Fusoris, Jean

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(b. Giraumont, Ardennes, France, ca. 1365; d. 1436),


Fusoris is the only medieval maker of astronomical instruments about whom there is considerable information on both his life and his work. The son of a pewterer, he left his native region to study at the University of Paris, where he earned master’s degrees in arts and medicine and a bachelor’s degree in theology. Named successively canon of Rheims (1404), of Paris (1411), and of Nancy, and curate of Jouarre-en-Brie, Fusoris resided in Paris until 1416. He directed a large workshop for the manufacture of astronomical instruments, and his clients included the king of Aragon, the duke of Orléans, the antipope John XXIII, the king of Navarre, and the bishop of Norwich. His relations with the last, however, proved to be compromising when the Hundred Years’ War with England resumed in 1415. Fusoris was charged with high treason but, in the absence of conclusive evidence against him, he was only sentenced to exile in Mézières-sur-Meuse, where he had spent his youth. His name reappears in documents in 1423, at which time he had a commission from the chapter of Bourges to build an astronomical clock. He also established trigonometric tables for King Charles VII and, in 1432, wrote a treatise on cosmography for the chapter of Metz.

Much of Fusoris’s work has survived, and comprises both brass instruments and books. His workshop must have produced a great many astrolabes, for at least eighteen examples still exist—a truly unique case for western medieval astrolabes. None of them is signed, but all display a characteristic error: the presence of the star Cornu Arietis (β Arietis) in the southern hemisphere. Fourteen of these instruments have already been listed;1 others are at the Collegium Maius in Cracow, the Bayerisches Nationalmuseum in Munich (this instrument is from the Bassermann-Jordan collection),2 the Astronomisch-Physikalisches Kabinett in Kassel, and the Nordiska Museet in Stockholm. This list is probably not definitive and other astrolabes may be added to it.

Fusoris built at least one equatorium, which he sold for 400 écus to the bishop of Norwich. It consisted of several “instruments”-one for each planet—each of which bears two brass disks: the “mother” which is analogous to that of an astrolabe but without an armil, and the equant disk. The equant of the instrument for Mars has been found: it was reused as a clock face in the seventeenth century.3

Other instruments made in Fusoris’s workshop include dials, armillary spheres, and clocks; they seem not to have survived The astronomical clock made for the chapter of Bourges has been preserved, however, although in poor condition. Its representation of the motions of the sun and the moon in front of an astrolabic dial is excellent, for the relationships of the teeth of the gear train yield an astonishingly exact periodicity for the solar and lunar motions.

Fusoris’s written work includes texts in both Latin and French. Among the latter are a treatise on the uses of the astrolabe, dedicated to Pierre of Navarre, comte de Mortain, and a treatise on its construction, as well as one on cosmography written for the canons of Metz. Fusoris was perhaps also the author of an elementary treatise on arithmetic and geometry, preserved in two manuscripts. All these texts are pedagogical and without great originality, but they attest the author’s concern with presenting difficult material to the layman in a clear manner.

The Latin texts are much more technical. The treatise on the equatorium describes the construction and use of the instruments des sept planèts sold to the bishop of Norwich. The device is a geometric equatorium, similar in its operating principle to that of Campanus of Novara: each “instrument” reproduces very truly the geometric decomposition of the motion of one planet. In Fusoris’s device, however, the graduations of the equant and epicycle in degrees are replaced by a movable chronological graduation.

Toward the end of his life, Fusoris undertook to reconstruct the astronomical tables, but he was able to execute only the trigonometric part of his project, which he dedicated to King Charles VII. The establishment of his trigonometric table, which contains only sines and chords, was based on the theorem that every chord subtended by a median arc of two arcs of which the chords are known is equal to the product of half the sum of these chords and a coefficient that depends only on the difference of the two arcs and not on the arcs themselves. The chords and, as a result, the sines were therefore calculated gradually, starting from the chords of certain noticeable arcs. The smallest arc to which this procedure could be applied in order to obtain its trigonometric lines is one of 45’ the lines of the arcs of 15’ and 30’ were simply taken to be equal, respectively, to one-third and two-thirds of those of 45’. In accordance with the Ptolemaic tradition, the quantities were expressed as sexagesimal fractions of the radius, with a precision, depending on the angles, of the third, the fourth, or the sixth sexagesimal division of the sixtieth of the radius.

Still surviving are Fusoris’s notes on gnomonics, concerning the construction of the traveler’s clock and the tracing of horizontal sundials for the latitude of Paris. Also preserved are Fusoris’s annotated drawings of an astronomical clock that, according to his pupil Henri Arnaut of Zwolle, was built for the duke of Burgundy.

Fusoris appears to have been a highly skilled practitioner of the astronomy of his time, and he provides excellent testimony to the solid training in science given by the University of Paris at the end of the fourteenth century.


1. E. Poulle, Un constructed d’instruments astronmiques, 20-21. The astrolabe in the Whitney Warren collection (Gunther’s no. I93B) is no longer at the Smithsonian Institution; the astrolabe of the Conservatoire des Arts et Métiers, Paris, is not from the collections of the Écoie d’Horlogerie, but is a gift from Gaston Briere, who obtained it from Vallet de Viriville..

2. This instrument bears a date, 1447, which seems to have been added subsequently.

3. A. Simoni. “Un fortunato rinvenimcnto,” in La vlessidra, 27. no.9(I97I), 25-27.


The documents relating to Fusoris’s trial were published, with a long intro., by L. Mirot as “Le procès de maitre Jean Fusoris, chanoine de Notre-Dame de Paris, 1415-1416: épisode des négociations franco-anglaises durant la guerre de Cent ans,” in Mémoires de la Société de histoire de Paris et de l’lle-de-France27 (1900), 137-287. Fusons’s scientific work is set forth and commented on in E. Poulle, Un constructeurd’instruments astronomiques au XVe siècle, Jean Fusoris (Paris, 1963; = Bibliothéque de l’École Pratique des Hautes Études, IVe section, fasc. 318); the book has editions of the treatises on the uses of the astrolabe and on its construction, as well as of the treatise on the equatorium, of the notes on gnomonics and of the intro. to the astronomical tables.

The following references supplement those given by Poulle. There is an Italian trans, of the treatise on the use of the astrolabe: Vatican Reg. 1732, fols. 44-59v. Fusoris’s star table-which is part of the treatise on the construction of the astrolabe but can be found separately in Salamanca 2621 (see Poulle, op. cit., 17- 18) — is also in Wolfenbuttel 2816, fol. 139, where there is the same reference to Fusoris and to the year 1428. Besides the two Paris MSS (Poulle, op, c/V., 42, 125), the treatise on the equatorium exists in Florence Magi. XX. 53, fols. I v-34 (the beginning of the first chapter is missing).

An ed. of Fusoris’s treatise on cosmography (preserved in Paris MS fr. 9558, fols. 7-20) has been prepared by L.-O. GruGalerkin, Boris Grigorievndt as a doctoral dissertation for the University of Bergen, but it has not been published.

Emmanuel Poulle