The GOLDEN NUMBER, the aureus numerus, refers to the 19-year Metonic cycle by which the phases of the moon recur on the same dates. The cycle begins when the new moon occurs on January 1. To find the moon phases for the years 1923-2084, go to Lunar Outreach Services at the following site: <http://www.lunaroutreach.org/> From that site, you can also see how many days old the current moon is and the percentage that it is luminous.
The EPACTA, which occurs in cycles of 30 numbers, is the age of the moon in days on January 1. Keep in mind that if the paschal moon (which is calculated from the system of golden numbers and epacts and does not necessarily coincide with the astronomical full moon!) occurs on a Sunday, Easter Sunday will be on the following Sunday -- thus falling between March 22 and April 25.
The DOMINICAL LETTER is a system that assigns the days to a rotation of seven letters, a-g. Thus, Jan. 1 is A, Jan. 2 is b, Jan. 3 is c, and so on. Because the first Sunday in 1999 fell on January 3, the Dominical Letter for 1999 is c. All the Sundays in 1999 will fall on the letter c. The Dominical Letter runs retrograde from year to year, g-a.
The LITTERA MARTYROLOGII: The table for the Lit. Martyro. is found in the Martyrologium Romanum. To find the Lit. Martyrol., count the remaining number of days in January that follow upon the 1st new moon, and then find the corresponding letter from the sequence of letters given in the Martyrologium Romanum.
The INDICTIO: This is a 15-year cycle. It was orginally started for the purpose of fixing the tax for a set number of years. It was originally a 5-year cycle under Diocetian and was increased to 15 years under Licinius. The start of the indiction on Jan. 1 started under Pope Gregory XIII. The indictio has no relation to astronomical phenomena.
For further reading, consult the following commonly available references:
CATHOLIC ENCYCLOPEDIA (1913), see articles, "Christian Calendar," "Indiction", "Dominical Letter" and "Computus" (science of determining time).
ENCLYCLOPEDIA BRITANNICA, see article on "Calendar." The 1910 edition contains several tables and formulas in its article on "Calendar". Also, the current on-line edition has articles on "Calendar," "Easter," etc.
BOOK OF COMMON PRAYER: Almost any edition contains a section on tables and rules for finding the date of Easter. Most libraries have the Book of Common Prayer. You can also find it in the pew racks of any Episcopal Church. No English-speaking Christian should be without a copy of the Book of Common Prayer.
Bro. Paul August Jasmer, O.S.B., <firstname.lastname@example.org> Managing Editor of The Ordo (17 July 1999)
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