Ecclesiastical new moon

An ecclesiastical new moon is the first day of a lunar month in an ecclesiastical lunar calendar. Such months have a whole number of days, 29 or 30, whereas true synodic months can vary from about 29.27 to 29.83 days in length. Medieval authors equated the ecclesiastical new moon with a new crescent moon, but it is not a phase of the true moon. If the ecclesiastical lunar calendar is accurate, the ecclesiastical new moon can be any day from the day of the astronomical new moon or dark moon to two days later. The ecclesiastical calendar valid for the Julian and Gregorian calendar are described in detail by Grotefend, Ginzel and in the Explanatory Supplement to The Astronomical Ephemeris.

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