What are the “Moon’s Phases”?

26 June 2014

The Short Answer (TSA)

            The “Moon’s Phases” are the shapes of the sunlit part of the Moon’s face as seen in the night sky.  I say shapes because there are a number of different shapes taken by the sunlit part of the Moon’s face during the regular cycle of phases.  We pass through a full cycle of phases about every 29.6 days.

The Moon's Phases (Northern Hemisphere)

            The cycle of Moon’s phases begins with the “New Moon,” a time when the Moon can’t be seen at all in the night sky.  Of course, the Moon is still there, but to a person watching the sky from the Earth, no part of the face of Moon is reflecting any of the Sun’s light during the New Moon. 

            After the “New Moon” phase, the first visible reflected light appears on the Moon’s face as a tiny sliver in the shape of a crescent.  The Moon is a “Crescent Moon” as long as “some, but less than half,” of the Moon’s face is sunlit. 

            After the first light appears on the face of the Moon, the sunlit part of the Moon’s face continues to increase or get bigger.  When the size of the sunlit part of the face of the Moon is increasing, the Moon is called “waxing.”  So, that first “Crescent Moon” after the “New Moon” is called the “Waxing Crescent Moon.”     

            The “First Quarter” phase is reached when one half (1/2) of the Moon’s face is sunlit.  The name “quarter” causes some confusion. If half the Moon’s face is sunlit, why is it called a “quarter?”  In the name, “First Quarter,” the “quarter” refers to time instead of the shape of the sunlit part of the Moon’s face.  At the time of the “First Quarter” the Moon is one quarter of the way through one full cycle of phases. 

            When “more than half, but less than all” of the Moon’s face is sunlit, the phase is called a “Gibbous Moon.”  And, when the sunlit part of the Moon’s face is increasing toward a “Full Moon,” the phase is called the “Waxing Gibbous Moon.” 

            When the sunlit part of the Moon’s face increases until the whole face of the Moon is sunlit, we have “Full Moon.”  After the “Full Moon,” the sunlit part of the Moon’s face begins to get smaller.  This decreasing in size is called “waning.”

            After the “Full Moon” phase is over, “more than half, but less that all,” of the Moon’s face is sunlit.  So, again, we have a “Gibbous Moon” phase.  But during this “Gibbous Moon” the sunlit part of the Moon’s face is getting smaller instead of getting bigger.  So, this phase is called the “Waning Gibbous Moon.”

            When the sunlit part of the Moon’s face has decreased to the point that only one half (1/2) of the Moon’s face is sunlit, we have the “Third Quarter Phase.”  Again, “quarter” refers to time.  So, at the “Third Quarter,” the Moon is three quarters (3/4) through the complete cycle of phases. 

            When the sunlit part of the Moon’s face gets smaller than a half – so that “some, but less than half,” of the Moon’s face is sunlit, the “Crescent Moon” phase is back, again.  But, because the sunlit part of the Moon’s face is getting smaller, this is the “Waning Crescent Moon.” 

            The sunlit part of the Moon’s face continues to get smaller until the Moon seems to disappear
from the sky.  Then, the first phase, the “New Moon,” is back, again, and the cycle starts over.

            Before modern calendars, clocks and watches, the Moon’s phases were used to tell time.  The “Full Moon” phase got a number of colorful names depending on the season of that particular “Full Moon.”  The “Harvest Moon,” for example, is the “Full Moon” phase around the time farmers harvest crops in the fall of the year. 

            The “New Moon,” also got some colorful names because the Moon seems to disappear.  The “Dark Moon” refers to the “New Moon.”   And certain “New Moon’s” are called “Black Moon’s.”

            The “New Moon,” “First Quarter,” “Full Moon” and “Third Quarter” are often called the “major phases.”  The major phases are most often seen on modern calendars.

            The minor phases include the Waxing or Waning Crescent and Gibbous Moons.