Johannes Simonis de Selandia

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by Fritz S. Pedersen

John Simonis of Selandia is the author of a tract composed in 1418, concerning the Speculum Planetarum, an intelligently devised instrument for computing the places of the planets at a given time. His apparent place of origin may be either Zealand in Denmark or Zeeland in the Netherlands, and there is little or no extra evidence to decide between these.


The following records may concern John: In 1403, one Johannes de Zelanda, a clerk of the diocese of Liège and a master in arts, had been studying medicine in Paris for three years (DENIFLE & CHATELAIN 1897, 93). In 1409–1410, one Johannes Symonis was a scholar of the medical faculty in Paris (WICKERSHEIMER 1915, 71). The Coeli Enarrant attributed to J. GANIVET (fl. 1431) contains an astrological interrogation by magister Ioannes Symonis, magister in artibus et licentiatus in medicina, concerning whether there was to be a papal election at the council of Constance (THORNDIKE 1934, 139; JACQUART 1979, 182). This contains a “figura coeli super Viennens.”, i.e., a horoscope diagram for “Viennensis...”, made for 17 August 1417, 6 p.m. The author is surely the same as John, since both the time and the location fits almost perfectly with the Speculum Planetarum. The houses in the horoscope diagram (e.g., ascendent = first degree of Pisces; fourth house = sixteenth degree of Gemini) seem to allow for a geographical latitude of 42°–47° and most probably 45° or a little less. Thus at least “Viennens.” is unlikely to denote Vienna in Austria (latitude, ca. 48°), whereas, e.g., Vienne (Isère; latitude, ca. 45½°; cf. above) is quite possible.

In short, John is likely to have been a Paris scholar in arts and medicine, practicing astrology as was usual among physicians. The Speculum Planetarum should in itself be seen as a tool for the medical astrologer to help cast horoscopes; indeed, the preamble to the tract states that it is especially for the use of physicians. No actual specimen of the instrument has been preserved, but diagrams of it exist in a few manuscripts, and it is easily reconstructed from the text.


Date and Place

The tract has been examined by PEDERSEN 1968 and very thoroughly by POULLE 1980; most of the following is based on the latter work, and detailed references are not given. The text exists in three versions, for which see below. Two of these exhibit the author’s name, and state that the tract was composed after 1417 years were elapsed since the birth of Christ, in Vienna [in Italia [ducalis dominii Subaudie]]; this location is found in various expansions as indicated. The dating is confirmed from a set of planetary apogee values in one version of the tract, which are derived from the Alfonsine Tables and are valid for the start of 1418. The stated location is doubtful; the latest guess at a place in Savoy is Avigliana in Piemonte (POULLE 1980, 170–71 n. 35). Otherwise, “Italy” and “Savoy” are scribe’s guesses even if persistent in the manuscripts, and the place could be Vienne (Isère) as guessed by THORNDIKE 1934; this, at least, is not far from Savoy territory. The version that shows the apogee values also mentions a set of radix values for planetary mean motions, valid for the same year and for Paris; the values themselves, however, seem to have been lost.

Summary of Contents

The treatise has never been printed. As was said, it exists in three versions, namely: (A) a main version, Ad utilitatem communem studentium in astronomia et specialiter medicorum ... habere veritatem de qua laudetur veritas eterna regnans per infinita secula seculorum (T&K, 64; thirteen known manuscript copies); (B) an abridged version, Instrumentum in quo velut in speculo ... permanere de quo laudetur veritas eterna per infinita secula (T&K, 753; ten copies); and another abridgment, Planetarum vero speculum taliter perficies ... (T&K, 1050; four copies). All three versions have some phrasing in common, and all describe the same instrument. Several manuscripts of both (A) and (B) have headings or subscriptions containing the phrase “Speculum Planetarum”, the author’s name (which is fairly stable though one manuscript adds de Alamannia), and in some cases also the dating to 1417 elapsed years and the location at “Vienna”. Version (C) is anonymous. Only version (A) contains the apogee values mentioned above; since these are securely datable to 1418, they are no doubt authentic, so (A) is likely to be the original version.

Version (A) is quite short, comprising some ten book pages. Like most such texts, it contains two chapters, describing the construction and the use of the instrument. These are separated by a short discussion of the parameters required for using the instrument, namely, the mean motions that are to be calculated for the time in question, by means of tables.

The instrument described is basically a normal planetary equatorium designed to simulate the Ptolemaic planetary models. As such, it consists of: (1) a large plate, the mater, containing graduated circular scales including one common equant scale and, inside it, an ecliptical scale for each of the planets; (2) a ruler of fixed length, with one end anchored in the deferent centre belonging to the planet in question, and with the other end revolving around this centre to form the deferent circle. This end carries (3) a circular disc graduated to indicate the place of the planet on the epicycle, and (4) a revolving ruler that stands for the epicycle, with each planet marked at its proper epicycle radius. To use the instrument, one uses tables to calculate the mean positions on the equant and on the epicycle; then one places the movable parts accordingly, which brings the planet mark on (4) in its true position. This can then be measured on the ecliptical scale for the planet in question, by stretching a thread through the planet mark and the “Earth” mark belonging to that planet. This method works for the planets apart from Mercury and the Moon; it also works for the Sun, since for this purpose the Sun is considered to have a concentric deferent plus an epicycle, instead of an excentric deferent as in Ptolemy. For the Moon and Mercury, whose deferent centres move on circles of their own, there is also required (5) a further revolving ruler, to control the position of the deferent centre.

Most of the Ptolemaic models for the planetary motions were the same in principle, but the dimensions were different for each planet. Accordingly, in the mid thirteenth century, Campanus of Novara started the Latin tradition for such instruments by presenting designs for seven separate instruments, one for each of the five planets plus the Sun and the Moon. Since then, various efforts had been made to unite these instruments into a composite device. John’s attempt is considered quite successful. A novelty is the use of a common equant, which entails positing a separate “deferent centre” and “Earth” for each planet. A ploy whose aim is solely esthetical is that all the ecliptical scales on the mater have been made concentric with the equant; but since each of them still has to be graduated evenly as seen from the relevant “Earth” point, which is now no longer its centre, the graduation will look oblique and uneven.

An instrument on the same principle is described in a tract in one manuscript examined by POULLE (1980, 178–79: Recipe tabulam ligneam pergameno contectam (T&K, 1337)). It has planetary radix values that are dated to year 1420 elapsed, thus three years after the Speculum tract. It is not called by that name, it has no ascription, and the instrument it describes is rather less practicable than the Speculum, so it is doubtful whether John is the author. The Speculum tract had the attention of a celebrity-to-be like Regiomontanus, who copied it in 1457 (ZINNER 1938, 52, 221 no. 31; MS Wien 5258), at an age of about twenty-one. A tract on another instrument, copied in the same environment, also got the title Speculum planetarum and an ascription to Peurbach, the teacher of Regiomontanus (ZINNER 1938, 220 no. 30: Quoniam experimentum sermonum verorum (T&K, 1275); cf. PEDERSEN 1968, 71–72 and POULLE 1980, 217 ff.; MS Wien 5203). It is true that the tract itself cannot be by Peurbach (1423–1461), since it also occurs in a fourteenth-century Erfurt manuscript (CA F 394, 119rb), containing a list of apogees dated 1347; this copy lacks a title. But at least the Speculum Planetarum tract had made impression enough in this important environment to lend its name to a pseudepigraphon.


  • DENIFLE, H. & CHATELAIN, E. 1897: Chartularium universitatis Parisiensis. Tomus 4: Ab anno 1394 usque ad annum 1452, Paris.
  • JACQUART, D. 1979: Dictionnaire biographique des médecins en France au moyen âge. Supplément (Hautes études médiévales et modernes 35), Genève.
  • GANIVET, J. 1508: Coeli enarrant (Amicus medicorum magistri Johannis Ganiveti, cum opusculo quod celi enarrant propter principium eius inscribitur), Lyon.
  • LEHMANN, P. 1936: Skandinaviens Anteil an der lateinischen Literatur und Wissenschaft des Mittelalters. Erstes Stück (Sitz.-Ber. der Bayer. Akademie der Wiss. Philos.-Hist. Abteilung 1936, Heft 2) (reprinted in Erforschung des Mittelalters, Band 5, Stuttgart, Hiersemann 1962, 275–330).
  • PEDERSEN, O. 1968: “Two mediaeval equatoria,” in Actes du XIe congrès international d'histoire des sciences 3, Wroclaw etc., 68–72.
  • PEDERSEN, O. 1970 ff.: “John Simonis of Selandia,” in Dictionary of Scientific Biography, ed. C.G. Gillispie, 16 vols., New York 1970–1981 (with list of manuscripts supplementing Zinner’s).
  • POULLE, E. 1980: Équatoires et horlogerie planétaire du XIIIe au XVIe siècle, Genève (especially 170–79; list of manuscripts on 799–800).
  • T&K = THORNDIKE, L. & KIBRE, P. 1963: A catalogue of incipits of mediaeval scientific writings in Latin, Cambridge, Mass., cols. 64, 753, 1050, 1337.
  • THORNDIKE, L. 1934: A history of magic and experimental science. Volumes III and IV: Fourteenth and fifteenth centuries, New York.
  • WICKERSHEIMER, E. 1936: Dictionnaire biographique des médecins en France au moyen âge, Paris.
  • ZINNER, E. 1925: Verzeichnis der astronomischen Handschriften des deutschen Kulturgebietes. München, nos. 3077–77a–78; 9629–9642.
  • ZINNER, E. 1938: Leben und Wirken des Johannes Müller von Königsberg genannt Regiomontanus, München.