Romani Calendarii a Gregorio XIII. P.M. restituiti explicatio, Clementis VIII P.M. jussu edita. Accessit confutatio eorum, qui Calendario aliter instaurandum contenderunt.

Rome: Luigi Zanetti, 1603.

First edition, the dedication copy, in a magnificent contemporary red morocco binding by the Soresini workshop, of the most important work on the construction and accuracy of the Gregorian calendar. “Sometime between 1572 and 1575, Pope Gregory XIII convened a commission to make recommendations on the reform of the Julian calendar, and the young Clavius was tapped to serve as the commission’s technical expert” (DSB). The result was the calendar in general use today, the Gregorian Civil Calendar, introduced by Pope Gregory XIII in 1582 in the bull ‘Inter gravissimas,’ and gradually adopted by most of the nations of the world in the course of the following centuries. It was actually devised by one Aloisius Lilius, a lecturer in medicine at the University of Perugia, and presented in summary form in a treatise entitled Compendium novae rationis restituendi Kalendarii (Rome, 1577), which also contained the plan adopted for the reform of the ecclesiastical calendar. The problem addressed by the commission was that the Julian calendar in use since Roman times had run out of step with the astronomical data in two ways. In its solar part, it had accumulated an error of ten days; the true average vernal equinox fell on March 11 rather than March 21 as the calendar assumed. This was corrected by omitting the ten calendar days October 5 through October 14, 1582. In its lunar part, the Julian calendar was wrong by three days; the true average age of the moon (the number of days elapsed since the last new moon) was three days larger than its calendar prediction. This was corrected by a sudden increase of the epact (the age of the Moon on 1 January) by three. After that one-time correction, a new algorithm was put in effect which is about twenty times more accurate in its solar part, and four times more accurate in its lunar part. The new calendar met with considerable opposition from mathematicians such as Maestlin, Scaliger and Viète, but within a year the change had been adopted by the Italian states, Portugal, Spain, and the German Catholic states. Gradually, other nations adopted the Gregorian calendar: the Protestant German states in 1699; England and its colonies in 1752; Sweden in 1753; Japan in 1873; China in 1912; the Soviet Union in 1918; and Greece in 1923. “The fundamental exposition of the Gregorian Calendar, civil and ecclesiastical, is Clavius, Explicatio Romani calendarii a Gregorio XIII P.M. restituti (Romae, 1603), reprinted in Clavius, Opera mathematica, V (Moguntiae, 1612). Clavius considers the arrangement and accuracy of the calendar in relation to current solar and lunar theory” (Swrdlow, p. 49). Although reasonably well represented in institutional collections, this work is rare on the market in any condition – ABPC/RBH list only three copies.

Provenance: Pope Clement VIII (1536-1605), the dedicatee (his coat of arms stamped in gilt on covers, ink monogramme on recto and verso of title).

In the Julian Calendar, named after Julius Caesar, every fourth year had 366 days rather than 365. Roman astronomers calculated that a year — the time it takes the Earth to revolve around the sun — had a duration of 365.25 days. This method of adding a "leap day" every fourth year averaged out to this determined value. “The Julian Calendar was naturally adopted by the successor of the Roman Empire, Christian Europe with the Papacy at its head. By about 700 CE it had become customary to count years from the starting point of the birth of Christ (later corrected by Johannes Kepler to 4 BCE). But the equinox kept slipping backwards on the calendar one full day every 130 years. By 1500 the vernal equinox fell on the 10th or 11th of March and the autumnal equinox on the 13th or 14th of September, and the situation was increasingly seen as a scandal. The most important feast day on the Christian calendar is Easter, when the suffering, death, and resurrection of Christ are celebrated. In the New Testament we find that Christ's crucifixion occurred in the week of Passover. On the Jewish calendar, Passover was celebrated at the full moon of the first month (Nissan) of spring. In developing their own calendar (4th century CE), Christians put Easter on the first Sunday after the first full moon after the spring equinox. If the equinox was wrong, then Easter was celebrated on the wrong day. Most other Christian observances (e.g., the beginning of Lent, Pentecost) are reckoned backward or forward from the date of Easter. An error in the equinox thus introduced numerous errors in the entire religious calendar. Something had to be done. After the unification of the Papacy in Rome, in the fifteenth century, Popes began to consider calendar reform. After several false starts, a commission under the leadership of the Jesuit mathematician and astronomer Christoph Clavius (1537-1612) succeeded” (Galileo Project).

“The principal architect of the Gregorian calendar was a physician and lecturer at the University of Pe­rugia, Aloysius Lilius (Lilio, Giglio) of Calabria (1510?­-1576) … After the death of Lilius, his brother and (partial) collaborator, Antonius, a personal physician of Gregory XIII, in 1577 presented Aloysius’s manuscript to a commission appointed by Gregory XIII to consider the reformation of the calendar. After some delays, a summary of the manuscript was prepared by an astronomer, Pedronus Chaconus of Toledo, perhaps initially for the use of the Calendrical Commission. The summary, Compendium novae rationis restituendi Kalendarii, was reprinted in the [present work]. Copies of the summary were sent by the order of Gregory to Cath­olic princes and universities in early 1578 to obtain support for the projected reform and invite comments. (Copies of the original Compendium were found a few years ago by Gordon Moyer and Th. B. Settle in Italian libraries in Florence, Rome, and Siena.) A number of comments and suggestions, some contradictory, were received and evaluated by the Commission.

“After much internal discussion of many different views, the Commission settled on the plan originally suggested by Lilius, with some modifications. The Commission had been charged with two basic tasks: to determine a method for computing the date of Easter, and to stabilize the vernal equinoctial drift in the cal­endar. An implicit assumption was that such changes as would be made would be in accord with the religious traditions and customs of the Roman church. Moreover, as the Jesuit Christoph Clavius of Bamberg was later to emphasize in his reply to the objections to the Gregorian calendar by Michael Maestlin of Tü­bingen, a teacher of Johann Kepler, the rules for the calendar finally adopted had to be convenient and rea­sonably simple – accordance to (existing) astronomical information was secondary.

“For the calendar, Lilius suggested a plan equivalent to that previously put forward by Petrus Pitatus [Vera Solaris atque Lunaris Anni Quantitas Aliarque Rerum, Basel, 1568], also based on the Alfonsine year. While Pitatus had left open the question of the dates of the vernal and autumnal equinoxes, Lilius suggested March 21 for the date of the vernal equinox – (nominally) that of the Council of Nicaea in 325 – and offered two alternatives to com­pensate for the equinoctial drift. One of these – the omission of 10 days in a single year – was accepted. This, and the change in the Julian four-year cycle of leap years to a modification of the Pitatus's scheme so that the 400-year cycle would begin with 1600 as a leap year, constitute the (present) Gregorian calendar.

“Lilius’s innovation was in the revision of the method of computing new moons so that the lunar calendar agreed more closely with the proposed solar calendar. This was the use of the epact, the age of the moon (in days) on January I, reckoned from the last new moon of the preceding year. This linked the two calendars and en­abled the date of the last full moon preceding Easter of the current year to be computed. Tables of epacts to cover the 30 possible ages of the moon on January 1 were included in the Compendium, as was a con­cordance, in tabular form, between the epact and the ‘Golden Numbers’ previously used in the 19-year cycle (equal to [year/19] + 1). (One explanation of this name is that in illuminated medieval manuscripts these numbers were frequently written with gold ink.) An extended table of epacts in the form finally adopted by the Commission was given by Clavius [in the present work].

“The Commission finally issued a report to the Pope (Cod. Vatic. 3685), about the beginning of 1581. The plan that Lilius had proposed was approved in a somewhat modified form. In regard to the Julian calendar, it was decided to omit 10 days in October and thenceforth three leap years in 400, as had been pro­posed. After further delay, the Papal Bull, Inter Gravis­simas, was issued dated February 24, 1581/2 (repro­duced in [the present work]) and was accompanied by a set of rules, the ‘Canones in Kalendarium Gregorianum per­petuum,’ which is the present ecclesiastical calendar of the Roman church. No officially sanctioned explana­tion of the new calendar was made until the appear­ance of the massive tome of Clavius in 1603. It was based on an earlier work of Clavius [Novi Calendarii Romani Apologia Ad­versus Michaelem Maestlinum] of 1588, which was a reply to the attack of Michael Maestlin on the new calendar.

“In the literature, the influence of Nicolaus Coper­nicus on the final form of the Gregorian revision is fre­quently discussed. Pitatus does mention Coper­nicus, but only in the sense that some observations of Copernicus and others confirm his own choice of the Alfonsine Tables for his basic astronomical data. In the Compendium, there is no mention of Copernicus – only the Alfonsine year length is mentioned as a mean among several, as having lesser error. Finally, and most important of all the evidence before 1582, is the official report of the Calendrical Commission (of which Clavius was the most active member). The report states that as of September 8, 1580, the Commission had reached a consensus to the effect that the Alfonsine year value, taken as 365d 5h 49m, etc., was assumed to be be­tween the maximum and minimum, so that the vernal equinox is anticipated (in the calendar) by one day in about 134 years, which becomes three days in 400 years. In regard to the lunar-calendar aspect of Lilius's original plan, Clavius states that the Pruden­tine Tables (of Erasmus Reinhold, a disciple of Coper­nicus) were used in preference to the Alfonsine Tables.

“After the introduction of the new calendar, considerable partisan controversy concerning it arose. It lasted in some countries for centuries, but is now principally only of historical in­terest. People gradually came to accept that the Grego­rian calendar was much more accurate, and thus ulti­mately more useful, than the Julian calendar it had re­placed. The error in the Gregorian calendar is less than one day in 3000 years, and Vatican astronomers, such as J. de Fort, occasionally reexamine the question in the light of more recent astronomical information, such as the slowing down of the earth's rotational rate by about ½ second per century” (Dutka, pp. 60-62).

The present binding was executed by the Soresini workshop – it shows the typical Soresini tools of winged mermaids with curled tails at angles and in the shield. The workshop was founded in 1570 by Francesco Soresini, who was appointed Papal binder in 1575 on the death of Niccolò Franzese. Francesco was the founder of a celebrated dynasty of binders (Francesco, Prospero, and Baldassarre were his heirs). Soresini counted among his clients popes, cardinals, generals of religious orders, and members of the most distinguished Roman families. Baldassarre Soresini kept the workshop in business for at least a half century until as late as 1630.

BL/STC 17th-century Italian I, p.240; Honeyman 717. Dutka, ‘On the Gregorian revision of the Julian calendar,’ The Mathematical Intelligencer 10 (1988), pp. 56-64. Swerdlow, ‘The origin of the Gregorian civil calendar,’ Journal for the History of Astronomy 5 (1974), pp. 48-49.



Folio (356 x 238 mm), pp. [xxxv], [1, blank], 680, [2]. Arms of Clement VIII on title, letterpress tables, some with border of printer’s flowers, xylographic ornaments. Contemporary red morocco by the Soresini workshop for Clement VIII, spine and covers richly gilt with the arms of the dedicatee on both covers.

Item #5124

Price: $50,000.00