MOON: What is a “Gibbous Moon”?

19 June 2014

The Short Answer (TSA)

Most have heard of the “Crescent Moon,” and most of us know what a crescent shape looks like, but a “Gibbous Moon” doesn’t ring any familiar bells. There is a “Gibbous Moon” when more than half, but less that all, of the Moon’s face is sunlit.

Gibbous Moon

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 with the smallest sliver of light on its face.  That is the “Crescent Moon.”  The light on the Moon’s face increases until half the Moon’s face is sunlit.  That’s the “1st Quarter” phase of the Moon.

But, before we go any farther, 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.

The Moon’s Phases (Northern Hemisphere)

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.  There are two “Gibbous Moons” in each cycle of the Moon’s phases.

The first is called the “Waxing Gibbous Moon.”  “Waxing” means increasing.   So, the “Waxing Gibbous Moon” is the phase between the “1st Quarter Moon” and the “Full Moon,” when the sunlit part of the Moon’s face is increasing in size.

There will be another Gibbous Moon.  But the next one will be called a “Waning Gibbous Moon.”  “Waning” means decreasing.  The whole face of the Moon is sunlit at the Full Moon but, then, the sunlit part of the Moon’s face begins to decrease in size.

Just after the Full Moon, we have another Gibbous Moon because, again, more than half, but less than all, of the Moon’s face is sunlit.  But the sunlit part of the Moon’s face will continue to decrease.  So this is a “Waning Gibbous Moon.”  The “Gibbous” phase will end when only half of the Moon’s face is sunlit, the “3rd Quarter Moon.”