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 ecclesiastical calendar valid for the Julian and Gregorian calendar are described in detail by Grotefend,[1] Ginzel[2] and in the Explanatory Supplement to The Astronomical Ephemeris.[3]
The ecclesiastical new moon which falls on or next 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 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.[4]
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.)[5]
These complaints were finally addressed by the construction of the Gregorian calendar.
A check can be made on the difference between the astronomical new moon and the ecclesiastical new moon. The following table gives a comparison for 2010. All times are Greenwich Mean Time. Coordinated Universal Time is the same with a tolerance of 0.9 seconds either way.
The long term accuracy of the Gregorian ecclesiastical lunar calendar is remarkable. It will be in error by one day in about 73 500 years while the error with respect to the tropical year will be one day in about 3320 years.[8]
The following table gives the ecclesiastical new moon used for determining Easter (in the Gregorian system) for a range of years.
Year
Gregorian paschal new moon
Days in paschal lunar month
2014
April 1
29
2015
March 21
29
2016
March 10
29
2017
March 29
29
2018
March 18
29
2019
April 5
30
2020
March 26
29
2021
March 15
29
2022
April 3
29
2023
March 23
29
2024
March 12
29
2025
March 31
29
2026
March 20
29
2027
March 9
29
2028
March 28
29
2029
March 17
29
2030
April 4
30
2031
March 25
29
2032
March 14
29
References
^Grotefend, Hermann (1891). Zeitrechnung des deutschen Mittelalters und der Neuzeit, Bd. 1. Hannover, Germany: Hahn'sche Buchhandlung.
^Ginzel, Freidrich Karl (1914). Handbuch der mathematischen und technischen Chronologie. Leipzig, Germany: Hinrichs.
^Explanatory Supplement to The Astronomical Ephemeris. 1961.
^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 qualis 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.
^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.
^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.
^Lichtenberg, Heiner (1994). "Die Struktur des Gregorianischen Kalenders anhand der Schwankungen des Osterdatums entschlüsselt". Sterne und Weltraum. 33 (3): 194–201. Bibcode:1994S&W....33..194L.
