Ecclesiastical new moon

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An ecclesiastical new moon is the first day of a schematic lunar month in a computus. Such months have a variable number of whole 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 its computus is accurate, it can be any day from the day of the astronomical new moon or dark moon to two days later. It itself is only a minor part of a computus—the critical day is thirteen days later, specifically, the fourteenth day of the schematic lunar month which occurs on or next after March 21. This fourteenth day was loosely called the Paschal full moon by medieval computists. Easter is the following Sunday.

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