THE LUNAR  PHASES

The Moon is a natural satellite of our planet, Earth.  It revolves around Earth. completing a full cycle every 29.5 days.  In addition, the Moon spins on its axis, and like Earth, any particular spot on the Moon experiences day and night.  However, unlike Earth, which completes one rotation on its axis every 24 hours, the Moon rotates on its axis once every 29.5 days.  This is a wonder of nature, that the Moon orbits Earth in exactly the same amount of  time that it completes one rotation on its axis.  As a result, the same side of the Moon is always facing Earth. 

So, the Moon revolves around Earth in 29.5 Earth days ( a "lunar month"), while Earth is revolving around the Sun in its 365.25 day cycle (an "Earth year," of course.)  Both Earth and Moon, then, receive light from the Sun.  The Sun continuously lights up one half of each body, Moon and Earth, as each one rotates on its axis.

Now, while Earth is slowly revolving around the Sun, the Moon is more quickly revolving around Earth.  So, the three bodies are continuously changing position relative to one another.  If you were to stand in the same spot every night from sunset to sunrise, here is what you would experience:  On a certain night, as the sky is darkening after sunset on the western horizon, if you turned and faced the eastern horizon, looking for the Moon to rise, you would be disappointed.  The Moon would appear to be absent from the sky, not rising on that night.  That night is the beginning of the lunar phase cycle.  It is called New Moon.

 

In fact, of course, the Moon does rise on the night of New Moon, as it does every night.  But the Sun, the Moon, and Earth are aligned so that the Moon is between the Sun and Earth,  facing the Sun.  The Moon's "dark side" (from Earth) is being bathed in sunshine.  The New Moon seems to be absent from the night sky because at New Moon, both the Sun and the Moon rise and set at roughly the same time, and they both rise roughly from the same place on the eastern horizon and set in the same place on the western horizon.  So on the day and night of New Moon (and of the two days before and after New Moon), to us on Earth, the Moon is invisible because it is moving across the sky in generally the same place and at the same time as the Sun with its powerful light.  The Moon is invisible to us on Earth in the glare of this sunlight 

On the first night of the week following New Moon, the Moon remains invisible.   Then, shortly after sunset on the second night, the Moon appears in the western sky, hovering above the setting sun, as though observing the sunset, like a "sunset supervisor." 

Slideshow:  The First Week of the Lunar Cycle -- Waxing Crescent --  Scenic Views

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At first during this week, the Moon appears as a thin crescent, visible for only a short time after the glare of the Sun fades as the sunset turns into darkness.  Shortly after sunset, the Moon also sets.  But during the following nights of this week, the crescent Moon rises later and later, and it grows in thickness until on the seventh night, one-half of the disk of the Moon is illuminated.  The Moon at that point is called the First Quarter Moon because one-quarter of its orbit around Earth has been completed.  You can now see the half of the sunlit  face of the Moon which is nearest to Earth.

The week between New Moon and the First Quarter Moon is the week of the Waxing Crescent Moon.  (The word "waxing" here means "increasing or growing in size.")

Slideshow:  The First Week of the Lunar Cycle -- Waxing Crescent

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In the week following the First Quarter Moon, you would notice that the illuminated portion of the Moon continues to get larger.  What used to be the darkened half of the First Quarter Moon begins to be illuminated.  The flat division between the lighted half of the Moon and the darkened half at First Quarter Moon begins to bulge, so that the lighted part of the Moon bulges out more and more into the darkened half.  What began as a small bulge at the center  of the lunar disk becomes, night after night, an increasingly larger bulge, until the whole disk of the Moon is covered in sunlight.  This, of course, is Full Moon.

Slideshow:  The Second Week of the Lunar Cycle -- Views of the Full Moon

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What has been happening during the two weeks between New Moon and Full Moon is that the Moon has swung around Earth so that Earth is now directly between the Sun and the Moon.  That means that the setting of the Sun in the west ocurs at roughly the same time as moonrise in the east.  And as the Full Moon travels across the sky above you on the night of Full Moon, the Sun is travelling across the sky in the opposite direction directly beneath you, as though on a plane which originates in the Moon above you, passes through you, then passes through the center of Earth, and ends at the Sun.

 

(When the three bodies are alligned in every direction, the Moon, now dark to us on Earth because the Sun is lighting up the whole side of the Moon which we cannot see from Earth, the Moon passes in front of the Sun and blocksthe Sun's light from reaching Earth.  When that happens, we experience a solar eclipse, in which Earth is in the shadow of the Moon for an hour or more.)

Slideshow:  The Second Week of the Lunar Cycle -- Waxing Gibbous

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The shape of the Moon during this second week, in which one half of the Moon is completely illuminated and the other half bulges toward complete illumination, is called the Waxing Gibbous Moon.  The Moon is "waxing" because the bulge continues to grow  larger.  And the Moon is "gibbous" because that's the name of that bulging shape, just as "crescent" is the name of the shape of the Moon before the First Quarter Moon.

During the third week, you would notice that the Moon rises in the east later and later after sunset.  You would also notice that the Moon's full face begins to shrink on one side, and a gibbous moon forms.  The Moon clearly bulges less and less on one half until the bulge becomes completely flat, so that one half of the disk of the Moon is lighted, and the other half is in complete darkness.  This weeklong period is called the Waning Gibbous Moon.  (The word "wane" means "to grow smaller or dimmer.")

The half-moon that results from the week of Waning Gibbous Moon is called the Third Quarter Moon.  The Moon has completed three-quarters of its orbit around Earth.

 

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To summarize, the four weeks or phases of the lunar cycle are:

Week One, after New Moon:   Waxing Crescent Moon, ending in the First Quarter Moon.

Week Two:  Waxing Gibbous Moon, ending in Full Moon (equivalent to the Second Quarter Moon.)

Week Three:  Waning Gibbous Moon, ending in the Third Quarter Moon.

Week Four:  Waning Crescent Moon, ending in the New Moon.

During the fourth week, less and less of the Moon is illuminated.  The half-moon shape of the Third Quarter Moon becomes a thick crescent, and by the end of this week it is a thin crescent.  The cycle of lunar phases is complete, and the next phase cycle begins with the New Moon.