Unit 3: Modeling Earth and Moon Together

Our first model of the lunar phases is an easy and exciting place to start, but there is something missing – the Earth! Whenever we talk about a moon in orbit, we automatically assume that there is a planet for the moon to circle around. Early lunar models had the same problem that our model did, they failed to account for the Earth. Rather like a fish ignoring the water that they swim in every day, it is easy for us to ignore the Earth; in spite of it being so large, it is all around us and under our feet every day. People often forget to consider the obvious!

Gravity will also emerge as a major theme of this unit. Most of my astronomy students are astonished at how much gravity affects everything in the cosmos – and the Earth-Moon system is their first introduction to that concept. Although the activities in this unit seem to address many separate facets of the Earth and Moon, gravity unites them all!

Our new models will help students understand that the Earth and Moon are a system – two planet-sized objects bound forever together in space by their mutual gravity. If we wish to understand how the Moon works and how the lunar phases we see every night are produced, then we must take into account the Earth beneath our feet. In fact, because the Earth and Moon are bound together, we cannot understand one without studying both of them together. While it may seem incredible to you, this fundamental scientific truth was not discovered until the late 1960’s when we first began to send men and robotic craft out into space to explore the Moon for the first time.

This new model will also begin to take into account the physical scale of the Earth-Moon system. The Moon is about ¼ the size of the Earth, but very far away – about 30 Earth diameters away. Both the large size of the Moon relative to the Earth and the great distance from the Earth is seldom appreciated. Our new classroom model will be quite large and is best explored outdoors or perhaps in a gymnasium-sized space. Because it is accurate both in terms of size and distance, it will correct common errors seen in most models and diagrams of the Earth-Moon system – it is almost certain to surprise and delight your students.

Activity 3: Making a Scale Model of the Earth-Moon System

Because this model is both larger, and more complex that what we have done before, we will divide up the construction of the model, and exploring it scientifically into two separate activities. It is assumed that Activity 3 and Activity 4 will be done sequentially with one following close upon the other.

Academic Standards

Science and Engineering Practices

  • Developing and using models.
  • Using mathematics.

Crosscutting Concepts

  • Scale, proportion, and quantity.

  • Systems and system models.

Next Generation Science Standards

  • Space systems (K-5, 6-8, 9-12).

  • The Earth-Moon system (6-8, 9-12).

  • Gravitation and orbits (6-8, 9-12).

For the Educator

Facts you need to know

  1. The Moon is about ¼ the size of the Earth (when you compare diameters).
  2. The Moon is about 385,000 km (250,000 miles) from Earth (average distance). This is about 30x farther than the Earth is wide.
  3. The Moon orbits the Earth in about 28 days.
  4. One side of the Moon always faces the Earth, we call this the near side; the side we never see from Earth is called the far side.

Teaching and Pedagogy

This model of the Earth-Moon system is one of the largest models we will construct in this book – in fact, the completed model is approximately 60 feet in diameter, perfect for an outdoor activity!

In spite of the model being of great size, the materials for the model will fit into a single grocery bag – it turns out that the Earth-Moon system is mostly empty space! This vast amount of space compared to the relatively small Earth and Moon is one of the main things that your students will learn about.

Diagrams of the Earth and Moon in a textbook have to be compressed to fit on a single page. Physical models of the Earth and Moon system have to be made compact enough to fit on a desk top. You could draw or construct such models to correct scale, but the drawing on your page would show an Earth no larger than a BB, and the Moon would be a single speck on the page.

Your first reaction might be: “Those models lie!” In fact, almost every scientific model makes many compromises and simplifications. Some of these compromises are deliberate, others are out of ignorance. Unfortunately, when we become used to the compromises – and no one tells us about them – we come to think of these things as facts.

This will certainly be the first true-to-scale model of the Earth-Moon system your students have seen. It is a wonderful experience to introduce the student to the vastness of space, but just as importantly, we must dig deeper and draw the student’s attention to the compromises that scientific models make. Awareness of how scientists present models will help your students interpret, and understand these models better!

Student Outcomes

What will the student discover?

  1. The scale of the Earth-Moon system is enormous!
    • Almost every diagram of the Earth and Moon depicted in textbooks is wildly out of scale. When we use a 12-inch vinyl playball as the Earth and a rubber T-ball as the Moon, the diameter of the lunar orbit is 60 feet!
  2. The Moon crosses the sky from East to West each night.
    • This east to west motion is called apparent motion, it is caused by the speedy rotation of the Earth on its axis and it is not actually how the Moon moves through space as it orbits the Earth.
  3. The Moon moves from West to East as it orbits the Earth in space.
    • This is the Moon’s true orbital motion which is in the opposite direction of the east to west apparent motion that we see each night. We can see this eastward movement of the Moon from here on Earth, but we must watch the Moon carefully over several nights to observe it!
  4. Unlike our consistent sunrise and sunset, the time of moonrise and moonset changes by about an hour each night.
    • Because the Earth is spinning as the Moon orbits our planet, our Earth must turn more than 360o each day before we can see the Moon again. This means it takes more than 24-hours from one moonrise to the next, and the Moon rises about 50 minutes later each day. Be patient, this fact will be much easier to see in one of our later activities than it is to explain now!
  5. We can only see one side of the Moon from Earth (the near side), in order to see the far side, we must physically travel through space.
    • This unique fact will be explored in several upcoming activities. We will discover both that it is true, and more importantly, why it happens.

What will your students learn about science?

  1. We will learn that by improving a scientific model, we can begin to answer the Why does that happen? and How does it work? questions – not just the What happens next? questions.
    • When we build on and improve a scientific model, we partake in a tradition of scientific inquiry that is literally thousands of years old. Science has no sacred ideas that cannot be challenged. In fact, to be good scientists, we must challenge every idea and scientific model. If the model is robust, it will be able to answer questions and respond to challenges; if it is not, then it must be changed, or even discarded all together!
  2. Playing with scientific models is important. It helps us ask (and answer!) questions in a kinesthetic way – even before we have the vocabulary and conceptual understanding to frame these questions properly in English!
    • Understanding a scientific model physically and kinesthetically often comes before a proper description exists in the language of mathematics or English. ESL students often respond particularly well to a good scientific model and demonstration because they can frame the ideas mentally in their native language first, then acquire the proper English vocabulary right along with everyone else in the classroom. Scientists often invent new vocabulary to describe their discoveries, these terms are generally adopted in almost all languages without translation. Science and mathematics are truly the universal languages of humanity!
  3. Science lets us explore the shape and structure of things – even when they are too far away for us to actually touch and explore personally.
    • By making models of faraway things like the Moon here on Earth, we can begin to understand how they are put together and how they work as they do. Of course, models are just scientific theories made flesh, so to speak; they aren’t perfect and never tell us everything we wish to know – but they do help us frame the next question and point us toward where we may find the answer! In this unit, we will also make comparisons between common things like rain drops and distant things like the Moon. Comparing the structure of these things can help us see the themes in nature and how simple forces like gravity shape almost everything around us!

Conducting the Activity

imageThis activity involves making a true-scale model of the Earth – Moon system, and this baby is gonna be BIG! Large scale things delight children, and this model is large enough that you will need extra space just to play with it and explore; unless you teach in a gymnasium, this model won’t fit in your classroom. Don’t worry though, the pieces to this model can fit in a plastic grocery bag – you’ll see what I mean as we start constructing the model! The preliminary construction of the model using glue and sharp instruments should probably be carried out by the teacher, especially with younger students; use your own judgement here! Students can decorate and operate the model after it is built.

Materials

  1. Classroom paints and brushes, or permanent markers
  2. Sidewalk chalk
  3. One 12-inch vinyl playball (blue is highly preferred)
  4. One light-colored, 4-inch rubber balls (I used a rubber T-ball baseball)
  5. 50 ft of stout, non-stretchable cord (clothesline or pull-cord for blinds works well)
  6. Duct tape (blue is preferred)

Building the Earth-Moon Model

  1. imageThe larger vinyl play ball will be our Earth, the T-ball will be our Moon. Note that the 4:1 size ratio between these balls reflects the true scale of the size of the Earth and Moon in space!
  2. [Teacher] Tie a knot in one end of the cord and use an ‘X’ of tape to secure it to the vinyl playball. Alternatively, you can use a suction cup such as those used to hold a soap dish to the shower wall. These vinyl balls usually have a dimple where they are inflated, you will want to keep this clear so the ball can be reinflated if needed. Tape your line to the opposite side of the ball.
  3. [Teacher] Measure out 30 feet of cord, plus an extra few inches. Cut the cord and save the remainder.
  4. imageimageimage[Teacher] From the remainder of the cord, tie two knots 6’-10” apart, secure the knots with a few drops of white glue and allow to dry completely. Trim off any extra cord and discard. This cord-measure will show us how far the Moon moves each day as it orbits the Earth!
  5. [Teacher] Put a knot in the end of the cord. (Optional: secure the knot with a drop of white glue and allow to dry completely before proceeding.)
  6. [Teacher] Secure the knot to the rubber T-ball Moon. The best way to do this is to take a sharp knife (a hobby knife works well) and cut a deep, ½ inch slot in the T-ball. Force the knot into the slot with a screwdriver and seal the slot shut with a few drops of superglue and pinch shut; hot glue also works well.
    • Now that the model is built, it can be decorated by students. Remind them that the two pieces are tied together and to be careful of pulling or tripping! The Moon should be painted white if possible – the teacher can do this with spray paint before allowing the students to decorate the Moon if you wish.
  7. Think of the place where the cord is attached to the Moon as a ‘South Pole’. Draw a bold red ‘equator’ line on the Moon.
  8. This line is not actually an equator; instead, it will separate the near side from the far side! Label the near side and far side neatly in red.
  9. Use dark-colored markers or paints to add craters, rays (faint splash marks leading away from a crater in all directions!), and maria (dark-colored seas of frozen lava, usually round or oval in shape.) Some students will wish to be very artistic and use photos of the Moon to make the model look more accurate. But don’t worry if your Moon doesn’t look like the real one in the sky – our model will work just fine the same!
  10. Now it’s time to decorate the Earth. Consider the place where the knot is attached to the ball to be somewhere along the equator, perhaps out in the Atlantic or Pacific oceans. Once again, some students will want to be very accurate and artistic, others may wish to make a wildly imaginative planet that exists only in their imagination; either way, our model will work just fine!

Exploring the Earth-Moon Model

Now that our model is built, we can do several activities with it, most of these work best outside on the playground area.  Although I do not recommend this as a first choice, you can build a smaller, ‘inside the classroom’ model if you wish.  If you teach in a school with little play area outside, of if you just wish to conduct the activities inside, this smaller model works just fine.  For the smaller model, use the rubber T-ball as the Earth and a glass marble as the Moon.  To keep the Earth-Moon distance to scale, remember to make the string connecting them shorter, just 7.5-ft long, and make your measuring string just 2’-4” long.  This smaller (and less impressive) model will fit in most classrooms, but at 20 feet wide, it will still cramp you for space in most standard classrooms!

Activity 4: Exploring the Moon’s Orbit

The Moon’s orbit is wonderfully complex, and yet the youngest child in your classroom can understand the essentials of how the Moon moves through space. One of the essential skills of successful STEM teaching is to be able to break down complex things into small components that are simple to understand. Once your students complete these simple activities, they will be building the pieces of a conceptual model of the Moon and its orbital motion around the Earth.

Academic Standards

Science and Engineering Practices

  • Asking questions and defining problems.

  • Developing and using models.

  • Using mathematics.

  • Obtain, evaluate, and communicate information.

Crosscutting Concepts

  • Scale, proportion, and quantity.

  • Systems and system models.

Next Generation Science Standards

  • Space systems (K-5, 6-8, 9-12).
  • The Earth-Moon system (6-8, 9-12).
  • Gravitation and orbits (6-8, 9-12).

For the Educator

Facts you need to know

  1. The Moon’s diameter is ¼ that of the Earth, about the same ratio as a small marble to a baseball. The Moon is a much smaller world than Earth is!
  2. The Moon’s orbit is 60 times wider than the Earth itself. This 60:1 ratio demonstrates the vastness of space, but obviously makes it difficult (but not impossible!) to show an accurate model in the classroom.
  3. The Moon orbits the Earth approximately every 28 days (moon and month are related words!) Each week the Moon travels ¼ the way around its orbit.

Teaching and Pedagogy

Now that we have built and decorated our Earth-Moon system model, let’s have some fun with it! These next four mini-activities can each be done in 20-30 minutes, perfect for a single class period. Because the model is so large (sixty feet in diameter!), these will obviously be outdoor activities. I strongly suggest that you try them on a paved playground area where you can use sidewalk chalk to mark things out. The distance scale we are working with is something that really has to be experienced directly to allow students to gain a substantive cognitive understanding. One can talk about dinosaurs for days and look at all the photos on the internet you like, but there is no substitute for going to a museum and standing next to a life-size model or a real fossilized skeleton to give one an appreciation of the size of the creature.

These activities strike to the very core of constructivist pedagogy. During these activities, students construct their own meaning and create their own (hopefully accurate!) mental models of the Earth-Moon system. You may see this as simply “play time” rather than real science – don’t be fooled! The cognitive work the students are doing as they play with these models is substantial! Your students are constructing mental models and maps of things like size, scale, orbits, planetary motion, rotation and revolution, space travel, and much more. We will be building on these ideas as we continue to build and refine scientific models throughout this book!

Conducting the Activity

Mini-Activity #1

Take your Earth-Moon model outside to the playground with some sidewalk chalk. Use the model as a giant string-compass and draw the lunar orbit out in chalk. Use chalk to draw in the Earth and Moon in their correct sizes on your diagram. Draw the student’s attention to the sheer size of the Earth-Moon system compared to the relatively small sizes of the Earth and Moon themselves! Interestingly, the planet Saturn and its ring system would just fit inside the distance between the Earth and the Moon!

Try and use some sidewalk chalk to draw Saturn and its rings on the playground. The planet is a circle ten feet in diameter, the outermost rings make a circle fifteen feet in diameter! The great difference in scale between the tiniest and largest planets is one of the things that makes modeling the solar system so challenging.

How about the Sun in our model? To be in scale, our Sun would be a 100-ft ball (as large as a ten-story building.) We would have to place this giant Sun model 2.1 miles away; from that distance, it would appear to be almost exactly the size of our T-ball moon!

Mini-Activity #2

Ask the students to try drawing their model Moon while standing in the Earth’s position. The apparent size of the 4-inch rubber ball from 30 feet is about the same size as the Moon appears in the night sky! Although our Moon looks large because it is a bright object on a dark background, it is really quite small! If you have decorated your Moon with maria and craters with rays, ask students if they can make them out when standing where the Earth is. If they cannot, this is an excellent time to offer them a chance to try out a pair of binoculars if you have one. Students will quickly see that binoculars do bring things closer, but holding them steady and drawing what you see in the eyepiece is still quite challenging!

Mini-Activity #3

Use the 6’-10” measuring cord to mark out the distance that the Moon moves each day. Number these daily positions of the Moon for one entire orbit. How many days does it take for the Moon to orbit the Earth? Surprise! It takes about 28 days (one month) for the Moon to orbit the Earth. Use your sidewalk chalk to draw in the lunar phases as we see them from Earth around your lunar orbit. Use your Lunar phase map from Activity #1 to help you!

Mini-Activity #4

Try for a moon shot! Use marbles or ping-pong balls as ‘spacecraft’ and try to roll your craft all the way from the Earth to the Moon! Alternatively, have everyone make a paper airplane and try ‘flying’ to the Moon as someone walks slowly around the lunar orbit representing the orbital motion. Getting from the Earth to the Moon is hard!

Discussion Questions

Now that your students have had a chance to play with this model of the Earth-Moon system, they should have a much better cognitive grasp of how large the system is, and what the relative size of the two bodies are and how they are related in space. Almost all drawings and illustrations from textbooks or internet sites are horribly distorted in this way. Artists invariably show the Moon being far too close to the Earth, and often much larger than it actually is in comparison to the Earth. There are good reasons for this of course, try to draw an accurate scale picture and most of the space on the page is not only empty, but the Earth and especially the Moon are really too small to show any detail at all! Never-the-less, these drawings encourage gross misconceptions about our planet and its nearest companion in space.

  1. Show your students a drawing or illustration of the Earth and Moon in orbit taken from any textbook or website. Ask them what is wrong with this drawing as a scientific model?
    • Answer: There are likely to be a great many things wrong with these illustrations! The relative size of Earth and Moon and the scale of the distance between them just for starters!
  2. Ask your students to hold up their drawings of the Moon made from inside the circle at Earth’s position. Ask them why observing and drawing the Moon is so difficult!
    • Answer: This question will help you see how far your students – and their cognitive models of the Earth-Moon system – have progressed. No doubt they will now realize that drawing small features on a small lunar globe from very far away is quite challenging – even when they originally drew the features themselves and know just what they look like!
  3. Show a photo or some video of the Apollo astronauts flying to, and landing on the Moon. Ask your students what they think of these explorers and the journey that they made!
    • Answer: To understand an achievement, you must first know something about the challenge that it represents. If I told you I had built and learned to play a Theremin, this might not mean much to you unless you first knew that a Theremin is an electronic musical instrument that one plays without touching it. Your students are likely to find the Apollo voyages much more exciting now that they understand a bit more about the Earth-Moon system!

Supplemental Materials

Going Deeper

The average distance to the Moon is 385,000 kilometers – compare this to a trip from New York City to Los Angeles which is just 4490 km! That trip would take you 41 hours by car (without stopping for gas of food!) The Moon is about 90 times farther away than our imaginary cross-country trip!

Apollo astronauts traveled at an average speed of 5500 kilometers per hour (kph). Imagine you were going to travel this great distance – 770,000 km, all the way to the Moon and back – in a very small car with two of your best friends. Remember that this is a spacecraft and that you can not stop or get out! Work together with your two traveling companions to answer these questions.

  1. How long would this journey take you? (Show your work!)
  2. What things would you want to take with you? Space is very limited, so divide your items up into a Must Have and Want to Have lists.
  3. Being in the car for this long without being able to stop or even open a window presents some very special problems; eating, washing, and going to the bathroom come to mind! What would you do to handle living in this very compact space for so long?
  4. If your compact car got very good mileage, say 65 km per gallon, how much fuel would you need for the entire trip? Find a 5-gallon gas can and measure it; use this to estimate the size of fuel tank you would need for this trip!

Being an Astronomer

Observing the Moon’s apparent motion is much easier than observing its orbital motion around the Earth – but both take some patience and clear weather! The best time to do this is in the two weeks after New Moon. With your teacher or parent’s help, use the internet to find the date for the next new moon, your observations will begin about 3 days after this.

Three days after the new moon, you should see a thin crescent moon in the western sky just after sunset as the sky gets dark. Watch the Moon for an hour or so starting at sunset and notice the motion of the Moon as it sinks into the west. If the weather is nice, a good way to do this is to have a Moon Picnic in the back yard with your parents and eat dinner as you watch the Moon! This east to west motion that you see is the Moon’s apparent motion. What we are really watching is the Earth spinning on its axis.

Being a Scientist

The Moon’s orbital motion is harder to see, and you must watch the Moon carefully several days in a row to see it. Begin by going out on a clear night about three days after new moon. Look for the crescent moon low in the western sky right at sunset and make a note of the Moon’s position. An easy way to do this is to notice where the Moon is compared to trees or buildings in your back yard. Take careful notes of what you see!

For the next 3-4 nights, go out again just at sunset and notice the Moon’s position. You will notice that the Moon appears farther east each night. This west to east motion is the Moon’s true orbital motion. We don’t notice it on one night because the Moon takes 29 days to make a complete revolution around the Earth – it doesn’t move much in just an hour or two!

Calculate the circumference of the Moon’s orbit. Circumference = 2 π r (your teacher can help you with this!) The radius of the Moon’s orbit is just the distance between the Earth and the Moon – 385,000 km. Use what you have learned to answer these questions:

  1. How far does the Moon travel in each orbit?
  2. How far does the Moon travel in just one day?
  3. How fast is the Moon moving in orbit in kph?

Following Up

Think about how challenging space travel is! To be an astronaut and travel to the Moon requires great planning, scientific knowledge, and tremendous courage! We will explore these ideas further in later activities in this book.

Activity 5: Rotation and Revolution

We are going to use the Earth-Moon system model once again, but this activity gets the children thinking about our scientific model in a different way; it also helps students understand the difference between rotation (a body spinning around on an internal axis), and revolution (one body circling around another). These two motions are generally independent of each other; our Earth, for instance, rotates 365.25 times (days) for each single revolution around the Sun (year); this is not a whole-number ratio. Planets are generally not synchronized, that is to say their rotation time and revolution time do not divide evenly into one another. Our Moon (indeed most moons) are exceptions to this and have synchronized orbits, as we shall see.

Academic Standards

Science and Engineering Practices

  • Asking questions and defining problems.

  • Developing and using models.

  • Planning and carrying out investigations.

  • Analyzing and interpreting data.

  • Using mathematics.

  • Constructing explanations.

  • Argument from evidence.

Crosscutting Concepts

  • Systems and system models.

Next Generation Science Standards

  • Forces and interactions (K-5, 6-8, 9-12).
  • Space systems (K-5, 6-8, 9-12).
  • The Earth-Moon system (6-8, 9-12).
  • Gravitation and orbits (6-8, 9-12).

For the Educator

Facts you need to know

  1. From here on Earth, we only see one side of the Moon, commonly called the near side. The only way to see the Moon’s far side, is to fly there in a space craft and take photos!
  2. Rotation and Revolution are different! Things rotate on their axis the way a carousel spins on its central axis. To revolve, you must circle around a point outside your body. A tetherball revolves around the pole and the Earth revolves around the Sun.
  3. All planets and moons both rotate and revolve; just as the Earth rotates on its axis once a day, and revolves around the Sun once a year.
  4. The Moon is interesting because it rotates only once on its axis each time it revolves around the Earth. Rotation and Revolution take the same amount of time – about 28 days. This is called synchronous rotation, and it is the reason that we only see one side of the Moon from Earth!

Teaching and Pedagogy

The concepts of rotation and revolution are often difficult, not just children, but adults often struggle with them. It is not that the concepts are inherently difficult, but I suspect that because we fail to introduce children to them at all, this sets them up to struggle later in life. Studies show that we must be exposed to novel concepts several times before we begin to internalize them; even more exposure and practice is needed to master a concept. These early exposures to the ideas of rotation and revolution will be critical for your student’s later success in science. In keeping with the philosophy of many exposures to achieve mastery, we will return to these ideas again as you work through this book.

We are going to use the Earth and Moon model we built in Activity #3. You can use either the larger or smaller size model, but this activity generally works better outside using the larger size model

It may help your students visualize what is going on if you color your moon model before you work with it. Hang the moon model from the string (it should look like you are hanging it from the North Pole.) Draw a line where the equator should be and color the southern hemisphere dark grey, and the northern hemisphere white. The white half will represent the near side of the Moon, and the darker half represents the Moon’s far side.

With the model in place in the playground, ask the students what the cord between the model Earth and model Moon represents? The cord represents the force of gravity that holds the Moon in orbit of course, but the students may need to be guided to this idea. In general, if the students can stand 5-10 meters (15-30 ft.) away from the model, it will be easier to see what is happening.

As you can see in the illustration below, rotation occurs when a body such as the Earth spins around an internal axis. Virtually all objects in space spin around their own internal axis; for the Earth, this creates the night and day cycle. Revolution occurs when one object orbits around another. The Moon for instance, revolves around the Earth once per month.

image

Student Outcomes

What will the student discover?

  1. One side of the Moon always faces the Earth.
    • Students may decide that this is caused by the string which attaches the Earth and Moon models together. Remind the students that it is actually the Force of Gravity which locks the Moon in a 1:1 synchronous orbit around the Earth.
  2. The Moon rotates or spins on its axis just once for each rotation around the Earth.
    • It is sometimes helpful to have a student hold the Moon over their head and walk the Moon model around the Earth. You will clearly see that the student faces a different direction each time they move ¼ the way around the orbit!
  3. The Moon’s ratio of rotations : revolutions = 1:1.
    • This 1:1 ratio is typical of very large planets with relatively small moons. This is more common than you might think! The moon Charon always faces the same side toward it planet Pluto! Several moons of Jupiter and Saturn are locked in orbit in this same way.

What will your students learn about science?

  1. Playing with models and exploring what they can tell you about the world around you is powerful science! Working with models is very powerful. The famous scientists James Watson and Frances Crick used models to discover the shape of the DNA molecule – and won a Nobel Prize for their efforts.!
  2. Models can help explain what we see in Nature. Sometimes we see something, but we don’t understand how it works. Always seeing the same side of the Moon is like that – we’ve all looked at the Moon in the sky hundreds of times, but few people wonder why do we always see the same side? Playing with models can help us understand what is happening, and help us plan new experiments!
  3. Models can help show us where to look for new ideas, and help us form good questions to ask as we continue exploring! Once we see the same side of the Moon always faces us, we begin to ask other questions. Is it always exactly the same? Can we see the Moon tip or wobble at all? We will deal with these, and other questions, as we move through this book!

Conducting the Activity

Materials

We will use the Earth-Moon model that we constructed in Activity #3. It should be modified as discussed in the Teaching and Pedagogy section above.

Exploring the Earth-Moon Model

With the model in place in the playground, ask the students what the cord between the model Earth and model Moon represents? The cord represents the force of gravity that holds the Moon in orbit of course, but the students may need to be guided to this idea.

  1. Explore how the cord in our model is similar to gravity – and different from it. This is another way to help children realize the difference between a scientific model or theory and nature itself. Our model has several difference and similarities to nature – how many can you find?
  2. Gravity is not a physical cord of course; it is a force, similar to magnetic force. We can feel gravity, and like a magnet, the force gets stronger as we get closer to Earth (Most of us never get far enough away from the planet to notice this, however!) Gravity is also elastic! Unlike our cord, gravity can stretch to hold a moon in orbit at almost any distance around a planet. (Use orbiting satellites to give students a sense of this. Most satellites orbit much closer to the Earth than the Moon is, but some orbit much farther away!)
  3. After moving the Moon in orbit around the Earth one or two times, ask the students if the Moon rotates on its axis as it revolves around the Earth in space. Two points of view are helpful here.
  4. Ask some students to stand close to the Earth’s position as someone moves the Moon around its orbit – can they ever see the far side?
  5. Ask other students to stand well outside the Moon’s orbit as it moves around the Earth – can they see the far side?
  6. If the students are having difficulty with this, try moving this indoors onto a table-top. Prepare a T-ball or tennis ball colored black on one side and white on the other. Set a globe, or even a soccer ball or basketball on the center of the table – this is the Earth. The smaller black and white ball will be the Moon, keep the white side always facing the Earth – this is the near side of the Moon which we always see; the far side which we never see is black in this model.
  7. Slowly move the Moon around the Earth, keeping the white side facing Earth at all times. Students will quickly see that the Moon must revolve on its axis once per orbit. To drive the point home, keep the white face pointed toward one particular wall at all times and orbit the Earth again – both the near and far sides would visible from Earth if the Moon didn’t rotate at all! This 1 rotation per orbit motion is called a synchronous orbit – it is caused by the strong gravity of the planet. Many moons in our solar system have this interesting feature! We will explore how and why this works in our next activity!

Discussion Questions

  1. We know that one side of the Moon forever faces the Earth. Is there any other speed the Moon could spin on its axis and still have this be true?
    • Answer: No. Try this with your table top model. Spin the Moon just a bit faster than one rotation per revolution and we begin to see some of the far side. Spin the Moon slower and the same thing happens! Only by spinning exactly once around its axis for every one orbit around the Earth can the Moon keep its near side facing Earth and the far side forever hidden!
  2. How does this exact one to one ratio work? Is it all coincidence or is there something causing it and controlling the Moon’s rate of spin upon its axis?
    • Answer: In fact, this deep scientific question plagued men and women of science for centuries. The answer was only discovered after we traveled to the Moon and sent explorers there to observe and gather data! We will see exactly what they discovered, and how this works, in our next activity!

Supplemental Materials

Going Deeper

If your students are studying ratios, the orbits of the planets provide wonderful material for this. If you use a search engine (Google, Yahoo, etc.) and type in: “What is the rotation and revolution period for the Earth,” you will find what you are looking for.

Try dividing the revolution time by the rotation time. For Earth this will give you 365.26 days / 1 day for a ratio of 365.26: 1. If you do this, you must be sure the numbers are in the same units.

Example: Jupiter’s revolution time is given as 11.86 years, while its rotation time is given as 0.41 days.

To make the units the same, multiply 11.86 years by 365.26 (the number of days in a year.) This gives Jupiter’s revolution time as 4,332 days.

Now divide revolution by rotation: 4332 / 0.41 = 10,566 : 1

In other words, Jupiter has 10,566 ‘days’ per year! Look up the facts for other planets and moons in our solar system, you will be astonished at what you learn!

Being an Astronomer

Our model has told us something about the Moon, but is it really true? This idea given to us by the model (one side of the Moon always faces the Earth) is called an hypothesis. An hypothesis is an idea that we use to try to understand how the universe works – but it must be tested!

Astronomers test ideas like this by making observations. Observations can be made by looking at the sky with just your eye, or by looking through a telescope or pair of binoculars; some scientists even use cameras to take accurate photographs which can be studied later!

Try observing the Moon for a month! If you start after new moon, you will find the Moon in the sky just after sunset. After the full moon passes, the Moon is best observed in the early morning sky. Winter is a good time to do this because the Sun does not rise too early in the morning, and the sky gets dark early in the evening.

Look at the Moon’s surface every chance you get. Can you verify that you always see the same side? How can you be sure? Write your ideas down in a journal, then make sketches of what you see.

Teacher’s Note: Help the students by showing them a globe. The globe has many features, but they are always in the same places – the continents never move around! The Moon has regular features too, some are bright and others are dark. If students look for these familiar features, they should be able to verify that they see only one side of the Moon.

Following Up

There is more than one way to observe the Moon! Do an internet search for images of the Moon. Look at each one and see if you can find common features to verify our hypothesis.

You can also search for images of the far side of the Moon. The far side looks nothing like the familiar near side. A comparison of the two images side by side should convince even the most skeptical student that they have never seen the Moon’s far side!

Activity 6: The Lop-Sided Moon

The mystery of the Moon’s synchronous orbit is very profound, it has puzzled astronomers and scientists for thousands of years. Even today, when you point out that we only ever see the near side of the Moon, many people will insist that this means that the Moon does not rotate on its axis. The precise match of rotation time to revolution time seems almost miraculous; in fact, it is no such thing. Although the mechanism remained mysterious until the 1970’s, it is quite simple – the Earth’s gravity controls the Moon’s rotation and keeps one side forever pointed toward our planet, and one side forever hidden from us. This activity will show your students both clearly and simply how this works.

Academic Standards

Science and Engineering Practice

  • Asking questions and defining problems.

  • Developing and using models.

  • Planning and carrying out investigations.

  • Constructing explanations.

Crosscutting Concepts

  • Cause and effect.

  • Systems and system models.

  • Structure and function.

Next Generation Science Standards

  • Forces and interactions (K-5, 6-8, 9-12).

  • Space systems (K-5, 6-8, 9-12).

  • Earth shaping processes (K-5, 6-8, 9-12).

  • The Earth-Moon system (6-8, 9-12).

  • Gravitation and orbits (6-8, 9-12).

For the Educator

Facts you need to know

  1. Like the Earth, the Moon has a dense core of metal and rock.
  2. The Moon’s core is not centered, Earth’s gravitational pull has shifted the lunar core so that it is off-center and closer to the Earth.
  3. The Moon’s off-center core locks the Moon into a synchronous orbit, causing one side of the Moon to always face the Earth.

imageEquipment you will need:

  1. A rubber T-ball
  2. A hobby knife
  3. Fishing weights
  4. Instant glue

Teaching and Pedagogy

This series of activities begins to explore gravity as a fundamental force that shapes our universe. The shape of the Moon, how it moves in orbit, the way one side always faces our planet, even the peculiar 1:1 ratio of rotation and revolution that we call a synchronous orbit – none of these things can be understood without understanding gravity first!

The first person to understand the intimate relationship between gravity and the motion of the Moon was Isaac Newton. The famous story of Newton being struck on the head by a falling apple was actually the moment he discovered that the Moon and the apple both fall because of the same force – gravity! Newton was the first to realize that gravitational force extends far out into space and effectively rules the cosmos!

Newton was perhaps the smartest man ever to have lived; he invented the mathematics we call calculus to help him understand the action of gravity and the motion of the Moon in orbit around the Earth. But we needn’t dive deep into mathematics to understand the fundamental action of gravity and how it controls the Earth-Moon system; this activity will give students a powerful, conceptual knowledge that will serve them well as they begin to explore mathematics later in life!

You will need several items for this activity – some of the model building must be done by an adult, or by older students with professional adult.

Student Outcomes

What will the student discover?

  1. The Moon is not a uniform body – its core is not located at the center of the Moon.
  2. Earth’s gravity affects the Moon in more ways than one. The Moon’s rotation on its axis is powerfully affected as well as the Moon’s orbit.
  3. The Moon’s synchronous orbit causes the near side to continuously face the Earth while the far side always faces away from us.

What will your students learn about science?

  1. The universe is a complex place, there is always something new to learn and to explore. Even so, just a few fundamental forces and principles such as gravity control virtually everything there is! Because this is true, models (and other objects!) here on Earth are controlled by, and function much the same as distant objects across the solar system.
  2. The idea of fundamental forces makes it possible for us to make models on Earth that can tell us about the structure, motion, and function of objects so far away we may never be able to reach them. It also gives us confidence that when we make a scientific model here on Earth, the same fundamental forces and processes are at work in the classroom or the laboratory as they are in deepest space. While our models (and our understanding of them!) aren’t always correct, we can have confidence in the scientific process in general.
  3. We also learn that the universe and our solar system is a complex place! It often takes more than one scientific model to understand something as complex and wonderful as the Earth-Moon system. Science always welcomes new models, new ideas, and new questions. Even so, no one will believe you just because you are smart, or famous, from a big important country, or because you have lots of friends who all think you are right! Science tells us that only experiments can tell us which idea is right. Men and women make models and theories, but Nature decides which ideas are correct.

Conducting the Activity

Materials

  1. A 4-inch, light colored rubber ball (Yes, another T-ball baseball!) – $3.
  2. A ½-inch lead fishing weight.
  3. An eye dropper (For older students, one eye dropper per group works well).
  4. Red food coloring (Optional – red drink mix powder or any red drink works for this).
  5. 4-inch square of aluminum foil.
  6. A clear piece of plastic (an overhead transparency works well) or 12-inch square of clear plastic wrap.
  7. Kitchen hot pad.
  8. Hobby knife.
  9. Classroom paints and markers.

This activity requires some preparation by the teacher beforehand, as in our other activities, students will paint and decorate the model before working with it. Depending on the age of your students, you may wish to make more than one lop-sided Moon model. For children in grades 3-6, this works well as a group activity with 2-3 students per group. This is also a discovery type activity, you should not share your preparation of the materials with the students before they begin – they will figure things out soon enough!

Building the Lop-Sided Moon Model

imageHow much your students can do assembling this model is up to the instructor’s individual judgement, your class’s age, familiarity with tools, and maturity must be taken into account. I have taken a conservative approach and reserved all tasks with tools for the teacher.

  • [Teacher] Take the hobby knife and carefully cut out a hollow in the rubber ball just large enough to completely hide the lead fishing weight. If you cannot find any fishing weights, a stack of three 3/8” nuts from any hardware store will do.
  • [Teacher] Our next step is to use hot glue to secure the weight inside the ball. Have the square of foil and the kitchen hot pad ready – you may wish to coat the foil with butter, Vaseline, or non-stick cooking spray before you begin!

imageimage[Teacher] Put a little hot glue in the bottom of the cavity and carefully press the weight inside – the weight must be completely inside the ball for this to work properly. Add more hot glue until the cavity is completely full, then put the square of foil on top and press it down with the kitchen hot pad for a minute or two until the glue hardens completely. You should now have a smooth spot that matches the curvature of the ball quite well, and the weight is sealed inside where the students cannot touch it.

Safety Note: Don’t ignore the hot pad! Hot glue can easily burn you and the foil will not protect your hand from the heat!

  • [Teacher] I recommend painting the ball flat-white before giving it to the students to decorate. Mark a dot where the weight is as one ‘pole’, place another mark on the opposite side. These points are not poles per se, rather they are antipodes; one marks the point on the Moon closest to the Earth on the near side, the other marks the point on the Moon farthest from the Earth on the far side.
  • Have the students draw a bold, red ‘equator’ line halfway between the two antipodes you have marked. This will represent the boundary between the near and far sides of the Moon.
  • Students can then decorate their Moon with craters, rays, and maria as they did before. The exact pattern of craters does not matter – let them be as creative as they wish!

Exploring the Lop-Sided Moon Model

  1. Now it’s time to play! Students will quickly notice that there is something odd about the new Moon model. It doesn’t roll straight, and it wobbles when bounced or thrown! Ask them what is wrong with the model and they will quickly tell you that the ball is lop-sided or off balance!
  2. Now ask everyone to roll the Moon model gently on the floor or a table top, you can even try spinning it like a top if you wish. Each time the Moon model will stop in roughly the same position – heavy side down! Have the students label the weighted (downward) side as the near side, and the upward facing side as the far side.
    • Ask the students which way the near side faces, and they will quickly say “Down!” But what is down? You may point out to them that the near side always faces the Earth – just as with our real Moon. Why does this happen, children? “Gravity!” Because the Moon model is lop-sided, one side is heavier than the other and the pull of gravity causes this side to always face the Earth. A fact we discovered with an earlier model is now explained with our new model!
  3. Now it is time for the eye droppers and colored water. Since you will be using food coloring, plenty of newspapers to cover the desks will be in order! Have the students take up some of the colored water and try to hang the biggest droplet they can without letting it fall. What shape is this? A tear drop shape, of course – no one will likely be surprised by this. Now ask them why the water drop isn’t round? The answer is gravity once again – gravity stretches the drop from a perfectly round shape into the familiar tear drop shape. Why does the droplet’s shape always point the same direction? The answer of course is: heavy side down, just like our model of the Moon.
  4. A clever student may point out that the Moon doesn’t look like a tear drop! Quite right! Now it’s time to use the sheet of plastic (an overhead transparency works very well for this.) Have one student look upward through the plastic sheet while another student makes a hanging droplet of colored water above their head. What shape does the droplet look now? Round! We are now looking up at the droplet exactly as we look up at the Moon far above our heads in the sky![1]Discussion Questions
  1. If the Earth’s moon is locked into a synchronous orbit by gravity, what do you think we will find when we look closely at other moons in our solar system?
    • Answer: Gravity works the same for all things and in all places! NASA has sent long-duration space probes to Mars, Jupiter, Saturn; keeping these spacecraft in orbit around these planets long enough to make detailed studies of their many moons. Every moon in our solar system has its rotational motion controlled by the gravitation of its planet! Although we haven’t seen every moon in our solar system, from what we know today this seems to be a universal effect.
  2. What would it look like if you were an astronaut on the Moon, looking back at the Earth in the night time sky?
    • Answer:  Since the near side of the Moon always faces the Earth, any observer on the Moon would simply see the Earth hanging in one place in the sky. It would spin on its axis and change phases every month just as our Moon does, but it would never move across the sky! The Earth is also four times larger than the Moon, so it would appear 4x larger than the Moon does to us. It would be easy to see oceans, continents, and weather patterns spinning across the globe!

Supplemental Materials

Going Deeper

The idea that just one side of the Moon always faces the Earth is sometimes hard for children to accept. The Earth spins on its axis every day, shouldn’t the Moon do the same? One way for children to see for themselves is to observe the Moon carefully over time. The pattern of dark spots or maria on the lunar surface gives us a clue to what we are actually seeing. If students take a look at a globe of the Earth, it becomes clear that Earth looks very different depending on which side of the globe we are looking at. The same is true of the Moon!

Have students look carefully at the pattern of maria on the Moon as it runs from new moon to full moon. Although the Moon crosses the sky, the pattern of marks and dark maria we see never changes; we never see the far side at all. You can do this with a globe in the classroom – point the Americas toward the students, no matter how you tilt the globe from side to side, the pattern of continents and oceans always remains the same – you are not showing them the opposite side of the globe! Their own observations of the lunar surface should convince them that they never actually see the far side of the Moon.

You can go farther and look up images of the Moon’s far side on the internet. It looks quite different! There are very few dark maria on the lunar far side, and the four that are there are quite small and unlike the extensive seas of frozen lava that create the dark markings on the lunar near side!

Being an Astronomer

This is an interesting activity for older, or more advanced students. While a telescope is quite useful, this activity can actually be done by exploring photographs of the Moon on the internet!

Let’s explore the idea of Libration – the slight wobble that the Moon experiences as it orbits the Earth. You might think that since one side of the Moon always faces the Earth, you could only see 50% of the lunar surface. In fact, because of the libration or wobble of the Moon, you can see almost 60% of the Moon if you are a careful and patient observer.

  1. Begin with your lop-sided Moon model and a cafeteria tray (you can also use a cookie sheet for this). Place the Moon on the tray, and gently shake the tray back and forth as you watch the Moon from directly above.
  2. If you wish, a classmate can take a video with a smart phone while you shake the tray. As you watch, you will notice that the wobble in your model allows you to see past the line dividing the near side from the far side from time to time.
  3. If you have access to a telescope, take a look at the Moon at 50-100x magnification and pay particular attention to the edges of the lunar disk – even a very small and modest telescope will work for this. Some of the terrain you see at the very edge of the Moon is likely to be part of the Moon’s far side!
  4. If you do not have a telescope that you can use, check on the internet to see if there is an astronomy club in your area. These clubs often have observing nights that are open to the public. Club members all bring their own telescopes and binoculars, and almost everyone will be happy to point the telescope toward the Moon and show you the lunar surface! Some members may even have lunar maps with them that will tell you the names of some features! Remember to say ‘Thank you!’ after you’ve had your turn at the telescope!
  1. In fact, the distortion of the Moon’s shape is quite small. The near side does indeed bulge and ‘hang down’ toward the Earth, but only by a few kilometers. This distortion is so small that it took painstaking radar measurements from lunar orbit to detect it! Even so, the distortion is large enough for Earth’s gravity to be able to control the Moon’s rotation. ↵

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Astronomy for Educators by Daniel Barth is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.

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