From artists renditions of the solar system, kids readily gain a “picture” of the relative positions of the sun, earth, moon, other planets, etc. What is more difficult to appreciate is the distances between these objects. The following modeling exercise, known as the cosmic distance ladder, may help:
In this model, the sun is the size of a grape (one half inch in diameter). The earth and other planets are grains of sand. Now for distances between:
Student representing the sun (grape): stands at the edge of the area
Mercury (tiny grain of sand) = 1 step from sun
Venus (grain of sand) = 2 steps from sun
Earth (grain of sand) = 2.5 steps from sun
Mars (grain of sand) = 4 steps from sun
Asteroid belt (dust particles) = 8 steps from sun
Jupiter (big grain of sand) = 13 steps from sun
Saturn (big grain of sand) = 24 steps from sun
Uranus (big grain of sand) = 49 steps from sun
Neptune (big grain of sand) = 76 steps from sun
Kuiper belt = 100 steps from sun
Remind students that these are relative dimensions within the solar system. Lets leave the solar system and go on to the nearest star, about 4 light years away. On the same scale, the student representing this distance would have to take close to 7000 steps, roughly 1.3 miles.
Return again, to the fact that this relative distance scale is based on the size of the sun being the size of a grape (one half inch in diameter) and the Earth bring a tiny grain of sand! Then the nearest star (represented by another grape) is over a mile away!!!! Impress on students how much of outer space is exactly that–vast stretches of empty space.
Measurement of Cosmic Distances
Giving these relative distances should lead students to ask: How are such distances measured? The following video provides a historical sketch of how such distances have been estimated.