Answer:
The superstitious belief (related to astrology) was that the Moon, like other heavenly bodies, affected the behavior of people on Earth. The modern version of this is the observation that "when the moon is full, all the crazies come out."
Primitive and not-so-primitive cultures believed that the Moon had a direct effect on people's actions. The word lunatic comes from the Latin luna, and literally means "controlled by the moon." Although irrational actions were no more likely during a full moon than at any other time of the month, the superstition remained.