What is a “Full Moon”?

25 June 2014

The Short Answer (TSA)

            The Full Moon is that time in the lunar cycle when the Moon’s face is entirely sunlit.  At the Full Moon, the full round face of the Moon is visible in the night sky.

  Full Moon

            In astronomy, a “Full Moon” happens when the Moon reaches that place in its orbit in which it is farthest from the Sun.  The Full Moon is also the time when the Earth passes between the Sun and the Moon.  Put yet another way, at the Full Moon, the Sun and Moon are “facing” each other with the Earth in the middle.

            Although the Earth is, “more or less,” in between the Sun and the Moon during the Full Moon, very, very rarely does the Earth pass exactly in between the positions of the Sun and Moon.  So, if you were standing on the Moon, during most Full Moons, the Earth and the Sun would appear to pass by each other, with the Earth passing just over or under the Sun.  So, during a typical Full Moon, we might say that the Earth misses the Sun’s as it passes by.

            But, on those rare Full Moons when the Earth actually passes directly in front of the Sun, there is a Lunar Eclipse.  During a Lunar Eclipse, the face of the Full Moon suddenly becomes dark as the Earth passes in front of the Sun and casts a shadow that passes across the face of the Full Moon.  Lunar Eclipses can only happen at the moment of the Full Moon.     

            The term “Full 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.”

            But, let’s pause for a moment.  We need to clear up some confusion of about the “quarter” Moons. At the phases called the “1st Quarter” and the “3rd Quarter,” exactly one half of the Moon’s face is sunlit.  So, if half the Moon’s face is sunlit, why is it called a “quarter?”  

            The word “quarter” refers to time instead of to the light on the Moon’s face.  So, the “1st Quarter” is one fourth (1/4th) of the way through the time (29.6 days) it takes to complete a full cycle of the Moon’s phases: from the New Moon to the next New Moon.

            There are other “minor” phases, as well, including the “Crescent Moon” and “Gibbous Moon.”  When the sunlit part of the Moon’s face is getting larger, the Moon is said to be “waxing.”  And, when the sunlit part of the Moon’s face is getting smaller, the Moon is said to be “waning.” 

Lunar Phases (Northern Hemisphere)

            Before our modern clocks and calendars, the Moon’s phases were used to tell time.  The bright Full Moons of different months and seasons were, sometimes, given special names like “Harvest Moon” – the Full Moon closest to the beginning of autumn.