Commercial maths, algebra, stars, mnemonic ‘hands’ and navigation: Michael of Rhodes ms online.

ships Michael of Rhodes detail

In 1434, Venetian mariner Michael of Rhodes began his notebook, which would eventually contain 440 pages of mathematics, astronomy, astrology and information for navigators’ needs. Its text is written in the Venetian dialect.

A selection of forty folios from the manuscript can be viewed online here.

Composition and purpose.

According to Alan Stahl,

.. once one begins to compare the contents of Michael’s book with that of the Zibaldone [da Canal] and, even more so, with other merchants’ manuals from the period, it becomes evident that this is a very different type of composition, with texts from divergent sources, and intended for a different audience. Rather than fitting into the developing tradition of merchant manuals, tariffs, and related compilations, the Book of Michael of Rhodes can best be seen as the starting point of a distinct genre, the maritime manual.” (p.202)

Alan M. Stahl, ‘The Book of Michael of Rhodes and the Merchant Manual Tradition’, Revue Numismatique, Vol. 167 (2011), pp. 201-10. (downloadable through

Hand Diagrams:

Michael’s work shows the prevalence in that environment, in the early fifteenth century, of using mnemonic ‘hand’ diagrams. On the linked site it says:

“Michael refers explicitly to hand diagrams in three of the topics he explores. One drawing was for the method of calculating the annual lunar epact. A second was for calculating what we would now call the solar concurrent (the number of the weekday of March 24 in a given year) which could be used to find the weekday of any calendar day in a year. The last concerned the date of “Hebrew Passover,” preceding Easter.

Unfortunately, although he left space for them, Michael never did draw the hand diagrams into his manuscript. He did include drawings of hand calculations in the later manuscript he wrote that bears the name of Pietro Di Versi; he also wrote in that manuscript that the methods for calculating the rising of the moon were made “by the hand of Michael of Rhodes in 1444”.

Astrological imagery.

Michael’s  ‘astrological’ imagery consists of  what is termed “the ‘soft astrology’ of the Middle Ages’,  which admitted the influence of the heavens on Earth, but did not involve horoscopes or the creation of detailed natal or predictive charts. Its focus on medicine renders it akin to the content of the Tacuinum Sanitatis, in which we also find ” the dominant element (earth, air, fire, or water) and quality (hot, wet, cold, or dry) for each sign.”

Diagram by Judith for Cerutti copy of Ibn Botlan Tacuuinum sanitatis

diagram from: Spencer, Judith, trans., The Four Seasons of the House of Cerruti, New York, Facts on File Publications, 1984. First shown in a post referring Voynich researchers to the ‘Tacuinum sanitatis’ – a work very often mentioned now, but at that time evidently unknown. It hadn’t even a wiki article then. (‘Findings’ September 15, 2010).

Michael of Rhodes also provides a general sketch of the character of the person born under each sign, along with brief medical advice and other information. In this we find some point of connection with the  “Children of the Planets” texts so popular from about the same time.

A series of time-keeping diagrams are also in Michael’s handbook. My own opinion (not shared by any other Voynich researcher, to my knowledge, at the time of writing) is that a similar purpose informs the supposed ‘zodiac’ in MS Beinecke 408. This consists of a series of diagrams,  across two fold-outs, which do not represent the zodiac as we know the zodiac, but their centres are inscribed with the names of ten months.

This is Michael’s  figure for Libra.

Michael of Rhodes Zodiac Libra.0Michael of Rhodes’ manuscript is in private hands, but the present owner allowed a group of specialists to study it in depth, and first page linked above (in red) will take you to that. A facsimile edition has been published.

On the subject of portolans and ‘portolan’ charts more generally, I recommend Tony Campell’s website here. Campell is a former Keeper of Maps with the British Library.

On the relevance of such charts to study of MS Beinecke 408 I am unable to find evidence of anyone prior – do let me know if you can think of anybody who deserves credit.

Finally, here are the words of  John Holmes,  in charge of manuscript maps in the British Museum  – now British Library –  until his death in 1854.

“Faced with tables and calendars offering, in almost equal measure, dates that either confirm or contradict those of the works they accompany, we should obviously approach with caution those atlases whose dating has depended entirely on their calendars”.

  • as quoted by Campbell, who refers to Holmes’ own notebook.


Additional References:

1. Annalisa Conterio (ed.), Pietro Di Versi,Raxion de’Marineri,Taccuino Nautico Del Secolo XV, Published by Comitato Per La Pubblicazione Dele Fonti Relative Alla Storia Di Venezia, Venezia, 1991.

2. Michele Bacci, Martin Rohde, The Holy Portolano / Le Portulan sacré: The Sacred Geography of Navigation in the Middle Ages. Fribourg Colloquium 2013 / La géographie religieuse de la navigation au Moyen Âge. Colloque Fribourgeois 2013

I can imagine some of the most determined  “all-Latin-Christian-product” theorists may fall on that last reference  with a gleam in their eye. What better than to be able to re-use previous observations and conclusions of maritime reference [only mine, if the truth be told] without having to acknowledge any ‘foreign’ elements as those which I, and other appraisers, have recognised in it.

But I think that my cynicism may be almost out of date. Since the establishment of ‘Voynich ninja’, people differing from the “all Latin Christian authorial composition” theory are being permitted to think about, and to discuss without being ‘corrected’ a fair range of alternative propositions.

which is not a bad thing, is it?


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