Moon Phases and Eclipses

Click on the photos to see larger images and additional information.


One of the most beautiful things in the night sky is also the easiest to see:  the Moon.  Nearly everyone has seen the Moon and noticed how it changes shape each month, sometimes appearing full and bright, other times disappearing altogether.  When ancient peoples of all cultures saw the Moon cycle, they did not understand it, so they invented myths.  While these myths are still fun to read, humans have now walked on the Moon, flown around it, and learned what really causes its fascinating phases and occasional eclipses. 


Understanding the processes that cause Moon phases and eclipses can be difficult, since it requires understanding light and shadow, and being able to imagine points of view that we never experience on Earth.  

Many people have the misconception that the phases of the Moon are caused by the earth's shadow across the Moon.  This is not true.  When the shadow of Earth moves across the Moon, we call it a lunar eclipse.  When the Moon moves between Earth and the Sun, it casts a small shadow on Earth, and people inside that shadow see a solar eclipse.  Both these events are rare, happening only about every 6 months.


Moon phases are the result of the Moon's own shadow -- like Earth, only one half of the body is lit at a time.  Sometimes the lit side is pointed toward Earth, and we see the reflected light.  Sometimes the shadow or night side is pointed toward Earth, and we only see a crescent Moon.  When the entire unlit side is toward Earth, we experience a New Moon and can see no Moon at all.

Modeling the Phases of the Moon

Several web pages have useful animated or still images to help explain the Moon's motion around the Earth.  To explain this phenomenon to children, start with something familiar:  the appearance of the Moon.  Without showing them any photos, have them create a Moon image.  Students will almost certainly draw many different phases, and all will be correct Moon images.  Discuss their theories about why we see Moon phases.

Before modeling the Moon phases, see if your students can correctly order a series of Moon images (but remember -- because the lunar cycle repeats continuously, there is no correct "start" or "end" point).  


Styrofoam balls work well, since sticking a pencil in the bottom makes grasping them easy.  Only one Sun will be needed for the entire classroom, but every child or group of children will need an Earth-Moon set.  Only one Earth and Moon are required for a demonstration, and a globe may be used for greater detail.

Modeling Activity

Begin by holding a round ball in a dark room.  This is Earth.  Now shine a single light at one side of the ball.  This is the Sun, and the side of Earth facing the Sun is in daylight.  Now rotate the ball counterclockwise, turning the side that had been day until it is in shadow, experiencing night.  Imagine that you are in one point on the ball (placing a sticker on one spot may help) and looking out at the sky.  Turning slowly, decide when you see daylight, and when it is dark around you.  When is sunrise?  Sunset?

Hold a smaller ball beside the Earth, near its equator (but not so close that it is ever in Earth's shadow).  This is the Moon.  Notice that the Moon, like Earth, has a day side and a night side.  Move the Moon around Earth.  Does is always have a day side and a night side?  The Moon does not make its own light, it only reflects light from the Sun.  The night part of the Moon is so dark that on Earth we cannot see it all -- we only see the day side.


Now imagine that you are standing on Earth.  Hold the Moon behind the Earth, as in the Full Moon image above.  Half the Moon is in daylight, half is dark, but all the night part is pointed away from Earth.  You don't see a round ball in the air, you see a flat circle, like a photograph.  When the Moon is full, you see all of the daylight side.

Now hold the Moon beside the Earth, as in the First Quarter image above.  Part of the Moon is still in shadow, and part of the shadow is pointing at you!  You see only see half a circle, or a quarter Moon.  Try creating the other phases of the Moon by moving the Moon around the Earth.

Extending the Activity

Think about when you see different phases of the Moon.  The Full Moon happens when the Moon is on the opposite side of the Earth from the Sun.  So, to see the Full Moon, you must be experiencing night.  What phase of the Moon do you see when the Moon is between the Earth and the Sun?  The New Moon, which we cannot see because only the night side is pointed toward Earth, is in the sky at the same time as the Sun -- daytime.

When you move the model Earth and Moon around your classroom, they go as fast as you want.  In the sky, though, they move slowly and predictably.  The Earth turns every 24 hours -- that is one day.  The Earth orbits the Sun every 365.25 days -- that is one year (the decimal part of the day is the reason we have leap years every four years).  The Moon circles Earth every 29.5 days -- that is about one month.  The word for month even comes from the word Moon.  (For fun, try saying "moonth" instead of "month".)  The Moon also turns around, but its "day" is a whole month long.  Because it takes as long for the Moon to orbit the Earth as it does for the Moon to rotate once, the same face of the Moon is always pointed at the Earth.  Humans had not seen the "far side of the Moon" until we were able to send spacecraft behind it.  Remember, though, that the FAR side of the Moon is not always the DARK side of the Moon.

  Far side of the Moon

Observing the Moon

Although the Moon is most impressive when full when its light alone brightens a dark night, it is still quite exciting to see the Moon during the day.  Use a calendar or this U.S. Naval Observatory site to determine what phase of Moon is currently visible.  Crescent Moons are easily visible mid-day, and even quarter Moons are in the sky during school hours.  Binoculars or a small telescope will reveal more detailed features, such as mare, but are not required.  The Astronomy Society of the Pacific has a chart detailing when and where different phases of the Moon are in the sky (look at the bottom of the page).

Images of the Moon and Moon Phases  This animation program allows you to freeze images at particular phases of the month.  Set point of view to "Both" for the most informative viewing.  U.S. Naval Observatory site about phases of the Moon, including a movie of the Moon progressing through one complete lunar cycle (one month).  This interactive animation combines three perspectives:  a view from the side of the Earth and Moon traveling around the Sun, an image of the Moon as seen from Earth, and a diagram of the Earth and Moon from over the North Pole.  The images can be controlled easily with forward and back buttons, but may move too quickly and be confusing when animated.  The Earth and Moon Viewer invites you to view our planet and its Moon as they look now from many different perspectives:  from the Sun, from the Moon, on the night side, on the day side, from a satellite, etc.  Use this U.S. Naval Observatory site to determine what phase of Moon is currently visible  See an image of the Moon as it will appear at the time and date you select.  The Astronomy Picture of the Day library of Moon shots, including pictures of the Apollo missions.

More Moon links are on the activity page Creating Craters.

About Moon Phases  Detailed instructions for observing and understanding phases of the Moon using spheres to model the motion of Earth, Moon and Sun in a classroom.  Includes downloadable worksheets and quizzes.  The Jet Propulsion Laboratory's Educator's Guide to Moon Phases describes a modeling activity similar to the one on this page in which students use their own head as Earth.  An online Moon quiz!  Match phases of the Moon to their names.  In addition to descriptions of the Moon's phases, the bottom of this page includes a chart describing when different phases of the Moon rise, set, and are highest in the sky.  This is useful for planning a class's Moon observations.  Mythology of the Moon from many cultures.

Eclipse Links  The Eclipse Page maintained at NASA/Goddard Space Flight Center has eclipse articles and maps for upcoming eclipses.  Sky and Telescope Magazine gives detailed information about upcoming eclipses and tips for safely viewing solar eclipses.  The Jet Propulsion Laboratory's Educator's Guide to Eclipses.  The Exploratorium's web site has eclipse myths, scientific information, and photos of recent eclipses.  The U.S. Naval Observatory provides information about rise and set times for the Sun and Moon, upcoming eclipses, solstices, and other interesting astronomical events.  Although "Mr. Eclipse" specializes in eclipse photography, he also has information about how eclipses work, how to safely view them, and when and where to see upcoming eclipses.

2001 Challe Hudson

This web page was produced for Morehead Planetarium during a North Carolina State University astronomy class taken in pursuit of a Masters in Science Education.