Ecclesiastical full moon

An ecclesiastical full moon is formally the 14th day of the ecclesiastical lunar month in an ecclesiastical lunar calendar. The ecclesiastical lunar calendar spans the year with lunar months of 30 and 29 days which are intended to approximate the observed phases of the Moon. Since a true synodic month has a length that can vary from about 29.27 to 29.83 days, the moment of astronomical opposition tends to be roughly 14.75 days after the previous conjunction of the Sun and Moon (the new moon). The ecclesiastical full moons of the lunar reckoning tend to agree with the (Gregorian calendar) dates of astronomical opposition, referred to a day beginning at midnight at 0° longitude, to within a day or so. However, the astronomical opposition happens at a single moment for the entire Earth: the hour and day at which the opposition is measured as having taken place will vary with longitude. In the ecclesiastical calendar, the 14th day of the lunar month, reckoned in local time, is considered the day of the ecclesiastical full moon at each longitude.

Beginning in medieval times, the age of the ecclesiastical moon was announced daily at the canonical hour of Prime during the reading of the martyrology. This was still done by Roman Catholics using the extraordinary form of the Roman Rite (according to the 1962 Roman Breviary).