Concept
An ecclesiastical full moon is formally the 14th day of the ecclesiastical lunar month (an ecclesiastical moon) 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 Gregorian lunar calendar tend to agree with the dates of astronomical opposition, referred to a day beginning at midnight at 0 degrees 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 full moon at each longitude. Schematic lunar calendars can and do get out of step with the Moon. A useful way of checking their performance is to compare the variation of the astronomical new moon with a standard time of 6 a.m. on the last day of a 30-day month and 6 p.m. (end of day) on the last day of a 29-day month. Beginning in the medieval period the age of the ecclesiastical moon was announced daily in the office of Prime at the reading of the martyrology. This is still done today by those using the extraordinary form of the Roman Rite adhering to the 1962 Roman Breviary. In the Book of Common Prayer of the Protestant Episcopal Church of the United States of America, the dates of the paschal full moons for the 19 years of the Gregorian Easter cycle are indicated by the placement of the Golden Number to the left of the date in March or April on which the paschal full moon falls in that year of the cycle. The same practice is followed in some editions of the Book of Common Prayer of the Church of England.