What is the Gibbous Moon? | Almanac.com

What is the Gibbous Moon?

Waning gibbous Moon
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The waning gibbous Moon

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What is the Moon’s strangest appearance? It’s the “gibbous Moon.” After all, everyone can spot a crescent or a half or a full Moon. The gibbous Moon phase is fatter than half but smaller than full. See for yourself—tonight or the next several nights. Learn more.

What Does Gibbous Mean

The current lunar phase is the only one whose shape is not universally identifiable. It’s kind of football shaped.

As for its name, “gibbous” is an old fashioned word that means humped or swollen like the back of a camel, but it’s really not used anymore for anything but the Moon’s appearance.

The Waning Gibbous Moon

This gibbous Moon rises between nightfall and midnight, so it’s conspicuous in the middle of the night.

Specifically, this is a “waning” gibbous moon, i.e., it’s between the full Moon and the last quarter Moon. (When the Moon’s between the first quarter and full, it’s a waxing gibbous Moon.)

Learn why we call it a “quarter Moon” and not a “half Moon.”

See the Moon Phase Calendar by your local zip code.

Its light is interesting too. The half Moon, coming up next Wednesday, September 6 and bearing the official term “last quarter,” has about one tenth the brightness of the full Moon, which we saw last Wednesday, August 30. 

This alone may seem odd, since you’d think a half Moon would be half as bright as a full Moon. But the way the Sun bounces straight off the lunar surface like a highway sign makes the period within 2 or 3 days of full Moon much brighter than any other lunar phase. 

Put another way, the first few days of the waning gibbous Moon, at the start of this first weekend of September, we get a far brighter Moon than will appear from Sunday onward, since its brightness is now declining at the fastest possible rate.

Since the Moon currently doesn’t even rise until a few hours after nightfall, this waning gibbous phase is mostly seen by insomniacs or couples returning from a date. It’s not a widely observed lunar phase like the full Moon, which is out all night long. See Moonrise and set times.

But it does offer sideways illumination, with sunlight hitting it from the left, and this makes its craters and mountains stand out beautifully for anyone with a telescope. Anyone, that is, who’s willing to drag themselves out after everyone else has hit the sack.  

Learn more about the magical crescent Moon phase.

About The Author

Bob Berman

Bob Berman, astronomer editor for The Old Farmer’s Almanac, covers everything under the Sun (and Moon)! Bob is the world’s most widely read astronomer and has written ten popular books. Read More from Bob Berman