[In the following essay, Nobis details criticism of the Gregorian calendar reform by contemporary scientists.]

This paper has a bearing not only on the history of chronology in particular, but on the history of science in general. The reaction of astronomers to the calendar reform provides us with a very good example of the general problems involved in such an undertaking, especially those deriving from the practical needs of society. Furthermore, we can see clearly how external non-scientific factors influenced scientists when their opinions were sought.

The Gregorian calendar reform has been studied by such authors as Kaltenbrunner,¹ Schmidt,² and Stiewe,³ to mention only the most important. I will restrict myself here to a few of the topics which, although touched upon in the literature, have never been clearly expressed. Not many astronomers reacted to the reform in anything like a critical way, and when they did so, we find that their objective criticisms were often colored by personal reasons arising from their different religious and metaphysical positions. Consequently the reaction of astronomers was not uniform. With few exceptions, none had an entirely negative reaction. One clear exception was that of mathematicians in Prague who refused to help the bishop change the calendar of feasts. There were those who opposed the reform for personal reasons or on general principles like the theologians Lucas Osiander (1534-1604) and Jacobus Heerbrand (1521-1600), and the astronomers Michael Maestlin (1550-1631) and Tobias Moller (2nd half 16th c.); but there were also supporters who, nevertheless, presented suggestions for changes.

Maestlin was the only astronomer to protest on general principles; other critics like Justus Scaliger (1540-1609), Georgius Germanus (2nd half 16th c.), Sethus Calvisius (1553-1617), and Franciscus Viëta (1540-1603), accepted the principle of reestablishing the calendar according to the traditions of the Council of Nicaea, but they submitted alternatives and corrections. Those in favour of the reform included Protestants like Bartholomaeus Scultetus (1510-1614), Tycho Brahe (1546-1601), and Johannes Kepler (1571-1630), as well as Jesuits such as Christoph Clavius (1537-1612), Paulus Guldin (1577-1643), and Dionysius Petavius (1583-1652).

To understand the reactions of astronomers we shall consider in turn the mentality of the astronomers, the arguments they propose and the circumstances under which their critique was expressed. Although the existing literature touches on these questions, they have never been treated separately and in detail. To understand the mentality of the various astronomers, we have to take into account their relationship to the “magisterium ecclesiae”, the liturgy, and the Christian faith itself. Of particular importance is the different relationship of Protestant and Catholic astronomers to the “Magisterium ecclesiae”. Maestlin's statements in his work, “Ausführlicher Bericht von dem allgemeinen Kalender”,⁴ an opinion addressed to the Pfalzgraf bei Rhein, are eloquent examples. According to him the Pope had no right to institute a calendar reform, and he did so only with a view to gaining power. The Pope was even a heretic. Maestlin argued that by calling the calendar a “Calendarium perpetuum” the Pope and the authors of the Canones⁵ showed their disbelief in the Last Judgement which he himself thought would happen soon, thus providing a theological argument against the necessity for a calendar reform. But in his book Elenchus Calendarii Gregoriani,⁶ Sethius Calvisius refuted this argument by stating that for the same reason it would be unnecessary to build any more houses in the future.

Totally different, of course, was the reaction of those critics with Catholic sentiments. Viëta, who submitted an alternative plan in his Relatio calendarii,⁷ believed that the true intentions of the Pope were not really fulfilled by this reform and that Clavius was responsible for the errors. Georgius Germanus, however, who called himself a Catholic in his Computus ecclesiasticus⁸ opposed the reform, criticizing Clavius, and he sent his paper to the Protestant mathematician, David Origanus (1558-1628) for examination, thus indicating that he hoped to unify the Catholic and Protestant opinions.

Such hope was also expressed by two other astronomers who were moderate critics of the Gregorian reform. The Catholic Jacobus Cuno in his These de calendario Juliano⁹ proposed a compromise between the old and the new calendar, as did the Protestant Sethus Calvisius. Also conciliatory was the rebuttal to Maestlin's opinion which the Protestant representatives of the “Pfalzgräfliche Schule” at Neustadt a.d. Hardt¹⁰ had composed for the Pfalzgraf.

The different attitudes of Protestants and Catholics towards the “Magisterium ecclesiae” become evident from the connection between the calendar reform on one side and liturgy and faith on the other. Luther in his tract “Von den Konziliis und Kirchen”¹¹ contested the very connection and Maestlin agreed with him. An anonymous opinion addressed to the Bavarian sovereigns affirmed the necessity of the reform, but required that the Protestants accept the new calendar only from the emperor. Maestlin is, however, the only astronomer to use this position clearly as a motive for his rejection of the entire reform, although the Lutheran preacher (not an astronomer), Jacobus Heerbrand, a representative at the Council of Trent, did likewise. Heerbrand, professor at the University of Tübingen, presented his objections in the Disputatio de Adiaphoris et Calendario Gregoriano.¹² He is even more strident than Maestlin. For Heerbrand the Pope was the Anti-Christ, who “putavisse posse mutare tempora”. Essentially he put the question of whether the calendar reform is really an “Adiaphoron” and he concluded that it was not indifferent to one's faith whether it was accepted or not. Thus, its acceptance is not allowed even if the Emperor gives the order to do so. On the other side the Confessio Augustana maintained that the Easter rule was not an article of Faith.¹³

From the opposite point of view, in his work, Explicatio ad Calendarium Gregorianum, Clavius admits that the Pope has the right to declare Easter an immovable feast. Paul of Middleburg (Paulus de Middelenburgo), the chairman of the calendar commission of Leo X, had likewise stated this opinion in the fourteenth book of his Paulina.¹⁴ For Rome the reform was primarily, but not exclusively, a religious concern. The bull, Inter Gravissimas, referred to the Council of Trent which had explicitly charged the Pope to reform only the breviary and the missal.¹⁵ The result was a small calendar reform in 1568 together with the new breviary. The full reform of the calendar occurred only after the completion of the new Martyrology, an indication of the Church's thinking concerning the connection of the liturgy and the calendar, a connection which has a long tradition beginning with an early pre-Nicaean Easter sermon by Hippolytus of Rome († 235), who connected the Easter events with the cosmos. To Hippolytus we owe our early Easter cycle and Scaliger commented on him in 1595 in the same work where he states his alternative to the Gregorian reform.¹⁶ Shortly before, in 1551, the cycle of Hippolytus had been recovered in Rome.¹⁷ Hippolytus, when speaking of the cross, says that it is the fulcrum of the universe, the point where all comes to rest. In regard to the death of the Lord we find that nearly the entire cosmos had collapsed and dissolved during the anguish of the passion.¹⁸ An eclipse was believed to be a supernatural event and a sign of the extinguishing of the pωs τó αληθινóν,¹⁹ a tradition which we find again in the late medieval commentary on the “Sphaera” by Sacrobosco, which was the primer of astronomy for all priests up to the seventeenth century.²⁰ This treatise ends with a citation from the “Breviarium Romanum” which remained valid until 1971: “Aut Deus naturae patitur aut machina mundi dissolvetur”.²¹ The Church Fathers at the Council of Nicaea adopted, it appears, the same convention as Hippolytus when they fixed the Easter date to the first Sunday after the full moon which occurs after the vernal equinox. But because no Canon exists to prove this we have to rely on traditions and indirect sources.

One's view of the relationship between science and faith influenced attitudes towards calendar reform. For example, a scholar of the Thomist schools deriving from Albert the Great would think differently on these matters than a student of Latin Avveroism. For instance, the prevailing view at the University of Paris was that the reform should not be put into the hands of astronomers lest they should decide to replace ecclesiastical cycles with astronomical calculations. The Roman Curia was, indeed, wary of depending on astronomers because of such opinions as those of Galileo, who considered that astronomers were justified in their interpretations of Sacred Scripture in so far as astronomical topics were concerned.²²

We now turn to the arguments proposed by various astronomers as to who should author the reform, how it was to be carried out and its consequences. Following the Council of Nicaea it was the Emperor Constantine who promulgated the encyclical which is preserved in the Vita Constantini by Eusebius.²³ Luther in his tract Von den Konzilis refers to this encyclical and appears to argue that the wordly sovereigns should be responsible for the calendar reform.²⁴ He thus initiated what was to become a traditional attitude for astronomers from the Protestant tradition. At the beginning of the Imperial Circular of 4/14 September 1583 it was noted that the new calendar had been established on the basis of the expert opinions and reflections of the court mathematicians employed by Christian sovereigns and rulers and with the previous knowledge of the emperor. Furthermore, it was acknowledged that uniformity in time calculation was necessary for commerce and traffic throughout the empire. The edict did not refer to the cycle of Church feasts or other ecclesiastical aspects of the calendar. The Elector of Saxony stated that time calculations were the emperor's task and, together with the Elector of Mainz, he claimed that in such matters the Pope should present his opinions to a council and that consultations should be had so that any reform would proceed with unison among the various parties involved.

As to the precise details of the reform, there were various alternatives proposed to the one that was officially promulgated by Gregory XIII. Viëta was severely criticized for publishing a calendar with his own modifications but attaching a copy of the official papal bull Inter Gravissimas, thus giving the impression that his was an official version of the calendar. His modifications concerned the calculation of the epacts, the manner in which the intercalation days were inserted between the hollow and full months and the date for the beginning of the year. The astronomer Calvisius also criticized the calculation of the epacts. Georgius Germanus attempted to devise a calendar so that Easter would never occur before the date fixed by the Prutenic tables and so that the vernal equinox would not occur before March 21. Although he avoided some of the difficulties met by Clavius, his calendar was not on the whole as manageable as the one ultimately adopted. Scaliger contested the rule for leap years because it permitted the vernal equinox to occur other than on March 21.²⁵ Although his method matched the celestial movements better it did not possess the convenience of the one adopted. In his Alterum Examen²⁶ Maestlin criticized the whole method of calculating the cycle of epacts since he claimed it would create as many errors as the calendar it purported to reform. He assumed that the Prutenic tables, already discarded by Tycho Brahe, were used in the reform. Nevertheless, he himself used the Prutenic tables to argue that the leap year rule of the calendar reformers would place the vernal equinox at the time of the Council of Nicaea on March 20 instead of on March 21. For this he was justly reproached by the Jesuit Possevinus (1534-1611). The reformers had, as a matter of fact, also used the Alfonsine tables.

On the other hand, Maestlin's strong desire that strict astronomical methods be applied in the reform was based upon a quite justified concern for the civil effects, such as trade and commerce, which a reform of the calendar entailed. It was in this spirit of responding to the necessities of civil society that Schulin published a calendar with both the old and the new style reckoning but, in so doing, he had to defend himself against reproaches for not allowing the new calendar to stand on its own even at the risk of creating confusion.²⁷ Well founded intentions, however, can go too far. Some Protestant intellectuals went so far as to claim that the papal edict was against nature. For instance, in the anonymous Bawrenklag uber des Romischen Papstes Gregorii XIII newen Calendar it was stated that farmers no longer know when to till the ground and the birds are uncertain as to when they should sing and when they should fly away. The argument of Maestlin and Osiander that the Pope had stolen ten days from the farmers' life is found in a copy of a proclamation in which a Bohemian priest tried to explain the loss of ten days to his faithful. On their part those of a Catholic tendency also went to absurdities such as the claim that a nut tree, in response to the papal promulgation of the calendar, blossomed ten days in advance in Gorizia, Italy.²⁸

At the request of Count Palatine Ludwig Philip, Paulus Fabricius prepared for the emperor a criticism of Maestlin's work whereby he accuses Maestlin of inconsistencies on several points and recommends that he should have reserved his criticism for the strictly scientific questions associated with the calculation of the cycles.²⁹ However, Fabricius' criticism was suppressed by the imperial court which was seeking conciliation between the various factions.

The first printed defense of the calendar, published in Mainz in 1585, is by the Jesuit Johannes Busaeus (1540-1587).³⁰ Some of his main arguments, directed against Heerbrand are: Constantine initiated but did not execute the rule for determining the date of Easter; it is a practical matter that the people should carry out the reform since neither the Protestant sects nor the German sovereigns agree among themselves and, furthermore, the world is larger than the German empire; the new calendar is not substantially but only accidentally different from the old and the difference pertains to mathematics and not to theology. Another defense of the calendar reform was given by Johannes Rasch of Manich³¹ but his arguments, directed mostly against Maestlin, are not very convincing. The Jesuit Possevinus also refuted Maestlin and announced the forthcoming reply of Clavius.³² In his Apologia to Maestlin³³ Clavius explained why the use of mean motions was adopted, the principal reason being that no tables giving exact true positions with time existed. He argued, furthermore, that local longitude differences would cause discrepancies and thus the traditional desire, since the Council of Nicaea, that all Christians celebrate Easter on the same day could not be realized except by the use of mean motions. Clavius also showed that in any reasonable calendar the variation of the date of Easter, namely that it could be celebrated at the beginning or at the end of the first full moon of spring, could not be avoided. In 1606 Clavius wrote a work against the criticism of Scaliger showing that his suggested method of intercalation was not practicable.³⁴ Scaliger later responded to Clavius in a personally offensive way.³⁵ Although Clavius advanced further objective arguments,³⁶ the discussions had by now reached the level of personal vituperation.

We would now like to discuss the ways in which consultations and eventually the promulgation of the calendar took place. Bishop Hugelinus Martellus († 1593) in two treatises argues that the Church hierarchy was to be respected in matters of the calendar³⁷ and, like Josephus Zarlinus,³⁸ he proposed that dates be fixed according to the vernal equinox and the celebration of Easter at the time of the Council of Nicea. This was also the thesis of Floridus Plieningerus but his his arguments were based mostly on a kind of mystical experience.³⁹

It is undoubtedly true that many difficulties could have been avoided had agreements been sought more diligently with the civil authorities and with the Protestants. However, the Compendium of Lilius was not sent for evaluation to the Protestant sovereigns and the universities.⁴⁰ Furthermore the new calendar was not presented at the Diet of Ausburg called by Rudolph II in 1582, as attested to by the archduke Ferdinand of Tyrol. When the calendar was promulgated with the issuance of the bull, Inter Gravissimas, the Canones and an example of the calendar for the last quarter of 1582, there were complaints that it was not available enough. For instance, the Elector of Brandenburg said that he could not introduce the calendar at the time requested because the issues printed in Rome were not available. The mathematician, Martin Chemnitius (1522-1586) regrets that he cannot comment on the reform because he has received only fragmentary information from Danzig, Posen and Munich.⁴¹ A further difficulty arose from the fact that the official explanation of the calendar, Liber novae rationis restituendo Calendarii, promised in the Canones never appeared. In fact it was only twelve years later that a substitute for it in the form of the Clavius⁴² was issued. Although the Explicatio contains much material reflecting Clavius' personal arguments with various critics, it is an official document of the Church issued with the authority of the Pope and containing all of the principal documents of the reform: the Bull, Inter Gravissimas; the Canones; an example of the calendar for October-December 1582; and the Compendium of Lilio.

Some mathematicians, feeling that not enough scientific rigor had been applied to the calendar reform, made other attempts of their own. Paulus Fabricius, an expert of the emperor on calendar matters, composed a calendar in 1583 proposing that it be gradually introduced by issuing it alongside the old calendar until 1600.⁴³ Theodorus Graminaeus, an astronomer, lawyer and theologian at the University of Cologne, wrote in 1583 his Exhortatio de exequenda Calendarii correctione⁴⁴ and in 1595 an article entitled Supputatio Ecclesiastica⁴⁵ appeared, written by the personal physician of the bishop of Würzburg, Adrianus Romanus (1561-1615), later court mathematician for the king of Poland. The reactions of individual astronomers to the promulgation of the calendar ranged from the ridiculous mercenary interests of a Tobias Moller to the interesting debates of Kepler and Brahe. Moller complained that he had not received the compensation he requested for having proposed a new calendar to the Diet at Augsburg in 1582 and he refused to list his arguments against the Gregorian Calendar for fear that they would be plagiarized and he would suffer financial loss.⁴⁶

Both Tycho Brahe and Johannes Kepler were imperial mathematicians and Protestants and these factors undoubtedly placed some constraints on their expressing opinions on the calendar. Brahe responded positively to the Gregorian reform and from the very beginning used the new calendar to date his correspondence and his observations. We have two letters from him in which he states that there are no problems with the new calendar except that it is difficult for the Protestants to accept that it was authored in Rome.⁴⁷ Kepler expresses the opinion that the Gregorian Calendar is the best of those which had been constructed.⁴⁸ He left a posthumous article which presents his arguments in the form of a dialogue between a Protestant chancellor, a Catholic preacher and an expert mathematician. His summary sentence is of some interest: “Easter”, said he, “is a feast and not a planet. You do not determine it to days, hours, minutes and seconds.”⁴⁹ In 1613 at the Diet of Regensburg Kepler unsuccessfully argued that the Gregorian Calendar be adopted, stating that it did not involve the acceptance of a papal bull but only the result of the work of astronomers and mathematicians. But the Protestant sovereigns did not begin to yield in their resistance to the Gregorian calendar until 1699, when the astronomer, Erhard Weigel (1625-1699), a teacher of Leibniz, suggested replacing the tables of Clavius with the Rudolphine tables of Kepler, which were first printed in 1617.⁵⁰ Nonetheless, in the century or so following the promulgation of the Gregorian calendar there were a number of publications by astronomers supporting it.⁵¹ In 1703, one hundred years after Pope Clement VIII's declaration on the calendar,⁵² Francesco Bianchini (1662-1729), secretary of the Church's calendar commission and former librarian of Pope Alexander VIII, pronounced a final official statement on behalf of the Roman Curia.⁵³ Still in the subsequent years, up until 1775, the controversy continued concerning the determination of the date of Easter. In fact, a calendar issued in February 1700, based upon an astronomically exact determination of the date of Easter, placed the Protestant celebration of Easter one week earlier than the Catholic celebration in the years 1724 and 1744. The last differences were finally abolished by decree of the German Emperor and of the King of Prussia, Frederick II.

Notes

1. Kaltenbrunner, F., “Die Polemik über die gregorianische Kalenderreform” in Sitzungsber. der Phil. hist. Classe der kaiserlichen Akademie der Wissenschaften. Bd. LXXXVIII (Wien, 1877), 485 ff.

2. Schmidt, I., “Gregorianische Kalenderreform”. Historisches Jahrbuch III. (Münster, 1882; 1884), 388-415; 543-595; V. 52-87.

3. Stiewe, F., “Der Kalenderstreit des 16. Jahrhunderts”, in Abhandlungen der bayer. Akademie der Wissenschaften. Hist. (München, 1880), Classe XV. 3. Abteilung 1-98.

4. Maestlin, M., Ausführlicher Bericht von dem allgemynen Kalender oder Jahrrechnung wie sie erstlich angestellt worden und was Irthumb allgemächlich dryn seyen eingeschlichen; item ob und wie dieselbige zu verbessern weren; sampt Erklärung der newlichen aussgegangenen Reformation von Bapst Gregorio XIII. und was davon zu halten sey (Heidelberg, 1583).

5. Canones in Kalendarium Gregorianum perpetuum added to the Romani Kalendarii a Gregorio XIII restituti explicatio (Romae, 1595).

6. Calvisius, S., Elenchus Calendarii Gregoriani, in quo errores, qui passim in anni quantitate et Epactis commituntur, manifeste demonstrantur et dupplex Kalendarii melioris et expeditioris formula proponitur (Francofurti, 1612).

7. Viëta, F., Relatio Kalendarii vere Gregoriani ad ecclesiasticos doctores. Exhibita Pontifice Maximo Clementi VIII. Anno Christi 1600 Jubilaeo Parisii.

8. Germanus, G., Computus ecclesiasticus sive Kalendarium triplex, Gregorianum, Antiquum et Novum, cum vero cyclo lunari et refutatione quorundam insignium errorum Christophori Clavii (Francofurtii Marchiorum, 1606).

9. Cuno, J., Theses de Calendario Juliano, quo ecclesia nunc utitur viciato, ejusdemque restitutione (Francofurti Marchiorum, 1583).

10. Bedenken, ob der Newe Bäpstliche Kalender eine Nothdurfft by der Christenheit seye unnd wie trewlich dieser Bapst Gregorius XIII. die Sachen damit meyne: ob der Bapst Macht habe, diesen Kalender der Christenheit auffzudringen. Ob auch fromme und rechte Christen schüldig seyn, denselben anzunehmen (Tubingae, 1583), Pars II.

11. Luther, M., “Von den Konziliis und Kirchen”, in Martin Luther. Ausgewählte Werke. III. A. Ergänzungsreihe Bd. 7 Hrsg. von H. H. Borchardt und G. Merz (München, 1963), 50 (559).

12. Heerbrandius, J., Disputatio de Adiaphoris et calendario Gregoriano (Tubingae, 1584).

13. Confessio Augustana 1540, Critical edition by Tschackert: Die unveränderte Augsburgische Konfession deutsch und lateinisch, nach den besten Handschriften aus dem Besitze der Unterzeichner (Leipzig, 1901), Article XXVIII.

14. Paul of Middleburg, Paulina, sive de reca Paschae celebratione et de die passionis domini nostri Jesu Christi (Forosempronii, 1513).

    Although he had admitted in the 14th book the right of the Pope to declare Easter an immovable feast, he thought it not adviseable because of the fact that the Montanist celebrate Easter on a fixed Sunday and also because an eclipse could occur on Good Friday.

15. Breviarium Romanum ex decreto concilii Tridentini restitutum Pii V iussu editum (Romae, 1568).

16. Scaliger, J. J., “Hippolyti Episcopi Canon Paschalis cum commentario et excerpta ex computo graeco Isacii Argyri de correctione Paschatis”, added to Elenchus et castigatio anni Gregoriani (Lugduni Batavorum, 1595).

17. Eusebius, Historia ecclesiae. VI, 22 Migne PG. XX, 1073 B - 1077 A; cf. E. Schwartz, “Christliche und Jüdische Ostertafeln”, in Göttinger Abhandlungen N.F. 8.6 (Berlin, 1905), 29-40 and Achelis, H., Das Christentum in den ersten drei Jahrhunderten (Leipzig, 1925), Plate 13.

18. Hippolytus, De Pascha, Homilia 6, Migne PG. 59, 743-746.

19. Gospel of John, I. 9.

20. Sphaera Ioannis de Sacrobosco emendata, aucta et illustrata (Coloniae, 1601).

21. “Lectio IV in II nocturno diei 9 Octobris SS. Dionysii Episcopi, Rustici et Eleutherii Martyrum”, in Breviarium Romanum, Ed. XI, Pars autumnalis (Turonibus, 1935).

22. Galilei, G., Lettera a Madama Cristina di Lorena. Granduchessa di Toscana (1615), Ed. Naz., V, 325.

23. Eusebius, Vita Constantini, III, 17-20, Migne PG. XX, 573 C - 575 A.

24. Luther, M., Op. cit. (Ret. no. 11), 46 (555) and 49 (558).

25. Scaliger, I.I., Elenchus et castigatio anni Gregoriani (Lugduni Batavorum, 1595).

26. Maestlin, M., Alterum Examen Novi Pontificalis Gregoriani Kalendarii quo ex ipsis fontibus demonstratur, quod novum Kalendarium omnibus satis partibus, quibus quam rectissime reformatus vel est vel esse putatur, multis modis mendosum et in ipsis fundamentis vitiosum sit (Tübingen, 1586).

27. Schulin, J., Entschuldigung und Ableinung wegen der Praefation oder Declaration den neuen päpstlichen Kalender betreffend, welche ohne sein wissen willen und meinung seinen Calendariis ist fürgesetzt worden (Tübingen, 1584).

28. Cf. C. F. D. Waren Bericht, “warumb das alt römisch Calender dieser Zeit notwendig ersehen und gebessert worden, wie im Nicenischen Concil vor 1255 Jahren auff begeren des grosszmächtigen Römischen Keysers Constantin Magni auch beschehen” (Mainz, 1584).

29. Cf. Cod. Vindob. 10711 fol. 71 f: Paulus Fabricius, “Kurzer … Bericht von dem julianisch-römischen Jar oder Calender … wider Michael Mästlin”.

30. Busaeus, J., De Calendario Gregoriano Disputatio apologetica. Doctoris Theologiae Disputationi Lutheranae Tübigensi opposita et in Academia Moguntia anno MDLXXX proposita (Mainz, 1585).

31. Rasch, D., Dernew Calender (München, 1586).

32. Possevinus, A., “De anni et paschae emendatione”, in Moscovia et alia opera de statu hujus saeculi adversus Catholicae ecclesiae hostes in officina Birkmannica (1587), Sect. IV.

33. Clavius, Chr., Novi Calendarii Romani Apologia adversus Michaelem Maestlinum Göppingensem in Tübingensi Academia Mathematicum tribus libris explicata (Romae, 1588).

34. Clavius, Chr., J. Scaligeri Elenchus et castigatio Kalendarii Gregoriani a Christophoro Clavio castigata (Romae, 1595).

35. Scaliger, J. J., Thesaurus temporum Eusebii Pamphili Caesareae Palestinae episcopi (Leodium, 1606).

36. Clavius, Chr., Responsio ad convicia et calumnias J. Scaligeri in Kalendarium Gregorianum (Mainz, 1609).

37. Martellus, H., De anni integra in integrum restitutione una cum apologia, quae est sacrorum temporum assertio (Florentiae, 1578).

38. Zarlinus, J., De vera anni forma sive de recta eius emendatione ad S. Gregorium XIII (Venetiis, 1580).

39. Plienninger, L. F., Kurtz Bedenken von der Emendation des Jahrs durch Bapst Gregorium XIII. fürgenommen etc. (Heidelberg, 1583).

40. Lilius, A., Kalendarium Gregorianum perpetuum cum privilegio summi Ponteficis et aliorum Principum (Romae, 1582).

41. Chemnitius, M., Bericht vom newen Päpstlichen Gregoriano Calendario an der Landgraffen zu Hessen, composed in 1582, printed in 1584 s.l.

42. Clavius, Chr., Romani Kalendarii a Gregorio XIII restituti explicatio (Romae, 1595).

43. Cod. Vindob., 10693 fol. 1r-23r; Paulus Fabricius, Annus et dies Jahr und Tag im alten und neuen Calender.

44. Graminaeus, Th., Exhortatio de exequenda Calendarii correctione (Dusseldorpii, 1583).

45. Romanus, A., Theoria Calendariorum (Wirceburgi, 1594); Supputatio ecclesiastica (Wirceburgi, 1595).

46. Moller, T., Widerlegung sambt eigentlicher Beschreibung der Restitution anni und Calendarii (Leipzig, 1583).

47. Cod. Vindob., 10686⁶⁶, fol. 77v and 84r: Tycho Brahe, Commercium epistolicum sive epistolae variorum uti … Johannis Maioris, Henrici Brucaei cum responsis Tychonis, a. 1572-1595.

48. Keplerus ad Maestlinum, in Kepleri Opera Omnia., Hrsg. von Chr. Frisch. VIII (1858-71), vol. IV., 6.

49. Kepler, J., “Ein Gespräch von der Reformation des alten Kalenders worauff die Correctio Gregoriana gegründet”, in Cod. Vindob., translated in to Latin by M. G. Hansch: “Liber singularis de Calendario Gregoriano sive de reformatione Calendarii Juliani necessaria et de fundamentis atque ratione correctionis Gregorianae e manuscripto editus Francofurti et Lipsiae apud D.R. Merz et J.J. Mayer 1726”.

50. Kepler, J., “Tabulae Rudolphinae” (Ulmae, 1627), in Johannes Kepler. Gesammelte Werke., Hrsg. von W. v. Dyck, M. Caspar u. F. Hammer (München, 1937 ff.).

51. See, for instance, the following: Hornstein, J., Reformierter Reichskalender oder ein neues kurzweiliges Gespräch vom alten und neuen Kalender (Ingolstadt, 1596); Kepler, J., Ad epistolam Calvisii responsio (Francofurti, 1614); Guldin, P., Refutatio Elenchi Calendario Gregoriani a Calvisio conscripti (Moguntiae, 1616); Petavius, D., Doctrina temporum (Parisii, 1627); Doglioni, G.N., L'anno riformato dove … può ciascuno facillimamente imparare tutto quello che più è utile e necessario, si d'intorno le cose del mondo celeste ed elementare come d'intorno quelle de' tempi e del calendario (Veneczia, 1599); Jun(i)us, U., Bedenken über die von Weigelio dem Reichs-Collegio zu Regensburg proponirte Calender Vereinigung and Anmerkungen über das unpartheyische Bedenken von Prof. Weigelii proponirter Calender-Concialiation, nebst eine Beantwortung derselben (Leipzig, 1699).

52. Clemens VIII, Breve dat. (Romae, 16/6 Martis 1603).

53. Bianchini, F., Solutio problematis Paschalis (Romae, 1703).