From the “leap day before the calends of March” to the 29th of February – Part II

Pilots and navigators of the 15th and 16th centuries needed the Sun’s Declination Tables to determine their daily position on ocean voyages, using the reading of the sun’s altitude values they got from the astrolabe or quadrant.

As the Julian calendar had a cycle of 4 years, three common and one leap year, the most rigorous Tables had to be quadrennial. The calculations were made by astronomers from astronomical tables such as the Tables of the place of the sun from Abraham Zacuto’s Almanach perpetuum (Leiria, 1496). Astronomers calculated and sailors learned to read and use; sometimes they copied them for their notes or seaman’s books, other times they used printed editions that began to appear in Portuguese, as in the Évora Nautical Guide (Guia Náutico de Évora) (Lisbon, ca. 1516).

The four-year Tables presented the 1461 days of the four years of the Julian cycle. There, the month of February of the “leap year” actually has 29 days, but without the dating format and Roman numerals by Calends, Nones and Ides, only with the series of days of the month written in Arabic numerals. In most handwritten documents — Livros de marinharia, Atlas and Códice Bastião Lopes - we find the quadrennial Tables of the Sun’s declination. Let's look at the “Treaty of the Sphere” section in Évora Nautical Guide, printed in Portuguese:

“Treaty of the Sphere” in Évora Nautical Guide (Lisbon, ca. 1516), BPE, Res 0404, fol. Biv, detail.

This Guide includes the values of the “Place of the Sun” and “Solar Declination”. On the 23rd of February of the leap year (1st column, 3rd row), it indicates the place of the leap year (lugar do bis.) and the vigil of Saint Matias, but the last day of the month is the 29th (1st column, 9th row), without a Sunday letter , only with the values of the place of the sun and the declination of the sun (9th row, columns 4th to 7th). In traditional Julian calendars, the leap day took the place of 24th of February, Saint Matias’ day moved to the 25th of February, and the month retained its count of 29 days.

Here, the day itself 29th of February is writen, but without a Sunday letter (or, if we want we could repeat the Sunday letter from the previous day, ‘c’). Only the values in degrees and minutes of the ‘Place of the Sun’ and the ‘Declination of the Sun’ are displayed. The “leap place” of the traditional Julian calendar, which repeated the 24th of February, no longer makes sense because the “added day” is now the 29th of February, as we use it today in the most diverse calendars, printed in diaries or electronically. [José Madruga]