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the kinetic energy of an impact object

Investigation II: Determining the relative distance between the earth and the moon

Once all participants have found a correct earth-moon partner, ask each pair to think about how far their moon should be from their earth. Remind people that different pairs of spheres will create different-sized models. Ask people to consider what they may already know about how far the moon is from the earth, and encourage them to share their thoughts and ideas. Again, make sure meter sticks and/or measuring tapes are available for them to use, though the use of non-standard measurements is also encouraged.

Once pairs have settled on an acceptable distance for their models, ask them to stand that far apart holding their spheres at their shoulders (for easy sight lines). As before, ask participants to compare their decisions with other pairs and make changes if they decide want to reconsider their choices.

Finally, reveal that the earth and moon, on average, are about 30 earth diameters apart. Ask people to check their models. Using a meter stick or measuring tape or another method, each pair needs to separate their earth from their moon by 30 times the diameter of the model’s earth sphere. Once that’s done, each pair will have made a scale model of the earth-moon system.


One challenge of presenting scale models in astronomy is the difficulty of modeling both the size and distance of two (or more) celestial bodies simultaneously. It’s helpful to think about the correct ratios and sizes by simplifying the numbers. In this activity, it’s easy to remember that four moon diameters will fit across one earth diameter, and thirty earth diameters will fit between the moon and earth.

Investigation I: Determining the relative sizes of the earth and the moon

Most people’s ideas of the relationship between the earth and moon come from photos and illustrations that may exaggerate the size of the moon in relation to the earth. As a result, participants probably started by creating pairs in which the moon was smaller than the earth (which is correct), but larger than the correct ratio dictates.

The moon’s diameter is actually 2,159 miles (3,476 km); the earth’s diameter is 7,926 miles (12,756 km). By simplifying these measurements (in miles), it’s easy to remember that the earth’s diameter is about 8,000 miles, and the moon’s diameter is about 2,000 miles. The ratio of the two is about 4:1, meaning that the earth’s diameter is four times greater than the moon’s diameter, and that four moon diameters will fit across one earth diameter.