SCIENCE20 - The Phases of the Moon

This science blog is about the phases of the Moon.  Night after night we see the Moon take different shapes in the sky.  These different shapes are called the phases of the Moon. 

How Moon phases occur, and how they are associated with eclipses of the Sun and the Moon, is somewhat complex.  I took on this scientific subject as a challenge to try to explain it coherently so that I understood it (always my first goal), and so that nontechnical readers would appreciate the details of these lunar phenomena.  I will leave it to the reader to decide if I was successful.

First, I will talk about the basic orbital geometry of the Sun-Earth-Moon system that produces the phases of the Moon.  Then I will identify the eight phases of the Moon, when they occur, and what causes them.  Next, I will talk about how lunar phases are related to eclipses of the Sun and Moon.  I will conclude by noting how the phases of the Moon affect life on Earth.

I will list my principal sources at the end.

Note:  For a broader discussion of “Our Moon and its Mysteries,” see my December 2, 2021 blog at https://bobringreflections.blogspot.com/2021/12/science5-our-moon-and-its-mysteries.html

 

Orbital Geometry of the Sun-Earth-Moon System

Let’s get to some basics about how the Earth and Sun move in their orbital paths, and how this relates to the phases of the Moon.  The schematic below shows the relationship of the Earth’s orbit around the Sun and the Moon’s orbit around the Earth - both orbits roughly in the same plane.  The Earth orbits the Sun once a year in a near circular orbit, at an average distance from the Sun of 93,000,000 miles.  The Moon orbits the Earth once a month on a near-circular path, at an average distance of 240,000 miles.  We only see one side of the Moon because it rotates completely around its axis in exactly the same time as it takes to orbit the Earth (more on this geometry later). 

Schematic of Sun-Earth-Moon orbital geometry.

Although the diameter of the Moon is about 400 times smaller than that of the Sun, it is also about 400 times closer to Earth.  That means that the Moon and the Sun appear to be approximately the same size in the sky as viewed from Earth. 

The Moon does not produce its own light.  There is only one source of light in our solar system, and that is the Sun.  Without the Sun, our Moon would be completely dark.  What we refer to as “moonlight” is just sunlight reflecting from the Moon’s surface.

Note:  Sunlight also reflects off the Earth to the Moon, providing a faint glow to dark areas of the Moon.

The part of the Moon that we see illuminated changes over the month in a regular pattern of phases.

Sometimes the entire face glows brightly.  Sometimes we only see a thin crescent.  Other times the Moon seems to disappear entirely.  So, what are these lunar phases and what causes them?

 

Lunar Phases

The part the of Moon that we see illuminated changes over the month in a regular pattern of phases, caused by the changing positions of the Earth and Moon in their orbits. 

In Western Culture, we divide the month into eight Moon phases:  four primary and four intermediate phases.  The primary lunar phases are: the new moon, first quarter, full moon, and last quarter.  These primary Moon phases, along with the intermediate phases, are shown on the chart below. 

Schematic diagram of how phases of the Moon occur.

In the figure, sunlight is shown coming in from the right.  The Earth is at the center of the diagram.  The inner ring shows the Moon at eight key stages during its orbit around the Earth.  The Moon phase name is shown alongside the image.  The dotted line from the Earth to the Moon at each lunar position represents your line of sight when looking at the Moon from the Earth.  The large Moon images on the outer ring show what you would see at that point in the cycle. 

Following the diagram, the new moon occurs when the Moon is positioned between the Earth and Sun.  The three celestial bodies are in approximate alignment (why approximate is explained below).  The entire illuminated portion of the Moon is on the back side of the Moon, the half that we cannot see.

At a full moon, the Earth, Moon, and Sun are also in approximate alignment, just as the new moon, but the Moon is on the opposite side of the Earth, so the entire sunlit part of the Moon is facing us.  The shadowed portion is entirely hidden from view.

The first quarter and third quarter phases, happen when the Moon is at a 90-degree angle with respect to the Earth and Sun.  So, we are seeing exactly half of the Moon illuminated and half in shadow. 

The intermediate phases between these four key Moon phases show the illuminated portion of the Moon gradually transitioning between the principal phases.

As the Moon moves in its orbit from the new moon position, a small sliver of the Moon becomes visible. It takes a crescent shape because of the Moon’s roundness; we only see the edge of the part that’s illuminated by the Sun, and that edge is on a round object so it appears curved.  In this intermediate phase, the sunlit portion of the Moon is increasing and is called the waxing crescent phase, where waxing means growing or expanding in illumination, and crescent means that less than half of the Moon is illuminated.  In the Northern Hemisphere, we see the waxing crescent as a thin crescent of light on the right.

As the Moon continues beyond the first quarter position, the sunlit part of the Moon is still increasing in the waxing gibbous intermediate phase, where gibbous means that more than half of the Moon is illuminated.

As the Moon moves beyond the full moon position, the sunlit part of the Moon begins to decrease in the waning gibbous intermediate phase, where waning means shrinking or decreasing in illumination.

Finally, as the Moon moves beyond the third quarter position, the visible sun-lit part of the Moon continues to decrease in the waning crescent intermediate phase, decreasing in illumination until reaching the new moon primary phase.  In the Northern Hemisphere, we see the waning crescent phase as a thin crescent of light on the left.

The figure below shows the progression of the phases of the Moon.

This video shows the progression of Moon phases over a month.

The lunar cycle repeats on average every 29.5 days.

So, the basic explanation of lunar phases is that they are created by changing angles (relative positions) of the Earth, Moon, Sun, as the Moon orbits the Earth.

 

Solar and Lunar Eclipses

Solar and lunar eclipses are special cases of lunar phases.  A solar eclipse occurs when the Moon is between the Sun and Earth, and the Moon's shadow falls on the Earth.  Viewers on Earth in the full shadow of the Moon will see a total solar eclipse, while viewers in the partial shadow region will see a partial solar eclipse.

A lunar eclipse occurs when the Earth is between the Sun and the Moon, and the Earth's shadow falls on the Moon.  Again, viewers on Earth in the full shadow region will see a total lunar eclipse, while viewers in partial shadow, will see a partial lunar eclipse.

Definition of solar and lunar eclipses.

Looking back at the diagram of how the Moon’s phases occur (and imagining it to scale), you may have wondered why, at a new moon, the Moon doesn't block the Sun (solar eclipse), and at a full moon, why the Earth doesn't block sunlight from reaching the Moon (lunar eclipse).  The reason is because the Moon's orbital plane about the Earth is inclined about 5 degrees from the Earth-Sun orbital plane.  Most of the time the Moon is either above or below the Sun, and the three celestial bodies don’t “line up” for eclipses.

The Moon’s orbital plane is inclined about 5 degrees from the Earth-Sun orbital plane.

However, at unique times during the year, the Earth, Moon, and Sun do in fact "line up.”   A solar eclipse can only happen during a new moon phase.  A lunar eclipse, and can only happen during a full moon phase.  This is shown in the figure below.

Orbital and Moon phase conditions for solar and lunar eclipses.

The plane of the Earth orbiting the Sun is shown at the center of the diagram.  The Earth is shown at four positions in its orbit, along with a depiction of the Moon’s orbit around the Earth inclined at 5 degrees.

If the Moon’s path crosses the plane of the Earth's orbit around the Sun while the Sun, Earth, and Moon are aligned at new moon, a solar eclipse occurs on the day side of Earth.  If the Moon’s path crosses the plane of the Earth's orbit around the Sun, while the Sun, Earth, and Moon are aligned at full moon, a lunar eclipse happens on the night side of Earth.  These favorable geometric conditions for eclipses are shown at the top and bottom of the diagram.

Unfavorable conditions for eclipses are shown at the left and right of the diagram.

A total lunar eclipse occurs where the Moon passes completely through Earth's shadow; all of the Moon appears a faint reddish-brown.  A partial eclipse occurs when only part of the Moon passes through Earth's shadow, so only part of the Moon appears reddish.

Moon’s reddish tint during total eclipse.

Partial lunar eclipses occur on average one or two times a year.  A total lunar eclipse occurs once every 2.5 years.  A lunar eclipse usually lasts a few hours, with totality (the duration of total obscuration of the Moon) ranging between 30 minutes to over an hour.

Note:  Pat and I viewed a total eclipse of the Moon on September 28, 2015, while vacationing in Phoenix, Arizona.

There are two to five partial solar eclipses each year, with a total eclipse taking place every 18 months or so.  Whether you can view that eclipse depends on where you are in the world.  Total solar eclipses last only a few minutes.

 

How Moon Phases Affect Life on Earth

The Moon has long been important in our culture and life.  Since prehistoric times, many cultures have personified the Moon as a deity.  The Moon's regular phases made it a convenient timepiece, and the periods of its waxing and waning formed the basis of many of the oldest calendars.  Even today, many festivities celebrate the Moon, particularly the “Full Moon” in the autumn called “Harvest Moon.”  

For millennia, the phases of the Moon have been followed to plant crops, using the general rule of thumb: during the light, plant what needs light.  This means during the moon's lighter phases - the two weeks between the new moon and the full moon, when moonlight is increasing - plant crops that grow above the ground, where they see the light.

In mystical practices, the different phases of the Moon are often used to symbolize stages of growth or the process of manifesting desires.  As a source of light in the darkness, the Moon can symbolize illumination, insight, and guidance.

Astrologists believe that understanding the Moon phase, or where it is in relation to the Earth over its recurring cycle, is key to dictating our mood and energy levels at that point in time.  In addition, many astrologers consider the Moon phase at birth to capture the emotional tone of one's life.  Someone born on a full moon, for example, will be open and expressive, while someone with a trailing-off waning crescent will value solitude.

For many animals, particularly birds, the Moon is essential to migration and navigation.  Others will time their reproduction to coincide with the specific phases of the lunar cycle.

 

 

 

Sources

My primary sources include: “The Phases of the Moon explained,” planetary.org; “The Phases of the Moon,” timeanddate.com; “Lunar Phases and Eclipses,” science.nasa.gov; “phase,” britannica.com; “Understanding The Moon Phases,” moonconnection.com; “What Are the Moon’s Phases?” spaceplace.nasa.gov; plus numerous other online sources.