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Ecclesiastical full moon

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An ecclesiastical full moon is the fourteenth day of a 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. If the ecclesiastical new moon, the first day of the ecclesiastical lunar month, is formally thought of as beginning one day after conjunction, then a formal average opposition will fall at about noon on the 14th day of the same lunar month. 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 (see table). 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. When it is January 11, 2009 in Shanghai, China, that is considered the day of the ecclesiastical full moon for Shanghai. When it is January 11, 2009 in Oklahoma City, that is considered the day of the same ecclesiastical full moon for Oklahoma City.

Day of lunar opposition 2009[1] Gregorian ecclesiastical full moon for a year of epact 3[2][3]
Jan 11 Jan 11
Feb 9 Feb 10
Mar 11 Mar 11
Apr 9 Apr 10[4]
May 9 May 9
Jun 7 Jun 8
Jul 7 Jul 7
Aug 6 Aug 6
Sep 4 Sep 4
Oct 4 Oct 4
Nov 2 Nov 2
Dec 2 Dec 2
Dec 31 Dec 31

The first ecclesiastical full moon of the year to begin on or after March 21 is of special importance, since it is the Paschal full moon, the 14th day of the Paschal lunar month. Easter is the Sunday next after the Paschal full moon. Or, in other words, Easter is the third Sunday in the Paschal lunar month.

In the medieval period age of the ecclesiastical moon was announced daily in the office of Prime at the reading of the martyrology.[5]

In the Book of Common Prayer of the Episcopal Church, 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 moon falls in that year of the cycle.[6] The same practice is followed in some editions of the Book of Common Prayer of the Church of England.


  1. Day beginning at midnight UT. From the U.S. Naval Observatory's Principal phase calculator
  2. From the Explanatory Supplement to the Ephemeris and the American Ephemeris and Nautical Almanac, H. M. Stationery Office, London, 1966, fourth revised printing, 1977, page 426.
  3. In the ecclesiastical lunar calendar, the day begins at sunset on the previous day.
  4. Paschal full moon
  5. At medieval Exeter Cathedral, it was the next day's date and age of the moon that were announced. Et omnibus in locis suis sedentibus sit ibi quidam puer...paratus ad legendum leccionem de Martilogio, absque Iube domine, sed pronunciondo primo loco numerum Nonarum, Iduum, Kalendarum, et etatem lune qulis erit in crastino... (And when all are sitting in their places let a boy be there ready to read the Martyrology beginning with Iube domine, but first saying the number of Nones, Ides, Kalends, and what the age of the moon will be on the morrow...) J.N. Dalton, ed., Ordinale Exon. vol. 1, Henry Bradshaw Society, London, 1909, p. 37.
  6. The Book of Common Prayer according to the use of The Episcopal Church, Seabury Press, New York, pp. 21-22.

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