Waning gibbous Moon. Image 19 of 27. Optical image of a waning (decreasing in apparent size) gibbous Moon 19 days into its 28day cycle. The lunar phases arise as the Moons orbit of the Earth shows the Earthfacing side moving into and out of the light of the Sun. This Earthfacing 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 daynight 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 R340644670.