What is a “Waxing Gibbous Moon”?

25 June 2014

The Short Answer (TSA)

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

Waxing Gibbous Moon

“Waxing” means increasing.  So, the Moon is called a “Waxing Gibbous Moon” when the sunlit part of the face of the Moon is (1) increasing in size and (2) covers more than half, but less than all, of the face of the Moon

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.  The light on the Moon’s face increases until it reaches the “1st Quarter” phase when half 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 half the Moon is light, why is it called a “quarter?”  

The word “quarter” refers to time instead of to the light on the Moon’s face.  So, the “1st

Quarter” is one fourth (1/4th) 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 “1st Quarter Moon,” the Moon is called “Gibbous.” Again, this means that more than half, but less than all, of the Moon’s face is sunlit.  Because the sunlit part of the Moon’s face is increasing in size, this is not just the Gibbous Moon, but 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.”