To begin, place all the spheres in the box and ask each participant to choose one. Make sure each person has one of a pair so all can participate.
Investigation I: Determining the relative sizes of the earth and the moon
Once the spheres have all been chosen, tell participants that each sphere can be either the earth or the moon. Ask each person to find a partner whose sphere could represent the earth to their moon or the moon to their earth. Remind people to be flexible in their thinking, as one person’s “earth” could be another’s person’s perfect “moon” (especially if many different kinds of spheres are involved)—or vice versa.
Once partners have paired up and settled on acceptable models, have them compare their choices. Have people who chose the same spheres made the same or different decisions about their partners? Usually, people will know that the moon needs to be smaller than the earth, but may not have much more information than that. Encourage participants to share their thoughts and ideas, and tell them they can switch spheres or partners if they decide they’ve put together incorrect pairs.
When everyone has settled into an earth-moon pair, reveal to the group that the diameter of the earth is four times the diameter of the moon. Have each pair check their decisions by making sure their moon fits four times across the diameter of their earth. Ask pairs to reconsider their choices once again. Do they need to change one of their spheres or their partner? Make measuring tapes available for those who want to check their choices, and help facilitate so all partners find correctly proportioned spheres for the next part of the activity.
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.