Ecclesiastical new moon


An ecclesiastical new moon is the first day of a lunar month 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. The ecclesiastical calendar valid for the Julian and Gregorian calendar are described in detail by Grotefend, Ginzel and in the Explanatory Supplement to The Astronomical Ephemeris.
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. 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.
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.

These complaints were finally addressed by the construction of the Gregorian calendar.
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.
YearGregorian paschal new moonDays in paschal lunar month
2014March 3030
2015March 2029
2016March 929
2017March 2829
2018March 1730
2019April 530
2020March 2430
2021March 1329
2022April 130
2023March 2130
2024March 1029
2025March 2929
2026March 1929
2027March 829
2028March 2629
2029March 1529
2030April 230
2031March 2329
2032March 1130