A lunar phase or Moon phase is the apparent shape of the Moon's day and night phases of the lunar day as viewed from afar. Because the Moon is tidally locked to Earth, the cycle of phases takes one lunar month and move across the same side of the Moon, which always faces 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.