Joe Shmoe from Kfar Saba’s Big Moment: Art of the Calendar IV

Why does G-d require that we have the testimony of witnesses who have seen the new moon in order to establish Rosh Chodesh -- shouldn't the astronomical calculations suffice?

Bonus: chasidic aphorisim from one of the hidden tzadikim sheds new light on the Kohen's blessing before the priestly blessings.