When bowing at “Barchu ess HaShem…” or at answering “Baruch HaShem hamevorach…” one should straighten up before saying HaShem’s Name.
K’lal Yisrael is compared to the moon. The moon reflects the light of the sun—we reflect the light of HaShem, of the Torah. The moon waxes and wanes—such is the history of Am Yisrael.
And so the moon’s rejuvenation at the start of the month is a sign for the rejuvenation of K’lal Yisrael, and therefore we are joyous and say Hallel on Rosh Chodesh.