Based on the above, we can clarify a precise nuance in the Rambam’s wording at the beginning of Hilchos Kiddush HaChodesh:
The years we follow are solar years, as implied by the verse: “Keep the month of the spring.”
The straightforward meaning of this statement emerges from the Rambam’s opening ruling that “the months of the year are lunar months.” That is, a month is not merely a twelfth of a solar year, but an independent lunar cycle consisting of approximately twenty-nine and a half days.
The Rambam then notes that Pesach must occur in the spring, yet the seasons are governed by the solar cycle. Because a solar year exceeds a lunar year by approximately eleven days, the years must therefore be calculated according to the sun. Accordingly, every two or three years the Beis Din intercalates the year by adding an extra month, as the Rambam writes in the following halachah.
Indeed, he explicitly states in later chapters {of Hilchos Kiddush HaChodesh}: “These are the primary grounds for making a leap year, so that the years will follow the solar calendar.”
From the plain sense of the Rambam’s language, however, it appears that this requirement — {that the years be solar years} — is not only for the sake of the Festivals, i.e., to ensure Pesach falls in the spring. In fact, the Rambam does not even mention this objective in this halachah; he cites only (the {relevant} portion of the) verse, “Keep the month of the spring.” This suggests that it is a general rule concerning the year itself: that it must be a “solar year.”
The verse “Keep the month of the spring” is {cited} only as the scriptural basis {from which this general rule is derived}.
This, however, requires further explanation. Although over the course of the nineteen-year cycle as a whole the calendar is aligned with the solar year through the addition of intercalary months — and by the end of the cycle the discrepancy between the lunar and solar calendars is reduced to “one hour and 485 units” — {the reality of any individual year is quite different}.
In practice, no single Jewish year ever equals a full solar year of 365 days. {A regular year consists of} twelve lunar months — {approximately} 354 days, {while a leap year consists of} thirteen lunar months — {approximately} 383 days.
{If the year itself never actually matches the length of a solar year, how can the years be considered “solar years”?}
