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

Taking up from where we left in our latest episode, and to draw some conclusions, we can see how the need for tables with astronomical references, such as those for the Sun’s Declination, led to a new representation of the month of February: in the three common years with 28 days, in the “leap” years with 29 days— actually switching the added or “leap” day to the end of the month, from being a duplication of February 24th, to the new and shiny 29th of February. But the result is the same: the month of February had 29 days every 4 years.

And even after the Gregorian Calendar Reform in 1582, we still have leap years and their model still has an intercalary day. Formerly, the leap day was introduced after February 23rd, now the leap day is added at the end of the same month of February. Formerly, February 24th was repeated and called the “leap day before the calends of March” (bis sextus ante calendas, remember?), but now the numbering of the days of February goes from 1 to 29 without interruption. It is as if there was no longer an intercalary “leap” day, but simply a month of 29 days, in which the 29th is an additional or “extra” leap day. The result is the same, as shown by the fact that intercalations at the end of the year were already used in the traditional Roman calendar and in Julius Caesar’s reform.

Perhaps this comes most naturally to us because we no longer organize the dates or the calendar in the manner of the Romans of the Republic or the Empire. But the basis of this solution, having years with days added every fourth, remains Julius Caesar’s because we prefer, like him, an annual solar calendar which follows the seasons, having equinoxes and solstices on (roughly) constant dates.

Here, in the Sun’s Declination Tables used by early modern mariners, we witness the transition from the medieval model of the month of February in the Julian calendar, still with Roman dates, to a month of February with the Indo-Arabic numerals we still use today.

Quadrennial table February page, from the Évora Nautical Guide, ca.1516.

This transition reflected, in some way, the transition from the use of Roman numerals to Arabic numerals, and the transition from handwritten to printed documents. The Évora Nautical Guide (shortly to be made available through our RUTTER Digital Library), may be among the first Portuguese calendars printed with the longer month of February including a 29th day—without a patron saint! [José Madruga]