Calendar page from a 13th-century liturgical book.(British Library, Cotton MS Cleopatra B IX, folio 59r). The Golden numbers, indicating the ecclesiastical new moons, are in the fifth column from the left-hand margin, to the left of the Sunday letters

An ecclesiastical new moon is the first day of a lunar month (an ecclesiastical moon) 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 (see table).

The fourteenth day of an ecclesiastical lunar month is the ecclesiastical full moon. The date of the ecclesiastical full moon is obtained by adding 13 days to the date of the preceding ecclesiastical new moon.

The first ecclesiastical new moon of the year to begin on or after March 8 is of special importance, since it is the Paschal new moon that begins the Paschal lunar month (see table). The fourteenth day of the same lunar month is the first of the calendar year to occur on or next after March 21. This fourteenth day was loosely called the Paschal full moon by medieval computists. Easter is the following Sunday.

Calendar pages in medieval liturgical books indicated the ecclesiastical new moons by writing the Golden number to the left of the day of the month on which the ecclesiastical new moon would occur in the year of that Golden number. In some places the age of the moon was announced daily in the office of Prime at the reading of the martyrology.[1]

When in the 13th century Roger Bacon complained about the discrepancy between the ecclesiastical moon and the observed lunar phases, he specifically mentioned the discrepancy involving the ecclesiastical new moon

Quilibet computista novit quod fallit primatio per tres dies vel quatuor his temporibus; et quilibet rusticus potest in coelo hunc errorem contemplari. (Any computist knows that the prime [of the moon] is off by three or four days in our time; and any rustic can see this error in the sky.)[2]
These complaints were finally addressed by the construction of the Gregorian calendar.

Day of lunar conjunction 2008[3] Day in 2008 on which the moon had an illuminated fraction of 0.0 at midnight UT (beginning of day)[4] Gregorian ecclesiastical new moon for a year of epact 22[5][6]
Jan. 8 Jan. 9 Jan. 9
Feb. 7 Feb. 7 Feb. 7
Mar. 7 Mar. 8 Mar. 9
Apr. 6 Apr. 6 Apr. 7
May 5 May 5/6[7] May 7
Jun. 3 Jun. 4 Jun. 5
Jul. 3 Jul. 3 Jul. 5
Aug. 1 Aug. 2 Aug. 3
Aug. 30 Aug. 31 Sep. 2
Sep. 29 Sep. 29 Oct. 1
Oct. 28 Oct. 29 Oct. 31
Nov. 27 Nov. 28 Nov. 29
Dec. 27 Dec. 28 Dec. 29
Year Gregorian Paschal new moon Days in Paschal lunar month
1995 April 1 29
1996 March 21 29
1997 March 10 29
1998 March 29 29
1999 March 18 29
2000 April 5 30
2001 March 26 29
2002 March 15 29
2003 April 3 29
2004 March 23 29
2005 March 12 29
2006 March 31 29
2007 March 20 29
2008 March 9 29
2009 March 28 29
2010 March 17 29
2011 April 4 30
2012 March 25 29
2013 March 14 29


  1. 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.
  2. Roger Bacon, Opus Tertium LXX, in J. S. Brewer, ed., Fr. Rogeri Bacon Opera quaedam hactenus Inedita. Vol. 1. H.M. Stationery Office, 1859 (Kraus Reprint 1965), p. 282.
  3. From the U.S. Naval Observatory's Principal phase calculator
  4. Computed using the U.S. Naval Observatory's illuminated fraction calculator. When the moon has an illuminated fraction of 0.0 on two successive midnights, the date in the table is the date of the second such day.
  5. 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.
  6. The ecclesiastical new moon begins at sunset on the previous day.
  7. No day of illuminated fraction 0.0 at midnight is given. May 5 and 6 are the two successive days of illuminated fraction 0.1 at midnight.

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