John Simonis of Selandia

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John Simonis of Selandia

(fl. France, fifteeth century)


It is still uncertain whether John Simonis of Selandia was Danish, as supposed by P. Lehmann, or Dutch, as maintained by G. Beaujouan. In fact, nothing is known of his life except that he was a doctor artium who in 1417 wrote his only known work at Vienne. It is entitled Speculum planetarum and has the incipit “Ad utilitatem communem studentium in astrologia et specialiter medicorum.” The treatise was quite well known, and both Regiomontanus and Arnald of Brussels copied it. At least twenty manuscripts are extant; besides those listed by E. Zinner there are Darmstadt 780; Paris, B. N. lat. 10226; Vatican Palat. 1340; Vatican lat. 5006; and Yale De Ricci Supp. 25. The text (an edition of which is in progress) describes the construction and use of an equatorium, or analogue computer for planetary longitudes, with several features that are new and interesting compared to the previous instruments of Campanus of Novara, Peter Nightingale, John of Lignè res, and Chaucer.

The new features are on that part of the instrument related to the motion of the sun. Here all earlier equatoria had been based on the theory of Hipparchus (no epicycle and an eccentric deferent); John of Salandia based his speculum on the equivalent theory of Apollonius (one epicycle and a concentric deferent). He represented all epicycles by imaginary circles produced by points on a ruler turning about the epicycle center and gave all the planets a deferent of the same size, produced by a knot on a thread revolving about a pin that was placed in holes on the mater (ground plate) of the instrument.

Engraved upon the mater was a system of graduated circles, the outermost of which represented the ecliptic of the sun and moon, as well as the equant circles of the five other planets. With regard to the latter the center of the mater accordingly represents all the equant centers, but not the center of the earth, as in previous instruments. This conception entails some curious consequences. First, each planet must have its individual “center of the earth” represented by a hole properly placed on the mater. Second, the instrument must be provided with five particular circles, each representing the ecliptic of a particular planet. Third, since all these ecliptics are concentric with the mater, they must be divided into 360 unequal degrees in such a way as to appear equal when seen from the corresponding “center of the earth.” This construction, highly ingenious and sophisticated, presumably was unique in the history of instrument making until this time; but the cumbersome division of scales into unequal degrees may well be the reason why no specimen of the actual instrument is known and why most of the manuscripts are without illustrations.


The following works may be consulted: G. Beaujouan, Manuscrits scientifiques mé dié vaux de l’ Université de Salemanque (Bordeaux, 1962), p. 166; Paul Lehmann, “Skandinaviens Anteil an der lateinischen Literatur des Mittelalters,” in Sitzungsberichte der Bayerishchen Akademie der Wissenschaften, Philosohisch- Historische Abteilung, 1936, 2 (Munich, 1936), 55; O. Pedersen, “Two Mediaeval Equatoria,” in Actes du XIe Congreè s international d’ histoire des sciences, III (Warsaw, 1968), 68- 72; E. Poulle, La bibliothè que scientifique d’ un imprimeur humaniste au XVe siè cle (Geneva, 1963), p. 64; and E. Zinner, Verzeichnis der astronomischen Handschriften. . . (Munich, 1925), nos. 9629- 9642; and Leben und Wirken des. . . Regiomontanus (Munich, 1938), p. 52.

Olaf Pedersen