User blog comment:KeisariApaSetä/About the moon/@comment-12333373-20180105153859

I believe there are some theories about several (6 or 7) full moons per month due to the amount of moons observed on the geocentric planetoid-model in the tree of knowledge (ohara). Could be more or less depending on the velocity of the moons as they have to have different distances to the main planet. I dont know if the number of moons has been confirmed by Oda, but as we have yet to see more than one moon in the sky at night I dont think its accurate (but could be).

Also, ca 1 full moon per month in reality is earth-moon specific and could be different in the op world, thus having extra full moons each month might be possible without having additional moons.