The Talmud observes that Jews use a lunar calendar, while the nations of the world make use of the solar calendar. The reason, says the Sfat Emet, is because the nations of the world can only function and exist when conditions are favorable, and their environment is sunny. When their sun sets, so do their empires. Jews, on the other hand, live and exist even in darkness; they continue to shine even when persecuted and humiliated, just as the moon lights the world even in the thickest darkness. "Nothing is new under the sun" - The sun is constant in size, rising and setting at predictable times, always the same. The moon, on the other hand, is forever renewing itself; disappearing from view and then emerging anew, creating months - Chodashim. Months in the solar system are arbitrary, insignificant units of time. Lunar months give rise to birty, to new life. The feminine cycle approximates a month (the vest hachodesh being a lunar month) and pregnance is measured in months (tisha yarchei leidah).

- from Kos Eliyahu by R. Eliyahu Safran, pg. 31