The Phases of the Moon

My son and I were gazing at the moon last night. My son's grade 4 class is currently studying the different phases of the moon. Although we regularly see the moon's appearance in the sky change over the course of every month, this concept is actually quite complex and misconceptions are quite common. For one, it is tempting to explain that the earth is casting its shadow over the moon. In one study involving preservice teachers, it is found that nine out of ten hold an alternative conceptual understanding of the reason behind the different phases of the moon.

The moon last night (11/16/2015) over Annandale, Virginia

There are videos available on the internet that attempt to explain the phases of the moon. Below is one example from the National Science Teachers Association: 

The video clearly states the necessary facts to help understand the different phases of the moon: First, the moon orbits the earth, and second, half of the moon - the one facing the sun is always illuminated. Unfortunately, the above does not address directly common misconceptions such as the earth casting a shadow on the moon. People, after all, are aware of eclipses.

Above copied from PBS Learning Media

To see that an eclipse is a unique configuration of the earth, moon and sun, the following fact must be introduced. The moon's orbit does not lie on the same plane as the earth's orbit around the sun:

Above copied from The Science Geek

Thus, clearing the misconception may actually require much more than just watching a video that attempts to explain the different phases of the moon. In a recent article in the Journal of Research in Science Teaching, one promising approach is offered: Have teachers design a "slowmation" of a model that explains the various phases of the moon. The authors use the following elements to decide whether a teacher has correctly understood this phenomenon.

One finding this study has made is that through this exercise, teachers begin to see the importance, for example, of keeping in mind the relative sizes of the earth, moon and sun. The following is the earth compared against the moon:

Above copied from Wikimedia

We often see the statement that the moon's diameter is about one-fourth of that of the earth. One fourth, however, is along one dimension only. In terms of volume, one fourth can easily translate into one sixty-fourth. Therefore, the volume of the moon is less than 2 percent of the earth's. In terms of mass, the moon is less dense so it is only 1.2 percent of the earth's mass. The comparison between the earth and the sun is much more dramatic.

Above copied from Wikipedia

In the above picture, the planet that one could clearly see is Jupiter. Earth is represented by the small dot right below Jupiter.

Understanding the phases of the moon is indeed complex. It requires attention to correct facts. Misconceptions may not be easily addressed without dealing with the complete story. Consequently, misconceptions can be easily handed down to students explaining in part their current preponderance in society.