What Causes Moon Phases?

Moon phases are caused by the motions of the Earth and moon as they relate to the sun. Phases occur as the Earth-facing side of the moon changes over the course of 29.5 days when the moon revolves around the planet. When the angle of sunlight reflecting off the moon changes, humans observe different lighting levels from the moon. These phases are full, waning gibbous, third quarter, waning crescent, new, waxing crescent, first quarter and waxing gibbous. Each phase lasts approximately seven days.