Calculus Temporum Romanorum

Quae sint haec — the data explained

Littera Dominicalis — Dominical (Sunday) Letter

The letter A–G that marks the Sundays of the year. Number the days of January A, B, C … G, A, B … (1 Jan = A); the letter landing on the Sundays is the year's Dominical Letter. A leap year has two — the first for January–February, the second (the preceding letter) for March–December, because 29 February shifts the weekdays back by one.

By hand: 1 Jan = A, 2 Jan = B … 7 Jan = G — take the letter that falls on a Sunday (in a leap year, step back one letter after 29 February).

Aureus Numerus — Golden Number

The year's place (1–19) in the 19-year lunar cycle, after which the moon's phases recur on the same calendar dates.

By hand: (year + 1) mod 19 — the remainder, or 19 if it is 0.

Epacta — Epact

The age of the moon on 1 January — the roughly 11 days by which the solar year overruns twelve lunar months. It locates the new moons in the calendar, and so fixes the date of Easter. Thirty is printed “*”.

By hand: (11 × Golden Number + B) mod 30 — with B = 18 for 1900–2199 (20 for 1583–1699, 19 for 1700–1899); 30 is written *.

Indictio Romana — Roman Indiction

A 15-year civil cycle of the late Roman Empire, kept only for dating documents; it carries no astronomical meaning.

By hand: (year + 3) mod 15 — the remainder, or 15 if it is 0.

Cyclus Solaris — Solar Cycle

The 28-year period after which the days of the week fall again on the same dates (in the Julian reckoning); it is the cycle that governs the Dominical Letters.

By hand: (year + 9) mod 28 — the remainder, or 28 if it is 0.

Littera Martyrologii — Martyrology Letter

The letter by which the Roman Martyrology announces the age of the moon (Luna) each day. It is the year's Epact written as a letter — and the case matters: small letters a–u stand for epacts 1–19, capitals A–P for epacts 20–30.

By hand: a = 1 … u = 19, A = 20 … P = 30 — write the Epact as its letter (the small alphabet skips j and o; small = epacts 1–19, capital = 20–30).