What is a waning gibbous moon? The waning gibbous moon, is defined to occur when the Moon appears to be more than one-half but not fully illuminated by direct sunlight. Because the fraction of the Moon’s disk that is illuminated is decreasing, we consider the Moon to be “waning.” Each new lunar cycle is measured beginning at each new moon and the waning gibbous moon occurs when the Moon is more than one-half but less than three-fourths of the way around the sky (i.e., along its orbit) from new moon.

Waning Gibbous Moon