This is an open access title distributed under the terms of the CC BY-NC 4.0 license, which permits any non-commercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited. Further information and the complete license text can be found at https://creativecommons.org/licenses/by-nc/4.0/The terms of the CC license apply only to the original material. The use of material from other sources (indicated by a reference) such as diagrams, illustrations, photos and text samples may require further permission from the respective copyright holder.Cover illustration: Vatican City BAV Vat. gr. 1291 f. 47r. Epact table in the Vatican copy of the Handy Tables of Ptolemy. The diagram's outer ring indicates the epact for each year from the 30th year of the Diocletianic era (AD 314) to the 257th (AD 541), counted clockwise in concentric circles starting from the innermost ring. At the center is Selene, the moon, who uses her left hand to drive a chariot drawn by two bulls. The four surrounding medallions present personifications of night and day. Image courtesy of the Biblioteca Apostolica Vaticana.

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