John of Sicily

Background

John of Sicily was born c. 13th century. His name indicates that he was probably born in Sicily, Italy. Nothing more is known of his personal life.

Education

John probably studied somewhere in Italy or France, but no other information about his education is available.

Career

Nothing is known of John’s life except that he was part of the Paris scientific community at the end of the thirteenth century. His only extant work is a commentary on the "Quoniam cujusque actionis quan-titatem," Gerard of Cremona’s translation of the canons of the tables of al-Zarqal. This commentary is generally dated 1290 but is more probably from September 1291 (anno Domini 1290 completo), to judge from the numerous examples calculated for that date.

After citing the opening words of each chapter of the canons, John of Sicily very methodically summarizes its purpose, states its plan, the different sections of which are carefully indicated by appropriate citations, and then comments at length following the plan of the original. For the most part, these developments give John’s work the character of a treatise on trigonometry and planetary astronomy. In accordance with al-Zarqal’s canons, the commentary is divided into three parts.

First, it discusses what Gerard of Cremona called the “measure of time,” that is, everything concerning the divisions of the year in the lunar and solar calendars and converting each of the four standard calendars, that is, the Arab, Christian, Persian, and Greek, to the others. It should be noted that the version of the Toledan tables of which the treatise "Quoniam cujusque actionis" forms the canons has replaced the tables giving the equivalence between Christian years and Arab years with tables for reducing the years of all calendars to days expressed in sexagesimal numeration. The Alfonsine tables later adopted this principle and systematized its applications. The astronomy of the primum mobile, which constitutes the second part, includes the application of trigonometry to astronomy. Everything relating to the movements of the planets forms the third part.

Two types of star table are included in the Latin version of the Toledan tables. One contains forty stars with their ecliptic coordinates; the other, thirty-four stars with both their ecliptic and equatorial coordinates. This second table is declared verified for the Arab year 577 (A.D. 1181-1182). The canons do not specify to which type of star table they refer. Without indicating the number of stars, John of Sicily alluded to the double coordinates: he therefore had before him the table verified for A.H. 577.

Achievements

• John of Sicily went down in history as a noted astronomer and author. He is probably best known for his commentary on the canons of the tables of al-Zarqal.

Works

More photos

• book

  • A Catalogue of Incipits of Mediaeval Scientific Writings in Latin

    (The book contains John's commentary for the canons of al-...)

    1963

Views

The canons of al-Zarqal discuss the consequences of the motion of “accession and recession” only at the very end of the section on planetary motions: instead of considering the effect of the motion of the eighth sphere on the positions of the planetary auges with respect to the ninth sphere, the true places of the planets are determined with respect to the ecliptic of the eighth sphere, as if the auges were fixed, and the correction resulting from the motion of the eighth sphere is applied to get the final result. This procedure permits the calculator not to commit himself as to the motion he assigns to the eighth sphere until he reaches the very last step in the calculation.

In commenting on al-Zarqal, a partisan of “accession and recession,” John of Sicily recalled the various hypotheses that had been proposed: a simple movement of precession, estimated by Ptolemy at one degree in 100 years and by al-Battani at one degree in sixty-six years; a back-and-forth movement (one degree in eighty years with an amplitude of eight degrees) disproved by al-Battani; and the movement of “accession and recession” suggested by Thabit ibn Qurra. John of Sicily rejected the last of these movements, invoking arguments the origin of which P. Duhem traced to Roger Bacon. He adhered to precession as Ptolemy presented it, conceding, however, that its exact quantity was uncertain and would become determinable only by extensive observations.

References

• Dictionary of Scientific Biography, Volume 7
  1973