This webpage belongs to, which is a website promoting [Jan Zuidhoek (2019) Reconstructing Metonic 19-year Lunar Cycles (on the basis of NASA’s Six Millennium Catalog of Phases of the Moon): Zwolle], and shows an open letter from its author Jan Zuidhoek to his colleagues in the field of computus (only those of them who could not be reached by email) Paul Butzer, Sandor Chardonnens, Lisa Lynn Chen, Georges Declercq, Bruce Eastwood, Brigitte Englisch, Menso Folkerts, Leofranc Holford-Strevens, Pavel Kuzenkov, Máirin Maccarron, Stephen McCluskey, Philipp Nothaft, Masako Ohashi, James Palmer, Marina Smyth, Kerstin Springsfeld, Sacha Stern, Peter Verbist, Faith Wallis, and Daniel Watson, in order to acquaint them with the existence of this book (ISBN 9789090324678).



Open Letter to Colleagues in the Field of Computus

Dear colleagues,

My contribution to the Third International Conference on the Science of Computus in Ireland and Europe in Galway in 2010, which resulted rather recently (2017) in the publication of my article entitled “The initial year of De ratione paschali and the relevance of its paschal dates” as a part of the conference proceedings entitled “Late Antique Calendrical Thought and its Reception in the Early Middle Ages” and edited by Immo Warntjes & Dáibhí Ó Cróinín, was a rather modest one, because, at least in 2010, as a retired teacher of mathematics (at the Gymnasium Celeanum in Zwolle), I was a relative outsider in the field of computus. So I finished that article by expressing my rather ambitious intention to, in due course, supplement my qualitative account for the 2-day gap established therein between the so called proto-Alexandrian and the classical Alexandrian 19‑year lunar cycle with “a quantitative one by performing a complete reconstruction of the classical Alexandrian cycle”.

Well, that originally intended quantitative account would ultimately run into a both qualitative and quantitative account for a similar 2‑day gap between two crucial lost ante-Nicene Alexandrian Metonic 19‑year lunar cycles. This resulted from my recent (2018) analysis by reconstructing both the Alexandrian Metonic 19‑year lunar cycles which must have been actively constructed still before the first council of Nicaea, namely Anatolius’ 19‑year lunar cycle (second half of the third century) and the (first quarter of the fourth century) common archetype of the three well‑known post‑Nicene Alexandrian Metonic 19‑year lunar cycles.

I am happy to report that the aforesaid (2018) analysis has been realized in the recent (2019) publication of my book entitled “Reconstructing Metonic 19-year lunar cycles (on the basis of NASA’s Six Millennium Catalog of Phases of the Moon)”, thus reducing the aforesaid (2017) article to a preparatory study for this book. If you would like to receive a complimentary copy of this pioneering book, which has been dedicated to Daniel P. Mc Carthy, please email your postal address to me at If you would also like to receive (also for free) a copy of it intended for your university library, please do not hesitate to mention this.


Jan Zuidhoek





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