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
readers 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 Moon
disappears from the sky at the New Moon.
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, southern India, 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.
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, is here.
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 happen 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.”
Again, the
Moon disappears at the phase called the New Moon. Then, the Moon reappears 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 it reaches the Full Moon
phase. At the Full Moon, the Moon’s
whole face is sunlit. 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” the sunlit part of
the Moon’s face goes back to the Crescent Moon phase. The Moon becomes a waning Crescent Moon because a part, but less than
half, of the Moon’s face is sunlit.
The waning “Crescent
Moon” gets smaller and smaller until the Moon disappears. That's the next phase:
the New Moon. With the New Moon, the cycle of phases
begins again.
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