LETTERA LUNARE. – In medieval calendars, among other elements, the age of the moon is also indicated. To determine the beginning of the new moon in the Middle Ages, the so-called system of lunar letters was used, analogous to that of the ferial letters.
This system comprised 59 letters divided into three groups: the first (from A to V, called litterae nudae), the second (from A to V, called litterae post-punctatae or subnotatae), and the third (from A to T, called litterae praepunctatae or supernotatae). These letters were used to indicate the days of two consecutive lunar cycles. By means of these letters, one could determine not only the beginning of the new moons but also the age of the moon for every day of the year; in leap years, the added day bore the same number as the previous day. The lunar letter was sometimes used in the dating of medieval documents to designate the year (littera annalis); in this case, the indicated letter corresponded to the day of the March full moon (terminus paschalis). The system of lunar letters fell into disuse in the 11th century.
Niccolò Del Re