For a class of 27
How big and how far is the Moon relative to Earth? Typical earth science and astronomy texts typically depict a moon
that is much closer and much larger than the real thing. The example below is typical of what is found in textbooks:
The balloon activity described here will allow students the opportunity to construct a scale model of the EarthMoon
system, both in terms of planetary size and distance. In addition, students make a scale model of Mars, and discover
how far one might have to travel to visit the most Earthlike planet in our Solar System. This is a good introduction
to any study involving Mars or Mars colonization. It is also a good icebreaker at the beginning of a semester, to get
students to interact with each other.
 Obtain balloons. The best are balloons with 21/2 inch diameter when deflated, but any balloons will work fine. An easy way to do this activity is to purchase balloons that are colored red, white, and blue, for Mars, Moon, and Earth (using green for Earth and yellow for the Moon are also fine).
 Discuss the question of size of the Earth relative to the Moon. Determine what misconceptions the students may have.
 Distribute balloons. It is best to provide one third of the class with "Earth" (i.e. blue), one third with "Moon" (i.e., white), and one third with "Mars" (i.e., red).
 Distribute Planetary Data handout, one per student.
 Tell students that the Earth balloon will have a diameter of 20 cm. Have them figure out the scale (divide the Earth's actual diameter by 20 cm. Earth is about 63,800,000 times larger than 20cm). Ask students with Earth balloons to inflate their model approximately 20 cm (obviously the balloon is not a perfect sphere).
 Ask students to look at the handout and calculate the size that the Moon and Mars should be, at the same scale as the Earth model. (Answers: the Moon should be about 5 cm, Mars about 11 cm).
 Have students inflate the Mars and Moon balloons.
 Ask students, at this scale, how far apart are the Earth and Moon? The diagrams seen in common textbooks might lead many of them to suggest that the Moon balloon should be held less than a meter from the Earth balloon.
 Have students calculate the distance from Earth to the Moon at the same scale as the balloon models. The distance is about 6 meters. Have students holding Earth models stand at one side of the room, and a partner holding a Moon model about 6 meters away.
 Point out to students that they now have a scale model of the EarthMoon system. Earth and its Moon are considered a double planet. The distance between the two is the distance traversed by the Apollo astronauts who went to the Moon in the 1960s and 70s. (Have students recall the film Apollo 13).
 Compare the size of the Mars model with the Earth, Moon, and distance between Earth and Moon.
 Ask students how far away they think Mars will be at this scale. Have students attempt to demonstrate it in the classroom.
 Have students calculate the distance to Mars at this scale. The answer is about 120,000 cm, which in more familiar terms is 3/4 mile! Have students identify a local landmark that is about 3/4 mile away.
 Discuss the relative distance between Earth and Mars in the context of a human trip. How long did it take for Apollo astronauts to get to the Moon? (3 days) How long would it take for astronauts using similar technology to get to Mars? Mars Pathfinder, which launched in December 1996, arrived at Mars on July 4, 1997 (7 months). Mars Global Surveyor, which launched in November 1996, arrived at Mars in September 1997 (11 months).
Scale Distances (km) divided by 638 = (cm)
Earth Moon 3.84 x 105 600 cm = 20 ft
Earth Mars 7.80 x 107 1.2 x105 cm = 3/4 mi


Planetary Data Handout Microsoft Word format
Ask students to make models of the martian moons, Phobos and Deimos, at the same scale as the balloon models. They can calculate their scale diameters from the enclosed chart. It turns out that they are about the same size of a small grain of sand!
Have students convert all metric measurements into the English system.
1 inch = 2.54 cm, 1 mile = 1.6 km
