What is a “3rd Quarter Moon”?

26 June 2014

The Short Answer (TSA)

The “3rd Quarter” is one of the “major” phases of the Moon.  At the “3rd Quarter,” one half of the face of the Moon is sunlit.

3rd Quarter Moon (Northern Hemisphere)

The sunlit part of the “3rd Quarter” Moon’s face is “waning” (decreasing in size). After the “3rd Quarter,” the sunlit part of the Moon’s face continues to wane moving to the next phase, the “Waning Crescent Moon.” 

Moon's Phases (Northern Hemisphere)

During the “Crescent Moon,” some part, but less than half, of the Moon’s face is sunlit.  And, during the “Waning Crescent Moon,” the sunlit part of the Moon’s face is getting smaller and smaller – until the Moon (or at least the sunlit part of the face of the Moon) disappears completely from the night sky with the next phase, the “New Moon.”  At the “New Moon,” one cycle of the Moon’s phases ends and another begins.

The “quarter” part of the “3rd Quarter” phase name, sometimes, causes confusion.  If one half of the face of the Moon is sunlit, why is it called a “quarter” instead of a “half?”  But the term “quarter” refers to time – not the light on the face of the Moon.  So, the “3rd Quarter” is three quarters (3/4th) of the way through the full cycle of the Moon’s phases.

After the Moon disappears from the sky at the New Moon, the cycle begins again with the “Waxing Crescent Moon.”  The Moon will appear as a small sliver of light in the shape of a crescent in the night sky.  The sunlit part of the face of the Moon will be increasing or “waxing” during that first “Crescent Moon” after the “New Moon.” 

The sunlit part of the Moon’s face will continue to “wax” or increase until the whole face of the Moon is sunlit at the Full Moon.  Then, the Moon is said to “wane” as the sunlit part of its face decreases in size passing, again, through the “3rd Quarter” phase and, again, disappearing with the New Moon.

By the way, if you see a half moon in the sky, there’s a way to tell, at a glance, whether it’s the 1st or 3rd Quarter Moon.  I can say that, if the right side of the Moon is light and left side dark, it’s the “1st Quarter.”  If the left side of the Moon is light and right side is dark, it’s the “3rd Quarter.”  And, as long as you are in the Northern Hemisphere, this is a good “rule.”

But, if you live in Australia, South Africa, Chile, Argentina, or parts of Brazil, this rule is reversed.  So, if you live in the Southern Hemisphere, the left side of the Moon is light and right side is dark at the “1st Quarter.”  If the right side of the Moon is light and left side of the Moon is dark, it’s the “3rd Quarter.”

3rd Quarter Moon (Southern Hemisphere)