The earth and the moon
This lesson provides an example of the use of a scale model to provide perspective on the distance of
the moon from the earth and their relative sizes.
Learning goals are;
What is a model?
How do we scale a model?
What are the relative sizes of the earth, the moon and the distance between them?
Materials;
Small earth globe model approx. 12 cm in diameter
Bouncy ball (the moon model) 3.0 cm in diameter.
Meter stick
Ruler, one ft/31cm
Procedure:
1. Divide the class into three teams
2. Describe to the class what a model is and how a scale defines the ratio of the measurements of
the actual object to the measurement of the model, e.g., cm (model of earth, globe): km (earth)
3. Write on the blackboard,
12,800 km = the diameter of the earth
3,440 km = the diameter of the moon
4. Have the students measure the diameter of the globe model.
5. Have the students divide the actual earth diameter by the model diameter (the result will be
about 1067)
6. Explain that the result is the scale of the model. (one cm : 1067 km)
7. Have the students calculate the size of a model of the moon in cm at the same scale by dividing
the actual moon diameter by the scale. (The result will be about 3.2 cm.)
8. Give the students a bouncy ball and have them measure the diameter in cm and compare the
calculated size of the moon model diameter with the diameter they measured. Discuss. (The
bouncy ball is smaller than the calculated size but it is close enough to get an idea of the relative
size of the earth and the moon.)
9. Ask the students how far from the earth they think the moon should be placed in the model to
accurately represent the distance between the earth and the moon at this scale. Write the
distances in cm on the blackboard.
10. Write on the blackboard,
352,000 km = the distance between the earth and the moon.*
11. Ask the students if they can convert the distance from the earth to the moon to the same scale.
Show them that they can use the same scale. (The result will be about 330 cm)
RE-SEED Program, Center for STEM Education, Northeastern University 520 INV, 360 Huntington Ave., Boston, MA 02115
Paul G. Conroy, pa.conroy@neu.edu
The earth and the moon
12. Have a student hold the earth model above a point on the floor and measure off 330 cm** (or
whatever result the students calculate) and have another student hold the moon model there.
Explain that at this scale the size of the earth and moon and the distance between the earth and
the moon are now consistent at that scale. You might also like to point out that virtually all the
illustrations and models they will be exposed to are not to scale and therefor do not represent
the real earth and moon relationship in relative size and distance from one to the other.
Challenge them to find illustrations and models and figure out how they are not to scale, i.e,
relative size? Distance between?
13. If time permits you can ask the students how far away the sun would be in a model of this scale.
After some guessing write on the blackboard 150,000,000 km, the earth to sun distance, and ask
how they would convert this to cm for their model. They should know to divide by the scale
1067 for a result of about 140,000 cm or 1.4 km (.87 mi). To get this distance down to less than
10 m so it will just fit in the room the earth and moon models would have to be reduced in size
by 1/140. This would make the earth model about .09 cm (or 0.9 mm) in diameter and the moon
model even smaller. Not a useful model.
* This is the distance from the earth to the moon at perigee (nearest to the earth)
**If the classroom has a 12 in tile floor you can count off eleven tiles since 12 in is approx. 30
cm.
Resources:
Below is the hyperlink to the Maine Solar System model.
http://www.umpi.maine.edu/info/nmms/solar/
12 cm globe version
RE-SEED Program, Center for STEM Education, Northeastern University 520 INV, 360 Huntington Ave., Boston, MA 02115
Paul G. Conroy, pa.conroy@neu.edu