Moon Phases: 3-dimension diagram

It is a rather well-known fact that many school and college students and adults with various degrees of education have difficulties of understanding the moon phases phenomenon. Probably, the best report on that was a documentary made about Harvard University graduates (and even some professors) "A Private Universe".

In my view, the problem is caused mainly by misleading two-dimensional diagrams that are used in school science textbooks that trying to explain the moon phases phenomenon. To assist my own college students (future teachers) to grasp the phenomenon, I tried to find alternative diagrams, reflecting the three-dimensional nature of the moon phases phenomenon, on the Internet using the Google search engine in February 2008. However, after reviewing about 500 first pages, I gave up in my hope to find a 3-dimensional diagram. Here, I present my effort to develop one (I would appreciate anyone who directs me to a better representation).

But, first let me discuss why I so dissatisfy with traditional two-dimensional diagrams of the moon phases explanation. Let consider the following and rather typical 2-dimensional diagram that I found on the Internet:

Typical misleading 2-dim representation of moon phases
Figure1. Typical misleading 2-dimension representation of moon phases

I argue that this two dimensional diagram is perceptionally misleading, for at least, three reasons:

  1. Based on the diagram, it is difficult to understand, why in the full moon phase the Moon, being behind the Earth, is not obscured by the Earth from the Sun's light;

  2. It is difficult to understand why the lunar eclipse, when the Earth shadows the Moon, does not occur every lunar month;

  3. It is difficult to understand why the solar eclipse, when the Moon shadows the Sun, does not occur every lunar month.

While searching on the Internet, I found that many (but not all!) web pages, addressing the moon phases phenomenon, discuss the 5-degree tilt between the Moon-Earth orbit and the Earth-Sun orbit. However, the diagrams that the authors of the web pages used do not reflect this fact. For people who understand well the three dimensional nature of the phenomenon, the two dimensional diagrams are useful reminder of the moon phases explanation. In this sense, the two-dimensional diagrams, like Figure 1, are not wrong. If one can imagine another plain for the Earth-Sun orbit that does not belong the plain of the Moon-Earth orbit, then he or she can understand the moon phases phenomenon. However, for people who do not understand the 3-dimension nature of the phenomenon, 2-dimension diagrams, like Figure 2, are pedagogically misleading. In other words, the traditional 2-dimension diagrams are good for knowers but bad for learners.

I offer the following 3-dimension diagram of the moon phases phenomenon:

Three-dimension diagram of moon phases
Figure2. Alternative 3-dimension representation of moon phases

My alternative diagram of the moon phases phenomenon (Figure 2) tries to reflect 5-degree tilt between the Moon-Earth and the Earth Sun orbit (in an exaggeration way, of course). Although, my 3-dimension diagram looks more complex than traditional 2-dimension diagram (like on Figure 1), this complexity might hopefully invite more discussion among learners to reveal the three dimensional nature of the moon phases phenomenon. In my view, it helps better understand the full moon, the lunar eclipse, and the solar eclipse. I hope that public and educators can improve my 3-dimension diagram and put them on the Internet. Of course, 3-dimensional models can be better for grasping this idea than two dimensional pictures.

I found one interesting hands-on lesson, simulating and modeling the moon phases phenomenon, developed by Linda Shore and Paul Doherty:

Figure 3. The illuminated side of the moon appears as a crescent.

The full moon phase is simulated by the following position between "the Sun", "the Moon", and "the Earth":

Figure 4. Viewing the fully illuminated ball. The full moon.

Notice, please, that the full "Moon" to be seen has to be lifted up above the person's head ("the Earth"), reflecting the 5-degree tilt between the orbits.

What do you think?

Eugene Matusov, Ph.D.
Professor of Education
School of Education
University of Delaware, USA

Contact: ematusov_AT_udel_DOT_EDU (to avoid spam email flood L)

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