sentences: a moon phase occurs 28 days after the last time it occurred
docs: half_moon is phase_of_moon. phase_of_moon is time_period. phase_of_moon is phase. january has 31_days moon has phases full_moon is phase_of_moon. moon's_orbital_period_around_earth is 28_days. blue_moon is long_time. august has 31_days new_moon is phase_of_moon.