Earth-Moon System Model
1 tennis ball (or baseball)
1 ping pong ball (or golf ball)
1 meter/yardstick or measuring tape, or ruler
Pretend the tennis ball (or baseball) is the Earth. Which ball, the ping pong ball (or golf ball) or the marble is about the correct size to represent the size of the Moon?
Answer: The marble! The Moon is about ¼ the diameter of the Earth. A tennis ball is about 2 ½ inches wide; the baseball is just a bit larger. The ping pong ball is about 1 ½ inches wide, so it is more than half as wide as the “Earth” ball. The marble is 5/8 of an inch wide, which is nearly exactly ¼ the width of the tennis ball.
To represent the distance between the Earth and the Moon, have one person hold the “Earth” ball and another person hold the “Moon” ball. If there is more than one child, get more marbles and have each child hold a Moon. Then move the Moon(s) away from the Earth until you think they are about the right distance apart for this scale model. How far apart should they be? Measure the distance you think they should be and write it down. Then check the answer.
Answer: They should be about 6 feet 3 inches apart! This is a lot farther apart than they seem to be in most pictures in books. A good way to figure it out is that the Moon is about 30 x the Earth’s diameter from the Earth. The tennis ball is 2 ½ inches wide.
2.5 x 30 = 75 inches
75 / 12 = 6.25 feet = 6 ft 3 in
Note that on this scale, the Sun would be a huge ball nearly 23 feet in diameter, and it would be nearly one-half mile away!
On April 7, the full moon was near perigee, or near its closest distance to Earth. (See: Easter Supermoon Blog) So how much closer is this than the average 30 x Earth’s diameter above? The average distance of the Earth to the Moon is about 239,000 miles. This Supermoon was closer than most, at 222,000 miles.
Answer: Figure out the ratio and multiply by 30
222,000 / 239,000 = 0.929 0.929 x 30 = 27.87
2.5 in x 27.87 = 69.67 in 69.67 / 12 = 5.8 ft = 5 ft 10 in
Extension: Try this out using different size balls for the Earth and the Moon. For example, if the Earth was the size of a marble, how big would the Moon be and how far away? If the Earth was a basketball or soccer ball, how big would the Moon be and how far away? Remember that the Moon is always ¼ the diameter of the Earth, and the Moon is always 30 x the Earth’s diameter from the Earth.