What is a “First Quarter” Moon?

26 June 2014 
The Short Answer (TSA)

            The “1st Quarter” is one of the major phases of the Moon in which half of the face of the Moon is sunlit. In the Northern Hemisphere, the right half of the Moon is sunlit at the “1st Quarter.” It’s just the opposite in the Southern Hemisphere with the left half of the Moon sunlit at the 1st Quarter.”

1st Quarter Moon (Northern Hemisphere)

The “1st Quarter” Moon is increasing in size — “waxing.”  After the “1st Quarter,” the sunlit half of the Moon’s face continues to increase in size until it reaches the next major phase, the “Full Moon.”  At the Full Moon, the whole face of the Moon is sunlit.

The “quarter” part of the “1st Quarter” phase name, sometimes, causes confusion.  If half the face of the Moon is sunlit, why is it called a “quarter” instead of a “half?”  Because the term “quarter” refers to time – not the sunlit part of the face of the Moon.  So, the “1st Quarter” is one quarter (1/4th) of the way through the full cycle of the Moon’s phases.

The major phases begin with the “New Moon” – the Moon disappears from the sky. Then, the Moon reappears increasing in size (waxing) until it reaches the halfway point – the “1st Quarter.”

After the “1st Quarter” phase, the sunlit part of the face of the Moon continues to increase (wax) until the Moon’s “full” face is sunlit – the “Full Moon” phase.

Then, after the Full Moon, the sunlit part of the Moon’s face begins to “wane” or decrease in size until, again, only half of the face of the Moon is sunlit.  That’s the next major phase, the “3rd Quarter” Moon.  The 1st and 3rd Quarter Moons look just a little different.  If you live in the Northern Hemisphere, the right side of the Moon’s face is sunlit at the 1st Quarter.  At the “3rd Quarter,” the other side — the left side of the Moon’s face is sunlit.

What is you live in the Southern Hemisphere?  The left-right rule is exactly the opposite.  The left half of the 1st Quarter Moon is sunlit, while the right half of the 3rd Quarter Moon is sunlit.

1st Quarter Moon (Southern Hemisphere)

After the “3rd Quarter” phase, the sunlit part of the Moon’s face continues to decrease in size (wane) until the Moon’s face disappears from the night sky.  When the Moon’s face disappears, the “New Moon” phase has come.  And, with the New Moon, a new cycle of the Moon’s phases begins.

The, so called, major phases are the “New Moon” (no Moon), “1st Quarter” (half Moon), “Full Moon” (whole Moon), and “3rd Quarter” (half Moon, again). After the 3rd Quarter the cycle starts over with the new Moon.

 The Moon's Phases

M Grossmann of Hazelwood, Missouri
(& Belleville, Illinois)
About the Author

26 June 2014

What is a “Waxing Crescent Moon”?

19 June 2014

 Waxing Crescent Moon (Northern Hemisphere)

The Short Answer (TSA)

            A Crescent Moon is one phase in the regular cycle of phases of the Moon.  The Moon is in the Crescent phase whenever part, but less than half, of the Moon’s face is sunlit.  “Waxing” means increasing.  So, the Moon is called a “Waxing Crescent Moon” when the sunlit part of the face of the Moon is (1) increasing in size and (2) appears in the shape of crescent.

            At the first phase, the “New Moon,” the Moon disappears from the night sky.  At the next phase, the Crescent Moon, the Moon reappears, but the sunlit part of its face as only a tiny sliver in shape of a crescent.  That tiny sliver of reflected light first appears on the right outer edge of the Moon in the Northern Hemisphere and on the left outer edge in the Southern Hemisphere.

            Night after night, the size of the sunlit part of the Moon’s face grows in size.  Again, because the Moon’s light is growing, it’s called a “waxing” Moon.  And because part, but less than half, of the face of the Moon is sunlit, it’s called a “Crescent” Moon.

            The “Waxing Crescent Moon” continues until the sunlit part of the Moon’s face increases to the half-way point.  The half-sunlit Moon isn’t a crescent any more.  So, the Moon with its face half-sunlit is in a new phase -- called the “1st Quarter” Moon. 

            In terms of phases, the waxing Crescent Moon comes after the New Moon and before the “1st Quarter” Moon.  Put another way, the Waxing Crescent Moon “phase” begins with the first appearance of reflected sunlight on the face of the Moon (in the shape of a thin crescent) and ends when half the face of the Moon is sunlit.

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What is a "Crescent Moon"?

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.    

M Grossmann of Hazelwood, Missouri
(& Belleville, Illinois)
About the Author

What is a “Waxing” Moon?

19 June 2014       

The Short Answer (TSA)

“Waxing” means to get bigger.  So, when the Moon is said to be “waxing,” it means that the sunlit part of the Moon is getting bigger.  The Moon goes through a complete cycle of “phases” every 29.6 days.

The cycle of phases begins with the New Moon, a time when the Moon cannot be seen in the night sky at all.  After the Moon disappears, it will reappear as a crescent – a small sliver of light on the right outer edge of the face of the Moon.  With each night, the sunlit part of Moon will increase in size until the face of the Moon is completely sunlit – the “Full Moon” phase.  From the time of the New Moon phase to the Full Moon phase, the Moon is said to be waxing.

In terms of phases, the Moon disappears at the New Moon.  Then, the Moon, first, reappears in the night sky as a “crescent,” a small sliver of light on the right side of the face of the Moon.  Next, at the “1st Quarter” phase, the right half of the Moon is sunlit and the left half is dark. The Moon continues to grow until the whole face of the Moon is sunlit – the Full Moon phase.

After the Full Moon, the sunlit part of the Moon begins to get smaller and smaller.  When the sunlit part of the Moon keeps getting smaller, the Moon is said to “waning” instead of “waxing.”

The light of the waxing Moon first appears on the right outer edge of the Moon
and moves to the left

Even if you don’t know or remember when the Moon was New or Full, there is a way to tell whether or not the Moon is “waxing” or “waning.”  If the Moon is sunlit from the right side, it is waxing.  If the moon is sunlit from the left side, it is waning.  In other words, if the right outer edge of the moon is sunlit, it’s waxing.  If the left outer edge of the Moon is sunlit, it’s waning.

M Grossmann of Hazelwood, Missouri
(& Belleville, Illinois)
About the Author
Thursday 19 June 2014              

*Note: The Moon actually takes 27.32 days to complete a single orbit — travel once completely around the Earth.  But the Earth is “gaining” on the Moon a little bit every day.  So, from the viewpoint of an observer on Earth, it takes about 29.53 days for the Moon to complete a full cycle of phases — for example, new moon to new moon.

Think of it this way. Suppose you are a runner. You can run a mile in 10 minutes. You find a mile long circular track and start running from the starting line. But, while you’re running, someone moves the finish line one tenth of mile farther down the track. Instead of 10 minutes, it will take you 11 minutes to reach the “new” finish line. You come back the next day and start running from the new start/finish line. But, while you are running, someone moves the finish line one tenth of a mile farther down the track – again. It will take you 11 minutes to reach the “new” finish line.

So, even though you run a mile in 10 minutes and the track is only a mile long, it will always take you 11 minutes to finish. This is because, as you are running, the finish line keeps moving farther ahead!

That’s what’s happening to the Moon

What is a "Solar Eclipse"?

19 June 2014

The Short Answer (TSA)

            “Eclipse” is an ancient Greek word meaning "the darkening of a heavenly body."  Two things must happen to produce a solar eclipse.  First, the Moon must move in between the Earth and the Sun.  Second, the Earth must move exactly into the shadow cast by the Moon.

            Solar eclipses don’t happen that often, but the Moon moves in between the Earth and Sun every month.  In fact, the monthly “New Moon” is the exact moment when the Moon is, more or less, in between the Earth and the Sun.  But if there’s a “New Moon” every month, why isn’t there a solar eclipse every month?

            There are two reasons.  First, the Moon casts a very small shadow and the Earth is a small target.  Second, the orbit of the Earth around the Sun and the orbit of the Moon around the Earth are not exactly “level.” 

            Before things get technical, let’s put it this way.  The Sun is too bright to give you a clear view.  But let’s say you could watch the Moon passing by the Sun every month at the “New Moon.”  Most often, you’d see the Moon pass over or under the Sun.  Once in a great while, you’d see the Moon hit the “target” exactly and pass in front of the Sun.  And, when the Moon did hit the target, you’d see a solar eclipse. 

            So, with most “New Moons,” the Earth, the Moon, or both are a bit above or below the “level” of the Sun.   Because the Moon casts such a small shadow and Earth is such a small target, just the slightest difference in angle will cause the shadow of the Moon to miss the Earth.

 Why Isn't There a Solar Eclipse Every Month?

            Again, once in a while, everything will line up, and you’ll see a solar eclipse.  And what will you see?   You’ll see part of the bright light of the Sun suddenly dim as a shadow moves across the Sun’s face.  The Moon isn’t big enough to block all the Sun’s light.  So, even when there is a full solar eclipse, the sky doesn’t become completely dark.  You can still see the bright outline of the Sun around the edges of the shadow of the dark Moon as it moves across the bright Sun.  

            In fact, because the Moon is so small, its shadow doesn’t cover the whole Earth.  That’s why each solar eclipse can “be seen” from some places on Earth, but not from others.   From some locations, you will see a full or “total eclipse” with the round shadow of the Moon visible against the Sun.  But, from other places, you’ll see a “partial eclipse.”    The Moon, in a partial eclipse, doesn’t pass directly over the Sun.  Instead the Moon’s shadow will “clip” the bright face of the Sun and cover only part of the top or bottom of the solar disk.  In other places, you might not see “any” eclipse at all.    

 Solar Eclipse

M Grossmann of Hazelwood, Missouri
(& Belleville, Illinois)
About the Author

19 June 2014

What is a "New Moon"?

19 June 2014

The Short Answer (TSA)

            The New Moon is that time in the lunar cycle when the Moon is not visible in the night sky.   The New Moon is sometimes called the “Dark Moon.”   In astronomy, the “New Moon” is that moment in the Moon’s orbit when it meets the Sun – the moment when the Sun and the Moon are in the same “orbital” position. 

            I say “orbital” position because, as seen from the Earth, only rarely are the Sun and Moon in exactly the same place in sky.  During most New Moons, the Sun and the Moon, as seen from the Earth, appear to “miss” each other with the Moon seeming to pass just above or below the Sun.

            On those rare New Moons when the Moon passes directly in front of the Sun, there is a solar eclipse.  A solar eclipse happens when, as seen from the Earth, the Moon passes across and in front of the Sun visibly darkening the “face” of the Sun and reducing the amount of sunlight reaching the Earth.

            The term “New Moon” also describes a formal “phase” of the Moon.  The “Phase of the Moon” is the shape of the sunlit part of the Moon as seen from Earth.   That shape changes, as the Moon goes through a full cycle of phases.  Each full cycle of the Moon’s phases takes about 29.6 days.  

            The Phases of the Moon you hear the most about are the “1st Quarter,” “Full Moon,” “3rd Quarter,” and “New Moon.”  By the way, the “quarters” refer to the cycle, not the sunlit part of the moon.  So, the "1st Quarter" is one quarter (or one fourth) through one complete cycle of phases.  But if you look at the 1st Quarter Moon, you’ll see that actually one half of the face of the Moon is sunlit. 

            Modern calendars use the symbol of a dark circle to refer to the new moon: 

            But there is another, older, meaning of the term "New Moon."  Originally, the time when the Moon disappeared completely from the night sky was not called the "New Moon."  Instead, the complete disappearance of the Moon was called the “Dark Moon.”  The Moon was only a “New Moon” when the first visible crescent of the Moon appeared in the night sky.

            Outside of formal astronomy, the original definition of the New Moon, the first appearance of the visible crescent moon after the “Dark Moon,” is still a common one.  The first crescent still marks the beginning of the month in the Buddhist, Hebrew, Hindu, and Muslim calendars.

M Grossmann of Hazelwood, Missouri
(& Belleville, Illinois)
About the Author

19 June 2014