N. M. Swerdlow in the analysis of Copernicus's De revolutionibus (1984).

The last subject Neugebauer took up was Ethiopic astronomy, chronology, and computus, that is, the ecclesiastical calendar. He had long been intrigued by the primitive astronomical section of the Book of Enoch, originally written in Aramaic and surviving complete only in Ethiopic (Ge‘ez), which appeared to contain simplified Babylonian elements, and he also noticed from the catalogue of Ethiopic manuscripts in Vienna, passages that suggested a relation with Hellenistic astronomy and calendars. The question was, what was this material about, and was there more of it? After learning Ge‘ez—the only Semitic language that is not perverse, he called it (since it includes the vowels)—and studying many manuscripts, he found that the astronomical content was slight, but the calendrical and chronological information preserved from late antiquity and the Middle Ages was very interesting indeed. Chronology had in fact always been his third subject besides astronomy and mathematics; earlier he had collaborated with W. Kendrick Pritchett on The Calendars of Athens (1947) and analyzed the calendar of the Très riches heures for Millard Meiss (1974). Now he again took up chronology seriously. Ethiopic Astronomy and Computus (1979) is the summary of what he found, organized by subject in alphabetical order. There is much of interest here, but to name only the most significant result, he was able to reconstruct the Alexandrian Christian calendar and its origin from the Alexandrian Jewish calendar as of about the fourth century, at least two hundred years prior to any other source for either calendar. Thus, the Jewish calendar was derived by combining the 19-year cycle using the Alexandrian year with the seven-day week, and was then slightly modified by the Christians to prevent Easter from ever coinciding with Passover, which would be