Halakhah sa Mga Awit 104:19: Mishneh Torah, Sanctification & Arukh HaShulchan

And you have already seen from all of these actions, how many calculations are in it and how many additions and how many subtractions to the point that we have toiled much before we could produce approximate ways that do not have great sophistication to their calculations. For the [trajectory of the] moon is crooked. And therefore the Sages said (Rosh Hashanah 25a), "'The sun knows its way' (Psalms 104:19); the moon does not know its way." And the Sages [also] said, "Sometimes it comes long, sometimes it comes short." As you can see from these calculations that sometimes you add and sometimes you subtract until there is the arc of the sighting; and sometimes the arc of the sighting is long and sometimes short, as we have explained.

And according to the plain meaning, the reason that we bless the moon more than the stars/planets, although it's smaller than almost all the other planets (excepting Mercury), is because it's closer to the Earth than all the other stars/planets; it's only about 43,000 parasangs away. The moon inspires in us a feeling of the greatness of the Holy Blessed One, since every night its path is perceived, and it orbits the earth every month. For this reason it's called "yareach" [ירח], as it is written "[God] made a moon [ירח] to mark the seasons" (Psalms 104:19) -- since it orbits the earth in every moon of days, and its waxing and waning are perceptible. And Israel, who count [days] by the moon, are compared to the moon; just as the moon has no light of its own, and is illuminated by the sun's light, so too Israel have no light other than the Light of the Holy Blessed One, which shines for us from the midst of the Holy Torah, as it is written "A sun and a shield is Lord God" (Psalms 84:12). And just as the moon darkens and lightens, so too is Israel; just as we say in the blessing "and to the moon They said, 'renew yourself!' - a crown of glory to the womb-borne, for they are destined to renew themselves like it." And just as the moon exists even in the days of its diminishment [waning], when it seems dark on Earth -- it however is light on its other face from above, as is known; so too Israel. And for all of these reasons, Israel accepted this commandment with joy.