-- Starting from nothing (New Moon), the moon's shape grows ('waxes')
for half of the cycle, until it's full, and then it shrinks ('wanes') for the next
half of the cycle.
-- The moon's complete cycle of phases runs 29.53 days . . . roughly
four weeks.
-- So, beginning from New Moon, it spends about two weeks waxing until
it's full, and then another two weeks waning until it's all gone again.
-- After a Full Moon, the moon is waning for the next two weeks. So it's
definitely waning at one week after Full.
