William of Saint-Cloud

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(fl. France, end of the thirteenth century)


All of the very little that is known about William of Saint-Cloud comes from his own writings. The earliest recorded date of his activity is 1285, when he observed a conjunction of Saturn and Jupiter (28 December), an event to which he alludes in his Almanach. He was undoubtedly well received in French court circles. for his calendar is dedicated to Queen Marie of Brabant, widow of Philip III (The Bold); and he translated it into French at the request of Jeanne of Navarre, wife of Philip IV (The Fair).1 There is no substantial evidence to support the hypothesis that William of Saint-Cloud is identical with a certain Simon of Saint-Cloud, canon of Meaux and a steward of the queen. Simon is mentioned several times in archival documents. Nor are there grounds for calling William of Saint-Cloud by the name of Lefebvre.2

William of Saint-Cloud’s known works are devoted entirely to astronomy. The treatise on the Directorium, or “adrescoir,” is the oldest of his preserved writings; it is referred to in the Calendrier de la reine. The instrument described in it is a magnetic compass with a graduation in unequal hours: it is provided with a table for computing the duration of diurnal arcs.

The text accompanying Queen Marie’s calendar deals with problems relating to the daily movement of the sun and to the astronomy of the primum mobile (inequality of days and nights according to the season and the geographic latitude; division of the inhabited world into “climats”), as well as to the nineteen-year lunar cycle. Beyond the information usually found in such works (the number of the decemnovennial cycle coordinated with the day of the month in which te new moon occurs during the year designated by this number; ferial letters; saints’ days; entry of the sun into the signs of the zodiac), the calendar also furnishes more technical data: the height of the sun at noon, duration of diurnal and nocturnal arcs, and hours of the new moon. The most notable aspect of this work is William’s firm resolve to establish his calendar on a purely astronomical basis. As a result, he contradicted the ecclesiastical calendrical computation, emphasizing its inadequacy and errors. For example, he presented–along with the traditional decemnovennial cycle (which he was still obliged to give since the rules of the ecclesiastical computation had not been abrogated)–another cycle that conformed to the scientific data of the astronomical tables and that he designated by the letters from a to t. Toward the same end, he appended a table that permits the user to make corrections–beyond the first solar cycle of four years– in the dates of the entry of the sun into the signs of the zodiac. The base year of the calendar is 1292 (not, as P. Duhem stated, 1296).

The starting point of William’s Almanach is also the year 1292. The purpose of this work was to furnish the effective positions of the planets, in contradistinction to the astronomical tables which gave only the elements for computing these positions. The introductory text is neither a theory of the planets, nor, properly speaking, canons (which, moreover, are scarcely necessary in an almanac), but rather an account of the observations and considerations on which the book is based. William takes this opportunity to point out the errors he has detected in the astronomical tables he used and to show how he has corrected them. The tables that he subjects to this criticism are those of Toledo (used in the Arab calendar) and of Toulouse (in which the preceding tables are applied to the Christian calendar). Nevertheless, in the end he adopted the tables of Toulouse, although not without making slightly arbitrary corrections in some of the mean movements listed in them.

In discussing the movement of the eighth sphere, William verified by calculation and observation the value he assigned to it for the year 1290. Noting that this value differs by nearly a degree from the one that would result from applying Thā ibn Qurra’s theory of accession and recession, he rejected the latter and opted for the Ptolemaic theory of simple precession. He recorded, however, the different values that astronomers assigned to the amplitude of that precession.

The Almanach gives the daily planetary positions for a period of twenty years beginning with 1292. For the sun and the moon, all the positions are accurate to within a minute. This same degree of accuracy is achieved for the positions of the outer planets every ten days and for the positions of the inner planets every five days. The latitude of the moon is also given.

William’s Almanach makes no reference to the Alphonsine tables. Yet Duhem, on the basis of a text printed with the works of Nicolas of Cusa, which he attributed to William, concluded that the latter was among the first, if not the first, in Paris, to know of and use the Alphonsine tables. (William supposedly learned about them a few years after composing his Almanach.) Nothing is less certain. Duhem’s analysis of this short text is insufficiently critical. Moreover, it seems certain that the Alphonsine tables were not introduced into Parisian astronomy before 13203, after which date it is not certain that William of Saint-Cloud was still alive.

The works of William had only a limited diffusion, undoubtedly because they were based on astronomical data (the tables of Toledo and Toulouse) that was soon superseded by the Alphonsine tables. Nonetheless, William holds a very significant place in the history of medieval astronomy. The many observations he made and the conclusions he drew from them, together with his criticism of the available tabular material, make him a genuine precursor, perhaps even the chief inspiration, of the Parisian astronomers of the first half of the fourteenth century.


1. E. Zinner, Verzeichnis der astronomischen Handschriften des deutschen Kulturgebiets (Munich, 1925), p. 413, mistakenly speaks of a Queen Elisabeth. Further, Zinner (ibid., nos. 2578–2594) confused William of Saint-Cloud with William the Englishman, the Marseilles author of De urina non visa and of several other astronomical texts, who lived in the first half of the thirteenth century.

2. A. d’Avezac, “Note sur Guillaume de Saint-Cloud,” in Mémoires de l’Institut national de France, Académie des inscriptions et belles-letters,29, pt. 1 (1877), 8–10, and in Acedémie des inscriptions et bells-letters, comptes-rendus des sé 1869, 29–31.

3. See the article on John of Murs, in Dictionary of Scientific Biography. VII.


I. Original Works. The treatises of William of Saint-Cloud are all unpublished. The MS of the directorium or “adrescoir” (“Presens ingenium directivum (sic) vocitavi . . .”) is Paris Arsenal 1037, fol. 7v–8v, together with the French trans. (“Très haute dame ci sont les proufis que l’en puet avoir . . .”). The Latin text is in verse. Another very summary French version, without the table of hours, can also be found in Paris Arsenal 2872, fol. 21v.

The calendar in Latin (“Testante Vegecio in libro suo de re militari . . .”) is in: Florence Laur. XXX.24, fol 99–109v and 110–123: Paris Arsenal 534, fol. 91–106: Paris lat.7281, fol. 145–148 (text only); Paris lat. 15171, fol. 88–101. Calendar in French (“Si come Vegeces tesmoigne en son livre . . .) in : Paris Arsenal 2872, fol. 1–21. Another French version, with a reduced commentary, is in Rennes 593, fol. 1–7

The MSS of the almanac (“Cum intentio mea sit componere almanac . . .”) contain either the text only or the tables only; the text is in: Cues 215, fol, 24–31v; paris lat. 7281, fol. 141–144v; Paris nouv. acq. lat. 1242, fol 41–44; summary note in Cues 212, fol. 405–406; the tables are in: Vatican lat. 4572: Paris lat. 16210.

The MS Erfurt 4° 355, fol. 44v preserves a star table “verified at Saint-Cloud near Paris in 1294” that may well be by William.

II. Secondary Literature. On William and his work, see E. Littré, in Histoire littéraire de la France, XXV (Paris, 1869), 63–74; P. Duhem, Le système du monde, IV (Paris, 1916), 10–24.

Emmanuel Poulle

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William of Saint-Cloud

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