Based on the above, we can suggest the following explanation:
Since the purpose of the mitzvah of Kiddush HaChodesh is to reveal that time exists for the sake of the Torah and for the sake of Israel — thereby transforming time itself into a Torah-defined reality — we can understand why the Rambam emphasizes that “the years we follow are solar years.”
The Divine intent is that this mitzvah should affect every dimension of Time. This includes not only the “month,” which is defined by the lunar cycle, but also the “year,” which is defined by the solar cycle — for the moon itself has no inherent concept of a year.
Consequently, it is not sufficient to calculate the tekufos (the seasonal markers) according to the sun merely {as a practical necessity — that is,} for the purpose of determining when to add a leap month so that Pesach and the month of Nissan fall in the spring.
{If this were the sole purpose, the role of the sun would be reduced to a secondary consideration,} relevant only as a technical requirement governing the months.
Instead, the year itself, in its entirety, must become a Torah-defined reality in its own right. By the Beis Din calculating the solar years themselves, the sun’s cycle is brought to the ultimate purpose for which it was created.
This understanding is reflected in the fact that the Torah contains laws and mitzvos that are directly linked to the solar year of 365 days. A {prime} example is the ketores, which was prepared {for the entire year} in the amount of “365 maneh... corresponding to the days of the solar {year}.”
{The Torah mandates this measure} every single year, even though, {in a regular lunar year of approximately 354 days,} a surplus of incense would remain, requiring redemption and removal from its sacred status.
{This demonstrates that} the year is {also} reckoned according to the solar cycle, in the literal sense.
