The lunar table

After the date, the reading of the martyrology may announce the day of the moon: Luna vigesima, "the twentieth day of the moon". To find it, the book prints a small table over every day's eulogies. This page explains why the moon is announced, how the table works, and how to find the moon for any year.

Why the moon is announced

The solar calendar is known all over the world, yet the 2004 edition keeps the old custom of announcing the day by the moon as well. Announcing it is optional in the liturgical reading (Ordo, n. 10).

The reason given is Easter. The great solemnity, with Lent before it and Eastertide after it, depends on the first full moon of spring. Keeping the lunar reckoning recalls, even in the celebration of the paschal mystery, the bond between the people of the Old Covenant and of the New. The lunar date is also in use in the Eastern Churches and in many religions and cultures around the world.

The letters over each day

Over each day's eulogies the book prints thirty letters, one for each epact (see below), and under each letter a number: the day of the lunar month, from 1 to 30 (or 29). The letters are:

The letter F (epact XXV) is printed twice, once in black and once in red; see Two exceptions. So the table of each day has 31 columns.

Golden number, epact and letter

Three numbers tie a year to its column:

• The golden number is the year's place in the 19-year lunar cycle, after which the phases of the moon fall again on nearly the same days. It is the remainder of the year divided by 19, plus one: 2005 ÷ 19 leaves 10, so the golden number of 2005 is 11.
• The epact is the difference of 11 days between the lunar year (354 days) and the solar year (365), carried over from year to year. It tells how old the moon is when the year begins: in 2005 the epact is XIX, and on 1 January the moon is 20.
• The letter of the Martyrology is the letter that stands for that epact in the table above.

The book prints a chronological table with these three for every year from 2004 to 2033.

Finding the moon

Take the year's letter, find it in the table over the day's eulogies, and read the number under it: that is the moon to announce, every day of that year. The example in the book: in 2005 the golden number is 11, the epact XIX, and the letter u. On 1 January the number under u is 20, so the reader announces Luna vigesima; on 2 August it is 26.

Pick a date to see its golden number, epact, letter and moon, as this site computes them:

Two exceptions

Golden number 1. In a year whose golden number is 1, from 1 January to the end of that lunation the moon is announced one less than the table shows. The book's example is 2204: golden number 1, epact XXVIII, letter M. On 1 January the table shows 29, but 28 is announced; on 2 January it shows 30, but 29 is announced; from 3 January, when the table shows 1, the table is followed again. The one exception is a year of golden number 1 whose letter is the capital P: then the table is followed as usual.

Epact XXV. For the epact XXV the letter F is printed twice. When the golden number is from 12 to 19, read the black F; when it is from 1 to 11, read the red F. In 2011 (golden number 17, epact XXV), on 13 April the black F gives the moon 10. In the traditional tables the epact XXV of such a year is written in Arabic figures, 25, and the finder above shows it so. For the red F the book's example is "2303, golden number 9", in both the Latin and the Italian print; by the book's own rule, though, 2303 has golden number 5 and epact XI. The year meant is 2307: golden number 9, epact XXV, and on 13 April the red F gives the moon 9.

How long a table lasts

The letters of the martyrology are valid until 2199. Beyond that the epact of each golden number moves by a day, as the calendar corrects for the sun and the moon, and a new table of letters is drawn up. The book shows how:

• the golden number is the year plus one, minus 19 times the whole quotient of the year divided by 19 (that is, the remainder of the year divided by 19, plus one);
• from now to 2099 the epact is found from the golden number times 11: if that product is greater than 13, subtract 13, take the remainder of dividing by 30, and add 1; otherwise add 18 to the product. From 2100 to 2199, use 14 and 2 in place of 13 and 1.

For 2200 to 2299 the book gives a table of its own. For example, in 2248, golden number 7, the epact is IV and the letter d.

The printed tables on this site

The editions that print the lunar table set its 31 columns in two rows: 17 and 14 in the 1630 edition, 19 and 12 in the 2004 edition. In the reader, a day's table opens under its heading with the column of the year marked, and the year can be changed there. See, for example, the 2004 Latin edition.