish lunar months with
the theoretical lunations of the Alexandrian 19-year cycle, thereby providing their own
method of Easter reckoning with a venerable pedigree that could be tracked back to the
days of Moses or even Noah.6
This anachronistic view made it possible to apply the Easter computus to biblical exegesis. One noteworthy example is an obscure chronicle of the world, written in about 814
by Claudius of Turin, a Carolingian exegete and bishop, who is otherwise better known
for his leanings towards iconoclasm. In order to highlight pivotal points on the timeline of
salvation, Claudius invested a lot of effort in translating into Julian dates the ‘lunar’ calendar data that could be occasionally found in the Old Testament. Basing himself on an assumed equivalence between ancient Hebrew lunar months and Alexandrian computistical
months, he furnished his chronicle with weekdays and calendar dates for events such as the
beginning of Noah’s flood (Saturday, 5 April), the reception of the Law at Sinai (Monday,
31 May), and the Babylonian capture of Jerusalem (Saturday, 23 June).7
Between Cruciixion and Calendar Reform
261
Claudius of Turin’s method is also present in the works of a number of eleventh and
twelfth century monastic writers, who can be grouped together under the label ‘critical
computists’. One of their common goals was to employ the Easter cycle in order to solve a
chronological problem that had been puzzling Christian scholars for centuries: the exact
date of Christ’s Passion and Resurrection. Their strategy was to search for a year in the
Easter cycle in which the Julian date of the crucifixion (believed to be 23 or 25 March)
and the 15th day of the moon (as posited by the Synoptic Gospels) both coincided on a
Friday. In doing so, they effectively assumed that their computistical luna 15 of first spring
lunation would automatically correspond to 15 Nisan as it had been observed by the Jews
in first-century Jerusalem.8
Needless to say, this hypothetical Jewish calendar of the computists was very different
from the type of calendar that Jews—first in Babylonia and Palestine, but eventually also
in Spain and the rest of Europe—had actually adopted by the tenth century. Although the
medieval Jewish calendar retained a 19-year cycle of intercalation, its method of fixing the
beginning of the year represented a clear departure from the simple ‘epact’-based calendrical arithmetic that had characterized late antique Easter cycles. The technical foundation
of the fixed Jewish calendar, as it is still in use today, is the time of the mean conjunction
or molad, calculated on the basis of a mean lunation of 29d 12h 44m 3 1/3s = 29.530594
days. Each date of Rosh Hashanah (1 Tishri) is computed by adding multiples of this value
to a fictitious calendar epoch known as molad baharad, whose time is equivalent to Sunday,
6 October 3761 BCE, at 23 hours, 11 minutes, and 20 seconds.9
For the loss of simplicity, the astronomical precision thereby achieved was considerable, especially if compared to the lunisolar calendar still in use among Christians. The
old Alexandrian equation of 19 Julian years (6939.75d) with 235 months implied a mean
lunation of 29.530851 days, which caused the calculated new and full moons to fall behind the observable ones at a rate of one day in roughly every three centuries. By the end
of the twelfth century, Christian computists had not only noticed this discrepancy, but,
aided by the influx of Arabic astronomy from the Iberian peninsula, had begun to look for
new models on which to base an astronomically improved Easter computus. Under these
circumstances, it could not take long until the conjunction-based Jewish calendar made
its appearance on Christian scholarly radars. A meticulous comparison between the ‘Hebrew’ (i.e. Jewish), ‘Latin’ (i.e. Christian) and ‘Chaldean’ (i.e. Muslim) values for the mean
synodic month can be found in a Compotus, written in 1176 by the English astronomer
Roger of Hereford. In the calculated version of their lunar calendar, frequently encountered
in astronomical tables, the Muslims distributed 11 intercalary days over 30 lunar years (30
× 354 + 11 = 10 631 days), thereby producing a mean month length of 29d 12h 44m =
29.530555 days. According to the divisions of time used by Roger (where there are 40 moments to an hour, 564 atoms to each moment), this was equivalent to 29 days, 12 hours, 29
moments, and 188 atoms. A slightly larger value was proposed by the Jews, who increased
the number of atoms to 208 8/9. That both estimates were significantly closer to the truth
than the inflated ‘Latin’ value of 29 days, 12 hours, 29 moments, and 348 atoms did not
remain hidden to Roger, who admitted that the tabulated ‘ecclesiastical’ moon could differ by up to four days from the observable moon.10
The accuracy of the Muslim lunar calendar became an important source of inspiration
for various medieval proposals to reform the ecclesiastical lunisolar calendar, as they were
advanced in the thirteenth century by Robert Grosseteste (ca. 1170–1253), bishop of
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Lincoln, and the famous Franciscan scholar Roger Bacon (ca. 1214/20–ca. 1292).11 An
even more suitable template, however, was provided by the Jewish calendar, whose lunisolar form resembled that of the Christian Easter cycle more strongly than the purely lunar
calendar of the Arabic astronomers. In particular, both Jews and Christians used the same
19-year intercalation cycle, with the exception that the Jewish cycle began three years later
than the Christian one—a fact that had already been pointed out by Abraham Ibn Ezra in
his Liber de rationibus tabularum, a Latin work written especially for a Christian readership in 1154.12
That there was in his own time a lively interreligious exchange on calendrical matters is
hinted at in Ibn Ezra’s ‘Letter of the Sabbath’ (Igeret ha-Shabbat, 1158), in which he criticized fellow Jews for communicating the details of their calendar to Gentiles in an inexact
manner.13 The most impressive twelfth-century manifestation of such contacts is the Compotus emendatus, written in 1170 or 1171 by the Westphalian cathedral canon Reinher of
Paderborn, who wanted his church to abandon its erroneous ways and return to what he
regarded as the astronomically sound paschal reckoning that had once been prescribed
by Moses himself. In order convince his readers of the great advantages that the Jewish
conjunction-based reckoning held in store, Reinher composed a competent and elaborate
exposition of its rules of operation (the first of its kind by a Christian author) along with
innovative tables for easy conversion between dates calculated on the basis of the molad
and the Julian calendar. These tables made ample use Hindu-Arabic numerals, including
zero, which puts the Compotus emendatus among the earliest known Latin texts to employ
such numerals for technical purposes.14
Both the Jewish calendar and the reform of the Christian Easter cycle continued to
play a prominent role in the writings of Roger Bacon, who sent his famous Opus majus to
Pope Clement IV in about 1267. Although he did not go as far as Reinher of Paderborn in
demanding that the church should adopt some version of the Jewish calendar, Bacon spoke
very highly of the Jewish astronomers and the accuracy of their value of the mean month.
In fact, he was so determined to convince the pope of the importance of studying the Jewish calendar that he spared no expenses to acquire a Hebrew calendar manuscript and send
it to the papal curia in Viterbo along with his pupil John as an interpreter. According to
Bacon, this Hebrew calendar was ‘a wonderful work of astronomical art and highly useful
for the understanding of the Law and the feasts prescribed by it. A person ignorant of it can
never hope to properly understand the Law, nor can he converse with the Jews about such
things, let alone convince them in any useful way’.15
That Bacon himself maintained some contact to learned Jews (and that he conversed
with them about the calendar) is quite evident from his own profound knowledge on the
subject, part of which reflects passages in the Babylonian Talmud. Amongst other things,
he referred to the practice of the Jews in ancient Palestine, mentioned in the tractate Rosh
Hashanah (22b–23b), of lighting beacons on high mountains in order to make known the
beginning of a new month.16 Yet while Bacon was without a doubt a brilliant and original
author, his interest in the Jewish calendar was not entirely exotic even in the thirteenth century. Another detailed and technical description of the Jewish calendar was composed in
about 1294 by a certain Robert of Leicester, who is presumably identical with the scholar
of the same name who later served as the 48th master of the Oxford Franciscans (1321/22)
and who also penned a treatise On the Poverty of Christ.17
Aside from the reform of the calendar, biblical chronology was the other main factor
Between Cruciixion and Calendar Reform
263
that motivated Christian interest in the Jewish calendar. Indeed, it is difficult to find a
medieval discussion of the subject that does not in some way include an application of
this knowledge to determine the exact date and year of Christ’s Passion. In order to do
so, scholars had to be able to calculate the Passover dates of the first century CE, a task at
which the ‘critical computists’ had failed because of their anachronistic use of the Alexandrian lunisolar cycle. Since Reinher of Paderborn rejected this cycle in favour of the medieval Jewish way of reckoning, it was only natural that he would also be the first to use
the molad-calendar in an attempt to re-calculate the date of the crucifixion. This method
was subsequently applied by several other medieval Christian writers, including the aforementioned Robert of Leicester, but not by Roger Bacon, who apparently understood that
the Jewish lunar month of antiquity was not based on the molad or mean conjunction,
but on the day of first visibility of the new moon crescent. Whereas Reinher of Paderborn
had presented a static picture of the Jewish calendar, which assumed that the molad-based
reckoning had already been instituted by Moses (a view presumably shared by his Jewish
informants), Roger Bacon implicitly acknowledged that the Jewish calendar had a history
that had to be studied carefully in order to understand biblical time reckoning. It is thus
no surprise if Bacon, both in his methods and in his conclusions, came remarkably close
to modern discussions of the Passion date. Using astronomical tables, he calculated that
Jesus had been crucified on 3 April 33 CE, the 14th day of Nisan, a date that is still widely
accepted among experts today.18
The problems posed by the chronology of the Passion also determined Christian interest in another important aspect of the Jewish calendar and the feast days attached to it: its
postponement rules or deΩhiyyot. In the contemporary Jewish calendar, they make sure that
the 15th day of Nisan can never fall on a Friday. If this rule had already existed in the first
century, Jesus could not have been crucified on the first day of the Feast of Unleavened
Bread, as the Synoptic Gospels seemed to imply. That the Jewish calendar thus favored
John’s account of the Passion was already acknowledged in the early twelfth century by
Rupert of Deutz (ca. 1075–1129), who regarded the deΩhiyyot as ancient Jewish practice.19
An opposing viewpoint was expressed by an anonymous English computist, who, in 1175,
dryly noted that the present-day Jewish calendar cannot have been in use at the time of
Jesus precisely because it conflicted with the synoptic version of events.20
An ingenious attempt to harmonize both positions was eventually made in about 1430
by bishop Paul of Burgos (ca. 1353–1435), who had been a famous Castilian Rabbi named
Solomon ha-Levi before his conversion to Christianity in 1390/91. Like others before
him, Paul assumed that the Jewish calendar he was acquainted with had already been in use
during the first century. Basing himself on the reckoning rules of this calendar, he found
that in the year of the crucifixion (which he took to be 33 CE) the 15th of Nisan had been
postponed from Friday to Sabbath. Yet according to the Synoptic Gospels, the Last Supper, held on Thursday evening, had been a Passover meal. Paul found it hard to accept the
idea that Jesus, as a Jew faithful to the law of his people (Matthew 5:17), would have ever
violated the Mosaic rules by celebrating Passover one day early. Luckily, all chronological
problems seemingly dissolved if one looked more closely at the deΩhiyyot and the reasons
for their existence. As a former Rabbi, Paul knew that the rule which prevented Passover
from falling on a Friday was a mere corollary of another rule, according to which Hoshana
Rabbah on 21 Tishri could never fall on a Sabbath. Since the interval between Nisan and
Tishri was always constant in the fixed Jewish calendar, a shift of Hoshana Rabbah from
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Sabbath to Sunday automatically implied that the preceding 15 Nisan had to move from
Friday to Sabbath. Yet this meant that the postponement of Passover in the year of the crucifixion was contingent upon a feast (Hoshana Rabbah) that still lay several months ahead,
belonging to a future in which, due to Christ’s self-sacrifice, mankind had been redeemed
and the law of the Old Covenant abolished. The calendrical rules attached to Hoshana
Rabbah, Paul argued, were hence null and void in this near future and this was the reason
why Jesus, knowing what would happen, refrained from postponing the Passover shortly
before his death.21
Paul’s fame as a commentator on the Bible ensured that his solution for the Passion
chronology would remain the object of scholarly debate for at least the next two centuries.
To name just one example, we find his views discussed at the University of Louvain in the
late 1480s, where Paul of Middelburg (1446–1534), a Dutchman who later became bishop
of Fossombrone, and the philosopher Peter de Rivo (ca. 1420–1499) fought a heated controversy over the date of Christ’s crucifixion. Paul of Middelburg added further substance
to the theory of his namesake Paul of Burgos by presenting an array of obscure medieval
Hebrew texts, which to him proved that the present-day Jewish calendar, including its
deΩhiyyot, had been in existence since the days of the founding of the Second Temple. His
Hebrew citations in this context represent the earliest known instance of Hebrew printing
in the Netherlands.22 Peter de Rivo, on the other hand, dismissed this evidence as well as
Paul of Burgos’s entire strand of reasoning. In his view, the present-day Jewish calendar,
with its astronomical precision, showed clear signs of Arabic influence and could only have
been conceived after the rise of Islam, maybe even only after the development of the Alfonsine tables in the thirteenth century.23
Discussions of this kind would continue well into the sixteenth century and beyond,
during a period that saw important developments in the field of Christian Hebraism.24 As
we have seen, however, scholarly interest in the calendar as a particular aspect of Judaism
clearly predates the Renaissance. Owing to the joint roots of both calendars, Christian
intellectual engagement with the Jewish calendar was kept alive throughout the Middle
Ages, ranging from the naive identification of Alexandrian Easter reckoning with the calendar of Moses that characterizes the earlier period to the refined and erudite treatment we
encounter in the works of Roger Bacon. To give up such pursuits was to impede a proper
understanding of Scripture, at least according to Bacon, who reminded the pope that ‘our
Lord and the Apostles were Jews [Hebraei], as were the Patriarchs and Prophets’.25
Notes
1. See Strobel (1977), pp. 17–64; Finegan (1998), pp. 353–358. The following abbreviations are used
throughout: PL = Patrologia Latina; CCSL = Corpus Christianorum Series Latina; CCCM = Corpus
Christianorum Continuatio Mediaevalis.
2. For a comprehensive overview, see Pedersen (1983) and Thornton (1989).
3. Zerubavel (1982), p. 286; Simon (1996), pp. 310–322; Steel (2000), pp. 98–101.
4. Peter’s arguments are cited in the seventh century Chronicon Paschale: Dindorf (1832), pp. 4–10.
5. See Paschasinus’s Epistola ad Papam Leonem in Krusch (1880), p. 248. Photius, in his Bibliotheca (cod.
115), mentions a book entitled ‘Discourse against the Jews and the heretics who follow them and against
those [...] who do not celebrate Easter in the first month according to the Hebrews’. See Henry (1960), p.
86. On the general background, see Gerlach (1998).
6. See Bede, De temporum ratione, CCSL 123B: 312–315, 326–327, 420–422; ps.-Bede, De argumentis
lunae, PL 90: 723, and Wiesenbach (1986), pp. 119–122. See also Warntjes (2010), pp. 242–243, for
Between Cruciixion and Calendar Reform
265
further source references.
7. Claudius of Turin, Brevis Chronica, PL 104: 917–926. See further Allen (1998) and Nothaft (forthcoming).
8. The standard monography is now Verbist (2010). See further the editions by Weikmann (2004) and Wiesenbach (1986), who coined the term ‘kritische Komputisten’.
9. For a useful introduction into the present-day Jewish calendar, see Feldman (1978), pp. 185–210. On
its medieval origins, see Stern (2001), pp. 180–210; Stern and Mancuso (2007); Carlebach (2011), pp.
11–24.
10. MS Cambridge, University Library, K.K.1.1, f. 238r; Moreton (1995), pp. 581–586.
11. See the texts edited in Steele (1926); Brewer (1859), pp. 274–295; Bridges (1897), pp. 269–285.
12. See the edition by Millás Vallicrosa (1947), pp. 99–100.
13. Friedländer (1894/95), p. 71; Sela (1996), pp. 216–217.
14. The Compotus emendatus was edited by van Wijk (1951). See also Honselmann (1962) and Herold (2005).
On the reception of Hindu numerals in the West, see Burnett (2006).
15. Brewer (1859), p. 220: ‘Et in hac tabula est mirum artificium astronomiae, et summa legis intelligendae
utilitas, et omnium festorum legalium, quam qui nescit numquam potest scire intellectum legis, ut oportet,
nec cum Judaeis conferre de talibus, nec eius persuadere utiliter’.
16. Bridges (1897), p. 196: ‘Et ideo Hebraei antiquitus per astronomiam certificaverunt primationem lunae,
et cum non fuerat in visione novae lunae, nec potuit per visum cognosci, accenderunt faces in Jerusalem in
monte alto, ut sciretur quod tunc fuit tempus primationis, quatenus homines essent parati facere solemnitates et festa quae habebant expedire’.
17. See MS Oxford, Bodleian Library, Digby 212, ff. 2r–10r; MS Erfurt, Universitätsbibliothek, Amploniana,
Quart. 361, ff. 80rb–85rb; Russell (1936), p. 139; Walmsley (1953); North (1992), pp. 132–133.
18. See Bridges (1897), pp. 195–198, 206–210. For modern estimates of the crucifixion date see Finegan
(1998), pp. 359–369. A comprehensive account of pre-modern attempts at dating the Passion is provided
in Nothaft (2012).
19. Rupert of Deutz, De sancta trinitate, CCCM 22: 900; Idem, De gloria et honore filii hominis super
Mattheum, CCCM 29: 300–301. On the deΩhiyyot, see Stern (2001), pp. 166–167, 194–195; Belenkiy
(2002).
20. MS London, British Library, Cotton Vitellius A.XII, f. 96ra. On the Compotus constabularii, see Moreton
(1999).
21. See the commentary on Matthew 26 in Paul of Burgos (1634), cols. 441–446.
22. Paul of Middelburg (1513), sigs. D6v–E6r; Offenberg (1974).
23. Peter de Rivo (1488), sigs. c4r–6r; (1492), sigs. e5r–6v.
24. Weinberg (2000). On Christian Hebraism in the early modern period, see Coudert and Shoulson
(2004).
25. Brewer (1859), p. 213: ‘Nam Dominus noster et apostoli fuerunt Hebraei, sicut patriarchae et prophetae’.
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