Waning gibbous Moon. Image 15 of 27. Optical image of a waning (decreasing in apparent size) gibbous Moon 15 days into its 28-day cycle. The lunar phases arise as the Moon's orbit of the Earth shows the Earth-facing side moving into and out of the light of the Sun. This Earth-facing side of the Moon is locked in place by the gravitational influence of the Earth. This results in a lunar day that equals the length of the lunar Earth orbit. On a waning Moon the day-night line (terminator) marks the lunar sunset. The Moon is gibbous when it is more than half full. Taken in October 2001. A sequence of the monthly cycle of Moon phases is shown on images R340/644-670.