Waning gibbous Moon. Image 12 of 15. Optical image of a waning (decreasing in apparent size) gibbous Moon nineteen days into its 28-day cycle. The Moon is said to be gibbous when it is more than half full. The phases of the Moon are due to it revealing different amounts of its sunlit face as it orbits the Earth. Image taken on 21st June 2000. For a sequence showing the Moon throughout its cycle of phases, see images R340/596-610.