**Johannes Campanus** was chaplain to three popes, Pope Urban IV, Pope Nicholas IV and Pope Boniface VIII. In fact he knew Jacques Panteléon, patriarch of Jerusalem 1255-1261, during the years when he was patriarch. Panteléon was elected pope on 29 August 1261, becoming Pope Urban IV, and he made Campanus one of his chaplains He also made him Rector of the Church of Savines in the diocese of Arles in 1263 and gave him a canonicate in Toledo's cathedral in 1264. Campanus also held other benefices being chaplain during 1263-64 to Ottobono Fieschi, the cardinal deacon of Saint Adrian's and papal legate to England. Fieschi was elected pope, becoming Pope Adrian V, on 11 July 1276 but died less than five weeks later. Through Fieschi's influence, Campanus was made parson of Felmersham in Bedfordshire, England, some time before 1268. Urban IV died on 2 October 1264 but Campanus had other leading ecclesiastical figures as his patron so continued to be able to devote his time to scholarship.

One should not think that because Campanus held benefices in France, Spain and England, he must have travelled widely. In fact, although he may have visited these countries, there is no evidence that he did so. It was certainly common practice at this time for leading ecclesiastical figures to give benefices to support their friends who never went in person to the places where they held the benefices. Roger Bacon, writing in 1267, stated that Campanus was one of the four greatest contemporary mathematicians but, despite some historians believing that Bacon knew Campanus personally, this looks very unlikely. If Campanus did indeed visit Ottobono Fieschi while he was in England then a meeting with Roger Bacon would be quite likely but, as we have just explained, this probably did not happen.

Campanus later held a canonicate of Paris and served as papal chaplain to the two more popes, Nicholas IV (1288-92) and Boniface VIII (1294-1303). He lived in the Augustinian Friars' convent at Viterbo for the last few years of his life and in his will he gave instructions for the construction of a chapel to St Anne in the Church of the Holy Trinity in Viterbo. Campanus wrote a Latin edition of Euclid's *Elements* in 15 books around 1260 and it was the standard Euclid for 200 years. In an edition of this book published in 1531 he is described as 'Magistro Campano' which may indicate that he taught in a university, but there is no other evidence to support this. The date we have given for his death is a guess. It is based on the fact that he drew up his will on 9 September 1296 and on 17 September 1296 Pope Boniface VIII wrote a letter saying that Campanus had just died. His death must, therefore fall between these dates and we have simply taken the midpoint to minimise any error.

Most of Campanus's writings were on astronomy. He wrote *Theorica Planetarum* which, since it is dedicated to Urban IV, must have been written between 1261 and 1264. In this work Campanus describes the construction and use of an equatorium, the first to be described by a European. This instrument was basically a mechanical calculator to give the planetary motions. The instrument was made of disks whose rotation recreated the motion of a planet using Ptolemy's deferent and epicycle description. As described by Campanus, the construction is not practical and it is unlikely that Campanus would have made an equatorium himself despite writing in his dedication to Urban IV that the instrument [2]:-

... is a novelty among my discoveries, and furthermore I think it will please your greatness by the usefulness of its amusing approach.

The work also contains detailed descriptions of Ptolemy's view of the universe giving the longitude of the planets as well as a geometrical description of the motion of the model. Campanus, using Ptolemy's data, calculated the dimensions of the universe including a calculation of the area of the sphere of the fixed stars. Another astronomical work by Campanus is *Computus maior* which was a treatise on time-reckoning based on solar and lunar cycles. In this work, among many computational descriptions, he provided details of how to calculate the date of Easter. Campanus also wrote the important text *Tractatus de Sphaera* which was an elementary astronomy book describing celestial phenomena seen during the 24 hour rotation of the heavens. He describes planetary motion in some detail, which is unusual for an elementary text at that time. Since the *Tractatus de Sphaera* refers to the *Theorica Planetarum* and the *Computus maior* it must have been written after these other two texts.

Other astronomical work included a text on using a quadrant and a text on using an astrolabe. Campanus also published his own set of astronomical tables. Data for the planets was taken from the *Almagest* Ⓣ and from the Toledan Tables edited by the Arabian astronomer Azarquiel in 1080, based on his own work and that of al-Khwarizmi and al-Battani. Campanus determined the time of each planet's retrograde motion and gave precise instructions on using the tables. He also made calculations in great detail of the distances to the planets and their sizes. In [9] Katherine Tredwell gives this summary of Campanus's astronomical contribution:-

As an astronomer, Campanus was not innovative in the modern sense. His importance lies rather in his ability to understand and utilise the recently rediscovered astronomy of the ancients. ... his works helped to establish the foundation on which later generations of astronomers would build their critical re-examination of Ptolemy.

Let us now look a bit further at Campanus's mathematical contributions. Most significant of these is his Latin edition of Euclid's *Elements* which we have already mentioned above. We have given the date of this work as 1260 and this was worked out by Charles Sanders Peirce (see [15]) from the fact that the manuscript text owned by Jacques Panteléon while still patriarch of Jerusalem is still extant. Peirce writes [15]:-

... we have in the commentary by Campanus, considering its respectable strength, occasionally its remarkable strength, additional evidence of the promising beginning of science which was made in the thirteenth century...

The edition by Campanus relied to some extent on Adelard of Bath's Latin translations of Euclid's *Elements* from Arabic sources. Another Latin Euclid by Robert of Chester was a compilation of writings by commentators on Euclid and did not contain a translation of the text as such. Campanus, however, borrowed most of his definitions, axioms, postulates and enunciations from this work by Robert of Chester. However, in Book VII, Campanus added definitions, postulates and axioms which he took from *De elementis arithmeticae artis* by Jordanus Nemorarius. Campanus shows that he is familiar with the work of Arab mathematicians and, in Book V, he quotes from Gherard of Cremona's Latin translation of Ahmed ibn Yusuf's treatise on ratio and proportion. In 1485 Campanus's Euclid became the first printed edition of Euclid's *Elements* when it appeared in Venice, and it was in fact the first printed mathematical book of any importance. Between its first printing in 1485 and 1560 fourteen editions of Campanus's text were printed.

Two further mathematical texts are thought with a high degree of certainty to be by Campanus, although a slight doubt remains. These are *De figura sectore* and *De proportione* which clearly form two parts of a whole and are based on the work *De figura* by Thabit ibn Qurra. We explained above that Campanus borrowed material from *De elementis arithmeticae artis* by Jordanus Nemorarius. It is fair to note that he also added to the work, for he produced a manuscript which contains three additional propositions. Other mathematical work has been attributed to Campanus, but for most the attribution must be considered rather speculative.

Finally we should mentions Campanus's contributions to astrology. His contribution here was a major one and most people today who have heard of Campanus know of him because of his 'house system' of dividing the horoscope into twelve houses. In fact it was almost certainly his reputation as an astrologer which led to his being chaplain to three popes. Certainly Urban IV made Campanus his chaplain so that he could advise him on astrological and medical matters. Much of his astrological knowledge came from the treatises written in Arabic by ibn Ezra around the middle of the 12^{th} century. However, Campanus used his mathematical skills to develop his 'house system' which is still used by astrologers today. Campanus's house system provided a method of determining a plane through the celestial sphere which could be used to make twelve equal divisions called houses. The plane he chose was the prime vertical, that is the plane through the zenith intersecting the horizon at its east and west points.

Toomer writes [1]:-

Campanus was a writer of considerable influence. His Euclid was almost the canonical version until the sixteenth century, when it was gradually superseded by translations made directly from the Greek. The continuing popularity of his 'Computus' and 'Sphere' is attested by their being printed several times in the sixteenth century. The 'Theorica' was never printed, probably because, unlike the others, it was not a popular work but a technical one that would appeal only to those with a professional interest. It was none the less influential: this is shown both by the large number of surviving manuscripts and by the references to it in astronomical works of the fourteenth and fifteenth centuries.

Note that although the *Theorica* had not been printed when Toomer wrote these words, shortly after this he, along with F S Benjamin, edited and published the work, see [2].

**Article by:** *J J O'Connor* and *E F Robertson*