- Lunar phase (Wikipedia)
A lunar phase or Moon phase is the apparent shape of the Moon’s directly sunlit portion as viewed from the Earth (because the Moon is tidally locked with the Earth, the same hemisphere is always facing the Earth). In common usage, the four major phases are the new moon, the first quarter, the full moon and the last quarter; the four minor phases are waxing crescent, waxing gibbous, waning gibbous, and waning crescent. A lunar month is the time between successive recurrences of the same phase: due to the eccentricity of the Moon’s orbit, this duration is not perfectly constant but averages about 29.5 days.