16 day old moon. Image 16 of 27. Waning (decreasing) gibbous Moon 16 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 a lunar sunset. The Moon is gibbous when it is more than half full.