19 June 2014
The Short Answer (TSA)
If you look up the word “crescent” in many dictionaries, you’ll find that the definition is “the shape of the crescent moon.” Most of us really don’t want to struggle with the mathematical/geometric definition of “crescent.” So, let’s try the astronomical definition.
Crescent Shape
In astronomy, a “crescent” is “the shape of the lit side of a spherical body that is less then half illuminated. So, the Moon is a “Crescent Moon” if some part, but less than half, of its face is sunlit.
The phases begin with the “New Moon” when the Moon disappears from the night. When the Moon reappears, only a small sliver of its face is sunlit. If you live in the Northern Hemisphere, the right edge of the Moon is the first sunlit part of the Moon to appear in the night sky after the New Moon.
(Waxing) Crescent Moon
If you live in Australia, Chile, Argentina, South Africa, etc., the “right-left” rule is reversed. In the Southern Hemisphere, the left edge is the first sunlit part of the Moon to appear in the night sky after the New Moon.
Now, back to the Moon’s phases.
Again, the Moon disappears at the New Moon. When it reappears in the night sky, it first appears with only a tiny sliver of light on its face. The tiny sliver is in the shape of a “crescent.” This is the phase called the “Crescent Moon.”
The size of that first crescent of light, the sunlit part of the Moon, will continue to grow until half the face of the Moon is sunlit. But a half circle isn’t a “crescent” shape anymore. So, when half the face of the Moon is sunlit, the “Crescent Moon” phase is over and the next phase, called the “1st Quarter” Moon, has come.
1st Quarter Moon
But this “quarter” Moon thing needs a little explanation.
If it’s a “quarter” Moon, why is “half” of the face of the Moon sunlit? Shouldn’t only a quarter of the Moon be sunlit at the “1st Quarter”?
Actually, when talking about the Moon’s phases, the “quarters” refer to time and not to the light on the face of the Moon. So, the “1st Quarter” is one fourth of the way through the full cycle of phases – from New Moon to New Moon. The “quarter” has nothing to do with the sunlit part of the Moon’s face visible in the night sky.
Just because the crescent phase ends with the “1st Quarter,” doesn’t mean the Crescent Moon won’t be back again. Not only will the Crescent Moon return again, but we won’t have to wait for the next round of phases. In every cycle of phases, there are two phases called the “Crescent Moon.”
After the Moon disappears at the New Moon, the “first” Crescent Moon begins with the sunlit part of the Moon’s face in the shape of crescent. The first slim crescent of light on the Moon’s face will continue to grow until it isn’t a crescent anymore. This growing Crescent Moon is called a “waxing” Crescent Moon. “Waxing” means increasing. The Moon is waxing whenever the sunlit part of its face is increasing in size.
So, the waxing Crescent Moon ends when the next phase, the “1st Quarter,” begins. After the “1st Quarter,” the sunlit part of the Moon’s face will continue to grow until the Moon’s whole face is sunlit — the Full Moon phase. After the Full Moon, the sunlit part of the Moon’s face begins to decrease.
“Waning” means decreasing. So, the Moon is called “waning” whenever the sunlit part of its face is decreasing in size. After the Full Moon, the sunlit part of the Moon’s face “wanes” or decreases. As it shrinks, the Moon reaches the phase called the “3rd Quarter.”
At the “1st Quarter” the sunlit part of the face of the Moon had waxed or increased to the half-way point. So, at the “3rd Quarter,” the sunlit part of the Moon’s face has waned or decreased back down to the half-way point.
After the “3rd Quarter,” less than half of the Moon’s face is sunlit. So, again, the sunlit face of the Moon takes the shape of a crescent. But this is a “Waning Crescent Moon” because the sunlit part of the Moon’s face continues to get smaller and smaller. Finally, the last sliver of light disappears from the face of the Moon. Then, the waning crescent moon ended and new phase, the New Moon, has come.
Moon Phases
With the New Moon, the cycle of the Moon’s phases begins again.
Mark Grossmann of Hazelwood, Missouri
Lots of Moons & Music 