What is a “Waning Gibbous Moon”?

26 June 2014

The Short Answer (TSA)

 A Gibbous Moon is one phase in the regular cycle of the Moon's phases.  The Moon is “Gibbous” when more than half, but less that all, of the Moon’s face is sunlit.

Waning Gibbous Moon (Northern Hemisphere)

“Waning” means getting smaller.  So, the Moon is called a “Waning Gibbous Moon” when the sunlit part of the face of the Moon is (1) getting smaller in size, but still (2) covers more than half, but less than all, of the face of the Moon.  The “Waning Gibbous Moon” is the second “Gibbous Moon” in the regular cycle of phases. 

Moon's Phases (Northern Hemisphere)

The Moon’s phases begin with the "New Moon" -- when the Moon disappears and can’t be seen in the night sky.  When the Moon “returns,” it first appears as a “Crescent Moon” with the smallest sliver of light on its face.  

This “Crescent Moon” is called the “Waxing Crescent Moon” because the sunlit part of the Moon’s face is getting bigger.  And “waxing” means getting bigger.  The light on the Moon’s face continues to increase, or “wax,” until it reaches the “1st Quarter” phase when half the Moon’s face is sunlit.

The sunlit part of the Moon’s face continues to get bigger.  After the “1st Quarter Moon,” more than half, but less than all, of the Moon’s face is sunlit.  So, this is a “Gibbous Moon.”  But during this “first” “Gibbous Moon” phase, the sunlit part of the Moon’s face is getting bigger, or “waxing.  So, this phase is called the “Waxing Gibbous Moon.”

The sunlit part of the Moon’s face will continue to increase until the whole face of the Moon is sunlit.  Then, the Moon will have passed from the “Waxing Gibbous” phase to the next phase, the “Full Moon.”

But right after the “Full Moon,” the sunlit part of the Moon’s face begins to get smaller – “Wane.”  And, again, more than half, but less than all, of the Moon’s face is sunlit.  So, the Moon is Gibbous again.  But, now, the sunlit part of the Moon’s face is getting smaller every night. So, this second “Gibbous Moon” is the “Waning Gibbous Moon.”

The sunlit part of the Moon’s face will continue to get smaller until it reaches the next phase:  the “3rd Quarter” when only half of the Moon’s face is sunlit. 

But, before we go any father, we need to clear up some confusion of about the “quarter” Moons. At the phases called the “1st Quarter” and the “3rd Quarter,” exactly half of the Moon’s face is sunlit.  So, if one half of the Moon’s face is light, why is the phase called a “quarter?”  

“Quarter” refers to time instead of to the light on the Moon’s face.  So, the “3st Quarter” is three-fourths (3/4th) of the way through the time (29.6 days) it takes to complete a full cycle of the Moon’s phases: from the New Moon to the next New Moon.

After the “3rd Quarter,” the Moon "wanes" to the next phase, the “Waning Crescent Moon.”  When the sunlight disappears completely from the Moon’s face, we have a New Moon, and the cycle begins again.