He applied himself, also, to the adjustment of the calendar, not with exactness, and yet not altogether without careful observation. For during the reign of Romulus, they had been irrational and irregular in their fixing of the months, reckoning some at less than twenty days, some at thirty-five, and some at more; they had no idea of the inequality in the annual motions of the sun and moon, but held to this principle only, that the year should consist of three hundred and sixty days.
But Numa, estimating the extent of the inequality at eleven days, since the lunar year had three hundred and fifty-four days, but the solar year three hundred and sixty-five, doubled these eleven days, and every other year inserted after the month of February the intercalary month called Mercedinus by the Romans, which consisted of twenty-two days.
This correction of the inequality which he made was destined to require other and greater corrections in the future.
He also changed the order of the months. March, which had been first, he made the third month, and January, which had been the eleventh under Romulus, he made the first month; February, which had been twelfth and last, thus became the second month, as now. But there are many who say that these months of January and February were added to the calendar by Numa,
and that at the outset the Romans had only ten months in their year, as some Barbarians have three, and as, among the Greeks, the Arcadians have four, and the Acarnanians six; the Aegyptian year had at first only a single month in it, afterwards four, as we are told. And therefore, though they inhabit a very recent country,1
they have the credit of being a very ancient people, and load their genealogies with a prodigious number of years, since they really count their months as so many years.