The term 'moon periods' refers to the different PHASES of the moon, which are determined by the relative positions of the Earth, sun, and moon. The most common PHASES are new moon, waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, third quarter, and waning crescent. Each phase lasts for a few days and is characterized by a particular shape of the illuminated portion of the moon's surface. The cycle of PHASES repeats itself every 29.5 days.