by Dr. Daniel Barth
The first step to understanding a solar eclipse is to understand the Earth-Moon system. Textbooks and websites generally do a poor job of this; the relative size of the Earth and Moon are often shown incorrectly and the scale of the distance between them is usually wildly off.
It is the distance between Earth and Moon that is the hardest to show in a text, on a poster or on a computer screen. The Moon is about 30 times farther away than the Earth is wide – make your drawing of the Earth and Moon small enough to show the true distance between them and you cannot see any features that you can recognize.
Our solution is to have the students construct a model of a reasonable size, one that will fit in a classroom, and then let the children play with that model to help them understand the relative size – and distance – of the Earth and the Moon. You can make this model in a couple of sizes, you can decide which one is right for your classroom!
The smaller version uses a rubber T-ball for the Earth and a standard glass marble for the Moon. This model uses 7.5 ft of string to connect the Earth and Moon together and the model will store nicely in a 1-gallon plastic storage bag. This size is small enough that you can play with it in an empty room or even in a school hallway, the Moon’s orbit will be 15 feet wide.
The larger version uses a school dodgeball (or a basketball) for the Earth and a rubber T-ball for the Moon. This larger version uses 30 feet of string to connect the Earth and Moon and will store nicely in a plastic grocery bag. The advantage of the larger size is not only that it is more impressive to your students (the Moon’s orbit is now 60-ft wide!), but the Moon and Earth are now large enough to be painted and decorated to make the model more realistic. Playing with this model requires an empty cafeteria, gymnasium or an open school yard.
This article will take you through making the smaller model, but there is really no difference between the two except for the balls that you use and the length of string. Materials involved in making the smaller model include: classroom paints and brushes, markers sidewalk chalk, a 3-inch rubber T-ball, a standard glass marble or .75-inch ball bearing, a spool of strong twine or string, strong glue or hot glue and flat blue spray paint.
In this model, the larger T-ball will be our Earth and the marble 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! Because there is cutting involved, adult supervision is necessary.
What you need:
- Paints and brushes, or permanent markers
- Sidewalk chalk
- 3-inch rubber T-ball
- One standard glass marble or .75-inch ball bearing
- Spool of strong twine or string
- Super glue or hot glue
- Flat blue spray paint
To build the model:
A teacher or other adult should cut off 8 feet of cord and tie a large knot in one end of the cord. Use a hobby knife or small kitchen knife to cut an X about half an inch deep into the rubber T-ball. Squirt a generous amount of super glue into the X, then use a screwdriver or popsicle stick to force the knotted end of the string into the ball. If you are using hot glue, force the string in first, then force the nozzle of the hot glue gun into the hole and fill with hot glue.
Go outside and use the blue spray paint on the T-ball. Use several thin, even coats – a heavy coat of paint will not look good. Place the ball on several large sheets of newspaper – a garage is a good place for this. You will have to spray one side, let it dry thoroughly, then roll it over and spray the other side. The final result will be much nicer if you take your time with this step.
Use a drop of super glue or hot glue to attach your Moon marble to the eraser end of a pencil.
Measure the cord and put a mark 7-feet, 6-inches from the T-ball. Tie the pencil with the Moon marble attached at this location to show how far away the Moon is from our Earth.
Now that the model is built, it can be decorated. Children can try to make a “realistic” Earth if they wish with all the continents, but they shouldn’t worry if their “Earth” doesn’t look a thing like our home planet. It won’t change how the model works at all!
The little marble Moon can be painted white if possible – a teacher or adult can do this with spray paint. Once the paint is dry, children can decorate it with a marker and draw craters on its surface.
Now that our model is built, we can do several activities with it, most of these require lots of space so it is best to use a large open area such as a playground or park.
In the smaller version of the model, the Earth is represented by a rubber T-ball. In step one of the process, a teacher or other adult should cut a half-inch-deep X into the 3-inch teeball using a utility knife.
if you are using hot glue to secure the twine to your “Earth”, insert the knot into the X and then have an adult insert the nozzle to fill the space around the knot.
Once your Earth-Moon model is complete you can use it for a number of activities.
Things to do with your Earth-Moon model:
Activity No.1: Have an eclipse!
For this activity, you will need your Earth-Moon model, and another marble glued to the eraser end of a pencil to represent the Sun. We use a marble for both the Moon and Sun because the two objects appear to be the same size in our sky.
The person who holds the Earth is the observer. The person who holds the Sun stands 6 feet beyond the Moon and tries to hold the Sun model as still as possible.
The person who holds the Moon tries to move the moon-marble smoothly so that it passes exactly in front of the Sun allowing the Earth observer to see an “eclipse.” The Earth observer is allowed to give directions — “a little higher”, “too low”, etc.
Let groups of three try this activity. How many tries will it take your team to make an eclipse where the Moon model completely blocks out the Sun model?
What did we learn?
The Moon’s orbit is tilted a bit (just 5 degrees.) This small tilt means that the Moon is usually a little above or below the Sun as seen from here on Earth – that makes eclipses very rare!
The Moon’s orbit carries it in front of the Sun, blocking out the light during an eclipse. Many people think that the Sun moves behind the Moon. In fact, this is not true. It is the Moon’s orbital motion that carries it in front of the Sun during an eclipse.
An eclipse is very brief. Because the Moon and Sun both appear quite small from Earth (about 1⁄2 degree wide) the eclipse does not last very long. The Sun is blocked for just 2-3 minutes during most eclipses!
Activity No. 2: The Moon’s Orbit in Motion
On a paved area outside, have one person hold the T-ball Earth in one place. Another person with a piece of chalk uses the string attached to the model to draw a circle on the ground – this is the Moon’s orbit. Notice how small the Moon is compared to its orbit!
Cut a piece of string 201⁄2 inches long. Lay this string along the big circle as a measure and put a mark on the Moon’s orbit every 201⁄2 inches. Count how many sections you have divided the Moon’s orbit into.
There should be 28 divisions – and the Moon takes 28 days to orbit the Earth. What does each division represent? It shows how far the Moon moves in orbit each day!
Pick one marked point and use the chalk to label it “New Moon.” Pick the point directly across the circle and label it “Full Moon.” How many days does it take to go from full moon to new moon? It should be 14 days between full and new Moon.
Teachers and parents can challenge children to plan an investigation to test their model of the Moon’s orbit. Can they carry out this investigation on their own and prove the model’s predictions?
Students might observe the Moon in their own back yard and count the days from new to full moon, or they might look up a calendar that shows moon phases and count the days between full and new Moon in several months.
What do we learn?
The Moon does, in fact, move in orbit around the Earth. We are all told this, but few students have the opportunity to make a model and play with it to see this fact in action for themselves.
The Moon moves at a relatively steady pace in orbit, covering about the same distance each day. This orbital speed determines how long it takes to get from full moon to new moon, and the approximate length of a month in our calendar year.
Activity No. 3: Fly Me to the Moon!
(This works best on a flat classroom or gymnasium floor.)
Use the string and place the Moon marble on the floor at the correct distance from the Earth model.
Participants kneel by the Earth model and tries to roll another marble across the floor to hit the Moon marble.
What do we learn?
As simple as this fun activity/game seems, the challenge will be to stop the children once they begin.
Hitting a very distant target with a small projectile is very difficult! Have the children keep score to see how many attempts did it take for them to “land” on the Moon?
Ask your students to remember that the real Moon is not stationary – it is moving quickly in orbit! The Earth doesn’t stay still either, it is spinning quickly on its axis when you launch your Moon rocket. It takes a great deal of calculation and precision to launch a rocket from Earth and have it land on the Moon!
Dr. Daniel Barth left a career as a research scientist to teach. He has spent more than 30 years teaching astronomy, physics and chemistry at the high school and college level. A successful science fiction writer, Barth is the author of Maurice on the Moon, Doomed Colony of Mars and other works. He is currently assistant professor of STEM Education at the University of Arkansas in Fayetteville, and author of the Astronomy for Educators program.
Images are courtesy of L. Eric Smith III
Article from Sky's Up Magazine | July-September 2017