It is perhaps odd, but quite true that when you ask most people to picture a planet, a moon, or the entire solar system, they tend to visualize a series of bodies frozen in place in a neat line as you might see on a classroom poster or a textbook illustration. Almost no one pictures moons and planets racing around in orbit, moving like horses careening around a track.

Even so, motion is one of the most fundamental qualities of our successful models of the solar system. Motion involves distance, time, velocity, and acceleration; it may be linear, circular, or even elliptical in nature. We’re going to skirt around all the math and physics that are implied in this and focus on one thing – movement! Our goal will be to get your students to incorporate movement into their fundamental mental picture of the solar system.

Activity 23: A Working Model of the Lunar Phases

We have looked at lunar phases before; this was one of our first activities, but we found that this model had flaws. While the Moon is round, our old clay model of the phases was completely flat. We also noted that while the old model predicted what was going to happen next with lunar phases, it was noticeably deficient in explaining how the phases worked or why they changed as they did. This helped our students to recognize that all scientific models have flaws and are incomplete in places.

Now it is time to create a new model, one that takes into account both the shape of the Moon, and its motion as it orbits the Earth. Our new model will also take light into account. The lunar phases are obviously a play of sunlight and shadow, so we will include the light from the Sun in our new model as well. It might seem at first glance that adding shape, motion, and the effects of a distant light source into our model would make it far too complex to understand easily – not so! The power of a good scientific model to explain and simplify is often greatly underestimated – as your students will soon show you!

For the Educator

Facts you need to know

1. Planets and moons are all in motion. Okay, this one seems obvious, but the implications of what that means when you are observing the cosmos from a spinning, orbiting platform are not as simple as they may seem.
2. Adding motion to our model of the Earth – Moon system will finally answer the “How do phases work?” question that has been nagging us throughout this book.
3. It is the very motion of the planets, and the invention of the astronomical telescope by Galileo, which allowed him to prove that the Sun-centered model of Copernicus was in fact correct, and the Earth-centered model of Aristotle and Ptolemy were wrong.

Teaching and Pedagogy

One of the more profound and difficult tasks we face when we start teaching students using physical models is making the transition from a static model to a dynamic one. Consider that many students today learn science from looking at pictures in a text or on a screen. It is rather shocking when one realizes how little activity based science occurs in most schools. We could endlessly ruminate about the causes of this state of affairs, but the point is that students (and many teachers) are completely unfamiliar with dynamic models.

A dynamic model in motion often helps create a wonderful ‘A-ha!’ moment that lifts an idea from the page and makes it part of a child’s everyday reality. Once again, play will be an important part of our teaching. Children may stare vacantly at a photo or a video, but I have yet to meet a child who plays with a toy by simply looking at it.

The student’s urge to pick up a model and play with it should be gratified. As teachers, our job at this point is not to stop the child from ‘playing with the science equipment’, but rather to guide the child to make useful observations and discoveries during play. This is a different model of teaching than I grew up with to be sure, but it has been a powerful and effective pedagogy in my own classroom for many decades!

Student Outcomes

Conducting the Activity

Materials

1. Three white ping-pong balls per student group
2. Six poker chips per group (you can substitute sports-drink caps if you like)
3. One set of 6-10 powerful magnets (for the teacher’s model)
4. One tube silicone glue
5. One tube super glue (optional)
6. One can flat black spray paint
7. One can gloss yellow spray paint
8. Roll of 2” wide masking tape
9. Wooden or plastic ruler (actually, almost any sturdy stick will do)

Building the Lunar Phases Model

1. You can reuse your Sun model from Activity #20 again here.
2. [Teacher] Your two remaining ping-pong balls must be colored half-black, and half left unpainted white; the black side will represent night, the white side will be the daytime side of the moon or planet. If you wish to save time, you can reuse the Venus model from Activity #20 as your Moon model here. As we explained in Activity #20, there are two fundamental ways to paint ping-pong balls half-black: one at a time (very neat and precise), or in batches of a dozen or so at a time (less precise, but saves a great deal of time.) See Activity #20 for more details.
3. Now it is time to decorate the Earth and Moon using markers. There are two approaches to this, the accurate and the creative – you must decide which will work best for your students! For an accurate model, use photos or maps of the Earth and Moon and draw in continents, oceans, mountain ridges, green prairies, islands, etc. You can even use a bit of white paint (or correction fluid!) to add storms and clouds to your model of Earth. The Moon will have no color, make it all grey and white (paler shades will work best). Draw in the maria and prominent craters and make the model as accurate as you can!  For a creative model, have students draw continents, islands, oceans any way they wish. You can even have them name their planet creations. A creative moon may have maria, mountains, craters, etc. Some moons even have oceans, although they are not always filled with water! For the purposes of our model, it will not matter which approach you take. Alien worlds with unexplored moons still have phases the same way, and for the same reasons, that we have them here on Earth with our Moon! When you are done decorating, glue the planets and moons to their bases with silicone glue. After they are dry (24 hours!), a quick coat of clear art sealer will not go amiss (old-fashioned lacquer hair spray works well for this if you can find it!) – it often helps keep marker from coming off again on little hands!
4. Your model is now ready to play with and explore!

Exploring the Lunar Phases Model

Now that students have made their models, it is time to have some fun with them. In spite of the desktop scale of this model, working with it is an active experience for students, and one that will help them appreciate our perspective of standing on the Earth and looking out into space in a new way. This is one of my favorite activities, the delight that it brings to young and old alike is refreshing and contagious!

If you have made a set of ping-pong planet models for yourself, go ahead and attach magnets to the bottom of the bases with some superglue or silicone glue. Don’t use cheap rubbery refrigerator magnets, they won’t do. Models made with these weak magnets slide right down the slick whiteboard surface; this is frustrating for the teacher and often seems quite funny to the student. A magnetized set of models on the class white board can help students to position their models and understand what they are to do; a visual model to follow can be especially important for ESL or special needs students in your classroom.

Since this model only works when you look at it from the right perspective, you must take care that the students understand how to look at the model. When using magnetized models, I often use a large colorful arrow on the white board (also held on with magnets) to show students exactly how (and from what direction) to look at the model.

1. Begin by having students place the Sun, Earth, and Moon on a piece of large construction paper on the table in front of them as shown here. Be sure the ‘lighted’ sides of the Earth and Moon face the Sun (obviously!) and the dark, unlighted side faces away. I usually ask a sort of trick question at this point; “What do we call the lighted side of the Earth that faces the Sun?” The answer, of course, is “Day”, which elicits both groans and laughter. It does serve the remind students that there is both a cosmic and pedestrian perspective to this model!
2. Now have students move the Moon around in orbit, reminding them to keep the lighted side of the Moon always facing the distant Sun. Where are the lunar phases? The answer to this relies on where your eye is relative to the model! From the perspective of the model Earth and Moon, your eye high above the desktop (your viewing position) is millions of miles out in space above the North Pole. Although no human has ever been this far out in space, if you were there, you would not see the Moon cycle through lunar phases either! Students at this point may be a little frustrated, but fear not, all will be revealed in the next step! Depending on the age and sophistication of your students, this may be a good time to remind them of the scale of things from the 1000-yard solar system (activity #18). Our model is ‘lying’ about the distance between Earth and Sun, as well as the relative size of Earth and Moon, but these little inaccuracies will not affect the experiments we are about to do, or the truth of what we are learning about.
3. Now ask the students “Where do we see the Moon from?” Okay, another tricky question, but we see the Moon from the surface of the Earth!  Have the students put their eye down near the Earth model and look over the Earth toward the Moon – now ask them what they see! Remember that colorful arrow on the whiteboard? This is where it comes into play! (This arrow is especially helpful with younger students.) Your students will see a full moon phase! Have them draw a full moon phase (an empty circle) at this position where the Moon is sitting on the paper and label it. You may wish them to trace out a circle with a coin or a sports drink cap to keep things neat.
4. Advance the position of the Moon anti-clockwise in orbit by 45 degrees as shown below, remembering to keep the lighted side of the Moon facing the Sun. Have the students put their eye near the Earth and once again look over the Earth toward the Moon. If they have kept everything lined up correctly, they will now see a gibbous moon phase. Once again have them draw a circle near the Moon’s position and shade in and label the phase as they see it.
5. Very likely, the students will be way ahead of you now and able to continue advancing the Moon 45 degrees each time, then looking past the Earth and drawing the phase as they see it. In no time at all, your students will have recreated the familiar map of the lunar phases which we began this book with in activity #1. This is not repetitive, instead it has great pedagogic value as we will soon see!

Supplemental Materials

Going Deeper

You can tell students “The angle between the Sun, Earth, and Moon creates the lunar phases!” all you wish, and have them study diagrams in textbooks or on posters, but nothing I’ve ever done in a classroom has been as powerful as this simple activity for helping students understand that it is the motion of the Moon around the Earth and the geometry of the lunar orbit combined with our unique perspective here on Earth the creates lunar phases.

If you really want a victory for STEM science in your classroom, have your administrator come to your room after this activity is over and ask your students to teach the Principal how the lunar phases work. Your students will be delighted to show off their knowledge and expertise to your boss, and the model is so impressive that young and old find delight in it.

Everyone seems to learn something new the first time they try it for themselves. If you have a back to school night or parent’s night at your school, this is an easy and powerful was to demonstrate exciting and active learning in your classroom. This activity never fails to impress; in fact, when I came to interview for my current position as Professor of STEM Education at my university, this is the lesson that I chose to present to my boss and future colleagues!

Being an Astronomer

If you have a telescope or an active relationship with the local astronomy club, it is an excellent time for another peek through the eyepiece. If you don’t have access to a small telescope, try looking at high resolution photos of the Moon online.

If you are lucky enough to be able to see the Moon in the first quarter phase, look along the terminator, the dividing line between light and darkness; this is the place where sunrise is happening on the lunar surface and shadows are the longest and most dramatic. Look at the shadows that lie inside craters near the terminator, then gradually sweep your view into the more lighted portion of the lunar surface.

If you look carefully, you will see that as you sweep away from the terminator and into the light, the shadows inside the craters become smaller – this is because the Sun is higher in the sky in these locations! You are actually seeing how shadows change when the angle of the Sun in the sky changes, and this is exactly how the lunar phases work! The changing angle of the Sun shining on the Moon as seen from our perspective on Earth causes the changing patterns of light and shadow which we call the phases of the Moon.

Being a Scientist

If we examine our lunar phase model carefully or take photos of it with a cell phone, you will notice that the terminator, the line that separates light from darkness on the Moon’s surface, always stretches from one lunar pole to the other.

The reason for this is simple, looking down on the Moon from high above the lunar equator, we astronomers on Earth can see both poles at once. When asking students to draw phases of the Moon outdoors in my astronomy classes, I noticed something curious, very few students drew the terminator shadow stretching from one pole to another.

Can you verify this curious fact in your own observations of the Moon? It is not difficult, all you need to is take time to look at the Moon with your naked eye, or through binoculars if you have them. See if you can extreme ends of the terminator lie 180 degrees apart on opposite sides of the Moon as the model suggests they must do!

Following Up

Lunar eclipses are much more common than solar eclipses, and usually far easier to see! If you and your students have the opportunity to observe a lunar eclipse, you will get to see an entirely different type of shadow move across the Moon’s surface.

Lunar phases occur because we can see both the illuminated (day) side of the Moon and the dark side (night) at the same time. During the normal phases – there is no shadow on the Moon – we simply get to see both day and night at the same time.

Eclipses are different – here the Moon is moving into the shadow of the Earth and there is no connection to the day and night sides of the Moon itself. As a result, the Earth’s shadow does not stretch from pole to pole as the lunar terminator does. This proves that an eclipse is a completely different phenomenon than the Moon’s normal phases.

Can you make careful sketches or take photos of the Moon during a lunar eclipse that prove this hypothesis?

Activity 24: Aristotle’s Flat Moon

There is an ancient theory – sometimes attributed to Aristotle – that accounted for lunar phases in a different way than we do today. This theory held that the Moon was in fact flat (or perhaps bulged out on one side rather like a warrior’s shield). One side of the Moon was silvery-white, the other side was black, and it was the orbiting of this half-black, half-white Moon around the Earth that caused the lunar phases.

Why did they say that the Moon was flat? It is very difficult, if not impossible, to actually see the spherical shape of the Moon. If you look at a ball at arm’s length, or even across the room, there are many subtle clues of shading and shadow that allow us to see that the ball is in fact round. This is not true of the Moon! The full Moon looks perfectly flat – just like people in Aristotle’s time, we claim to see what we are taught to see!

If this seems silly to you, let me remind you that the most common misconception among adults about the lunar phases is that they believe that the Earth’s shadow falling on the Moon somehow causes or creates the lunar phases! Maybe that Aristotle fellow wasn’t as silly as he appears at first glance! In any case, let’s test Aristotle’s theory as Galileo did and see what happens.

For the Educator

Facts you need to know

1. The Moon is actually round, not flat. (Okay, you already knew that one!)
2. A model makes predictions – we record these predictions and test them against what we see in Nature. Good models make accurate predictions!

Teaching and Pedagogy

This activity teaches much more about the process of science as a cultural activity than it does about the Moon. There is no controversy today about how the phases of the Moon work, how far away the Moon is, or what the Moon is shaped like – but this was not always true!

We have areas of science today which have powerful controversies swirling about them. Theories about global climate change, how (and if!) vaccines work, the evolution of species, and life on other planets are just a few of these that students may have seen in the news.

When students see one group of adults shouting that “the science is settled!”, or “96% of scientists agree with our theory!” on one side of the issue. On the other hand, there are those who insist just as vehemently that the prevailing theory is wrong; the climate never changes, species do not evolve, and vaccines cause autism but do not actually protect people from disease.

Many students (and adults!) find these arguments very disconcerting. I have had hundreds of students and adults approach me as a scientist and ask, “Which one of these is true?”, or even more to the point, “How do we know which one of these ideas is correct?”

Science isn’t about votes or polls of course, and people do not decide which theory is valid. Real scientists use experiments and data to decide these things, and Nature cannot be argued with! Even so, sometimes the experimental results are not clear to us; more often, we simply do not know how to interpret and understand what the experiment is telling us.

Never the less, sometimes we do get definitive results; powerful experiments can show us that a theory is clearly wrong. At this point, no matter how fond we are of a particular idea or theory, it is time to discard it in favor of more accurate and powerful ideas. Teaching students how we decide between theories, keeping one and casting the other aside, is a powerful lesson about science that armors children against future misconceptions and manipulation.

Student Outcomes

Conducting the Activity

Materials

1. A ping-pong planet model of the Earth, Sun, and Moon (See Activity #19)
2. Three poker chips, one white, two black. (You can paint these the necessary colors if you don’t have ones of the correct color. Painted coins may also be substituted.)
3. Epoxy, hot glue, or super glue

Building the Flat Moon Model

1. Glue one black and one white poker chip together face to face. This will serve as Aristotle’s black-and-white Moon.
2. [Teacher] Epoxy or glue the double chip from step #1 on edge on the second black chip as shown below. I filed a flat spot on the edge to make the gluing easier. You can use hot glue or epoxy for this, I have found that silicone glue isn’t strong enough for this edge-on application, and superglue needs more surface to grip effectively. Regardless of what glue you use, be sure to hold the edge-on chips in place until the glue is completely hardened.

Exploring the Flat Moon Model

1. Now it is time to try a version of Activity #23 (Modeling Lunar Phases) using Aristotle’s flat Moon instead of a round one. Let the students play with this model, and ask them to see if they can get anything that looks like the familiar lunar phases out of it.
2. Try as they might, they will not be able to do this successfully. There is no position that works and shows us the familiar gibbous, quarter, and crescent phases. The flat Moon with one white face and one black face conflicts with everything we know about the Sun lighting planets and creating day and night.
3. Because this model of the flat Moon does not show us what we see in Nature, we must reject this model. The model may be interesting, but it becomes clear that Nature does not work this way, so our model is useless to us as scientists.

Supplemental Materials

Going Deeper

While it may seem strange to set students to trying out an experiment that is quite unworkable and doomed to failure, this activity does serve an important purpose. Aristotle’s idea of a flat Moon were simply accepted based upon the thinker’s great name and left untested for centuries. These untested (and incorrect!) ideas were taught in colleges, written down in books, and accepted without question for almost two thousand years! It was Nicholas Copernicus who developed the first modern heliocentric model of the solar system, but he never promoted his ideas during his lifetime and in fact held back the publication of his work until almost his dying day.

Galileo was cut from a different cloth altogether. He marveled at Copernicus’ Sun-centered theory, and set out to test it. Galileo not only invented the modern astronomical telescope, he single-handedly gathered the needed experimental data to prove Copernicus’ ideas were correct. Galileo developed many simple activities much like those in this book and wrote about them in simple language so that everyday people could try these experiments for themselves and see that Copernicus’ theories of how the solar system worked were superior to those of Aristotle. Galileo fought for the acceptance of these ideas and stood fast, refusing to give up in the face of terrifying opposition.

Galileo’s fight for scientific truth cost him his job, his fortune, and even landed him in jail for the rest of his life, but he never relinquished the truth. Galileo’s stubbornness freed us from the tyranny of false ideas and launched the modern scientific age. Every time we ask for data, and not blind belief, we too stand fast and support the truth. When the data demands it, Galileo taught us that we must abandon old established ideas in order to move forward. We do not throw out theories because they are unpopular or uncomfortable, we do not accept them because our teachers or civic leaders tell us to do so. We stand fast and support the truth, backed by sound scientific data and successful experiments.

You may have guessed by now that Galileo is something of a hero of mine, I hope he will become one for you and for your students as well. Go ahead and find a picture of the old gent and hang it up in your classroom. Even better, ask the children to draw their own pictures of Galileo and write a bit about what he did and what we owe him for his stubborn stance and determination to protect and promote scientific truth!

Following Up

Sometimes teachers are uncomfortable about teaching scientifically controversial subjects and choose to avoid them – other times teachers present these subjects as though they are not controversial at all; the phrase “The science is settled” springs to mind here.

I believe that both of these pedagogical models do a disservice to the student. When we avoid controversial topics all together, we teach students to think of controversial issues as unpleasant and to avoid them when possible. There is also an underlying current of disrespect, an implicit claim that the student is not capable of dealing with or understanding the issues at hand.

On the other hand, when we stoutly proclaim that there is no controversy, that scientists know the Truth, we implicitly lie about the nature of science. In the time of Copernicus and Galileo, 99% of the educated populace agreed that the Sun revolved around the Earth which was itself the center of the cosmos. Galileo was dismissed as a dangerous crank – today Galileo is also venerated as one of the greatest heroes in scientific history.

We can teach controversial subjects. We can teach them, if nothing else, as an example of how science works, how men and women challenge each other’s ideas, and struggle to gain a better understanding of Nature. We can teach that science is never 100% certain, and that no idea is above criticism or challenge. Einstein became famous in 1905 because he was the first scientist in 250 years to challenge Newton’s ideas about gravity. One hundred years later, scientists are still busily engaged in designing and carrying out experiments to prove (or disprove!) Einstein’s ideas and predictions.