A waning gibbous moon is an intermediate or secondary phase of the lunar cycle that starts right after the full moon and lasts until it becomes a third quarter moon (half-full moon).
During a waning gibbous moon, the illuminated area of the moon decreases from 99.9% to 50.1%. So the moon appears less than fully illuminated but more than half illuminated.
Meaning of the waning gibbous moon
A waning gibbous moon consists of two terms: waning and gibbous.
Here, “waning” means decreasing or shrinking. After a full moon, the illuminated area of the moon decreases day after day; that’s why it’s called a waning moon.
Another term, “gibbous,” comes from the Latin “gibbosus”, meaning “hump-backed”. During a waning gibbous moon, the moon looks like a “hump” shape or a “convex” shape on both sides.
So a waning gibbous moon signifies the shape of the moon.
The age of a waning gibbous moon
A full moon is seen in the sky about two weeks after the new moon, and a third quarter moon is seen in the sky about three weeks after the new moon.
So the age of a waning gibbous moon is greater than two weeks but less than three weeks.
A waning gibbous moon is best visible on the 3rd or 4th days after the full moon from the late evening through the early morning.
Rise and set time of a waning gibbous moon
A waning gibbous moon rises in the evening (between sunset and midnight) in the east, reaches its highest point (overhead point) in the sky after midnight, and sets in the morning (between sunrise and noon) in the west.
Please remember, a waning gibbous moon does not rise and set at the same time every day. On consecutive days, a waning gibbous moon rises and sets about 50 minutes later.
A waning gibbous moon is easily seen in the daytime
A waning gibbous moon is easily seen in the western sky in the morning, before the waning gibbous moon set.
As we see more than 50% of the lighted part of the moon during a waning gibbous moon, that’s why a waning gibbous moon is easily seen in the daytime, unlike a waning crescent moon.
A full moon immediately becomes a waning gibbous moon
A full moon immediately becomes a waning gibbous moon because a full moon has no duration.
However, you have to wait two or three days after the full moon to see a hump-shaped waning gibbous moon with your unaided eye.
Duration of a waning gibbous moon
The moon takes approximately 29.5 days to complete a lunar cycle. A lunar cycle has eight phases in total: the new moon, waxing crescent moon, first quarter moon, waxing gibbous moon, full moon, waning gibbous moon, third quarter moon, and waning crescent moon.
The new moon, first quarter moon, full moon, and third quarter moon have no duration as these are the primary or major phases of the moon.
So the duration of the four secondary phases (waxing crescent, waxing gibbous, waning gibbous, and waning crescent) is 29.5 days in total.
So a waning gibbous moon lasts around 29.5/4 = 7.3 days.
Read about all eight phases of the moon: New moon Waxing crescent First quarter Waxing gibbous Full moon Waning gibbous Third quarter Waning crescent
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