ust after Full Moon, when the face of the Moon is 100% illuminated, the intermediate phase called Waning Gibbous Moon starts.
Waning means that it is getting smaller. Gibbous refers to the shape, which is less than the full circle of a Full Moon, but larger than the semicircle shape of the Third Quarter Moon.
With some exceptions, the Waning Gibbous Moon rises after sunset but before midnight and doesn’t set until after sunrise.
During this period, the lit up portion of the Moon goes down from 99.9%% to 50.1%.
Technically, this phase starts as soon as the Full Moon moment has passed. However, it can be difficult to differentiate the first stage of a Waning Gibbous Moon from a Full Moon when as much as 98% to 99% of the Moon's surface is illuminated.
Sun Lights Up the Moon
The Moon does not radiate its own light, but the Moon's surface reflects the Sun’s rays. Half of the Moon’s surface is always illuminated by direct sunlight, except during lunar eclipses when Earth casts its shadow on the Moon. Just how much of that light we can see from Earth varies every day, and we refer to this as a Moon phase.
Primary and Intermediate Moon Phases
In Western Culture, we divide the lunar month into 4 primary and 4 intermediate Moon phases.
The Moon phases start with the invisible New Moon. The first visible Moon phase is the thin sliver of a Waxing Crescent Moon. Around a week later, half of the Moon’s surface is illuminated while the other half is in darkness at First Quarter Moon.
The illuminated part continues to grow into a Waxing Gibbous Moon, until 14 to 15 days into the cycle, we see the entire face of the Moon lit up at Full Moon.
The illuminated part then gradually shrinks into a Waning Gibbous Moon, and when it reaches Third Quarter, the opposite half from the First Quarter is illuminated. From there, it fades into a Waning Crescent Moon. Finally, the Moon disappears completely from view into another New Moon phase, only to reemerge and repeat this cycle over and over.