Ecclesiastical new moon

An ecclesiastical new moon is the first day of a schematic lunar month in a computus. Such months have a variable number of whole 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 its computus is accurate, it can be any day from the day of the astronomical new moon or dark moon to two days later (see table).

The first ecclesiastical new moon of the year to begin on or after March 8 is of special importance, since it begins the Paschal lunar month: The fourteenth day of the same schematic lunar month is the first of the 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. [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.]

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.) [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.]

These complaints were finally addressed by the construction of the Gregorian calendar.

References

External links

* [http://aa.usno.navy.mil/data U.S. Naval Observatory Data Services]
* [http://home.telepath.com/~hrothgar/lunar_almanac.html Gregorian ecclesiastical new moons]
* [http://www.bodley.ox.ac.uk/dept/scwmss/wmss/medieval/mss/lat/liturg/e/006.htm Oxford, Bodleian Library MS. Lat. liturg. e. 6. (14th century)]