1. Review planet order and relative sizes in our solar system.
Display the NASA illustration: All Planet Sizes. Ask students to point out the location of Earth. Then challenge them to identify all of the planets, outward from the sun (left to right): inner planets Mercury, Venus, Earth, Mars; outer planets Jupiter, Saturn, Uranus, Neptune, and Pluto. Remind students that Pluto is no longer considered a planet in our solar system; it was downgraded to the status of dwarf planet in 2006. Point out the locations of the asteroid belt (between Mars and Jupiter) and Kuiper belt (past Pluto) if they were included in this illustration. Explain to students that the illustration shows the planets in relative size. Ask: What do you think relative size means? Elicit from students that the pictures show how big the planets are when compared to each other and to the sun. Ask: Which planet is the smallest? (Mercury) Which is the largest? (Jupiter)
2. Have students gather data and compare planet sizes.
Divide students into small groups. Distribute one copy of the worksheet Planetary Size Comparison to each group. Have groups use the Planet Size Comparison interactive to find and record data on planet diameters and ratios. Ask:
 What do you notice about the size of the planets? (Possible response: The inner, rocky planets are smaller than the outer, gaseous planets.)
 How do you think the sizes of the planets compare? (Possible response: There is a big difference in the sizes of the planets. Some are fairly small and others are extremely large.)
 Would it be easy to model the planet sizes? Why or why not? (Possible response: No, because of the great differences in size.)
 How could we model differences? What everyday objects could represent planets and the sun? (Possible responses: peas/beach ball; grains of sand/orange)
Have students discuss the answers in their small groups. Then regroup as a class to discuss students’ ideas.
3. Build background about the astronomical unit (AU).
Explain to students that an astronomical unit, or AU, is a simplified number used to describe a planet’s distance from the sun. It is a unit of length equal to the average distance from Earth to sun, approximately 149,600,000 kilometers (92,957,000 miles). Only Earth can be assigned AU 1. Planets farther away would have AU greater than 1; planets closer would have AU less than 1. Ask: Why do you think scientists find it helpful to use astronomical units? (Possible response: Distances in the solar system are very large. Using AU helps keep the numbers manageable, or smaller, so we can easily calculate very large distances.) What are the challenges of using kilometers or miles instead? (Possible response: Using kilometers or miles would make calculations more difficult and could produce errors in measurements required to accurately send a probe or lander to another planet.) Explain to students that the astronomical unit provides a way to express and relate distances of objects in the solar system and to carry out astronomical calculations. For example, stating that the planet Jupiter is 5.2 AU (5.2 Earth distances) from the sun and that Pluto is nearly 40 AU allows you to more easily compare the distances of all three bodies.
4. Introduce the modeling activity.
Tell students that they are going to stand in for the planets and planetary objects in order to create a model of relative planetary sizes and relative distances. Display the NASA illustration: How Big is the Sun? to give students a sense of the relative sizes of planets compared to an everyday object like a basketball. Make sure students understand that the distances between the planets are very large compared to the sizes of each planet. This makes it extremely difficult to create an exact scale of our solar system, so this activity will focus on the distance comparison.
5. Have groups create models of relative planetary distances.
Divide students into groups of 9, 10 or 11, depending on class size. (If 9, one student represents the sun and the remaining students represent 8 planets; If 10, the sun, planets, and asteroid belt; If 11, the sun, planets, asteroid belt, and Kuiper Belts) Take students to a large area, such as the gymnasium or empty parking lot. You’ll need enough space for each group to spread out and create their model, using the following scale, with each step equaling approximately 1 meter (about 3.28 feet):
 Sun: stands at the edge of the area
 Mercury = 1 step from sun
 Venus = 2 steps from sun
 Earth = 2.5 steps from sun
 Mars = 4 steps from sun
 Asteroid belt = 8 steps from sun
 Jupiter = 13 steps from sun
 Saturn = 24 steps from sun
 Uranus = 49 steps from sun
 Neptune = 76 steps from sun
 Kuiper belt = 100 steps from sun
Emphasize that at this scale, the sun would be less than 1.3 centimeters (0.5 inches) in diameter. Ask students to describe what they notice about planetary distances from the model. If needed, allow one student from each group to put an object in their place and walk around their group’s model to make observations.
6. Have students make a math connection.
Distribute copies of the worksheet Stepping Out the Solar System to each group. Have students recalculate the number of paces for each planet’s orbit, as constrained by the size of the available area. Use the provided answer key to check groups’ work. Then have students recreate the model.
Informal Assessment
Have students work independently to summarize, in writing, what they learned about our solar system, including:
 locations of planets in relation to the sun and one another
 relative sizes of planets, including Earth
 relative distances of planets
 any conclusions they can draw about the locations of the asteroid belt and Kuiper belt
Extending the Learning
Encourage students to practice backyard astronomy. At certain times of day and year, it's possible to view planets Mercury, Venus, Mars, Jupiter, and Saturn with the naked eye. Students can use Sky and Telescope Magazine's Sky at a Glance feature to find out what planets are visible in the night sky and where to look. Ask students to report back to the class with what they observed.
Subjects & Disciplines

Earth Science
 Astronomy
 Experiential Learning
 Mathematics
Learning Objectives
Students will:
 calculate and model planetary distances by converting astronomical units (AU)
 compare planetary sizes using data
 analyze relative planetary sizes and distances using a kinesthetic model and data
Teaching Approach
 Learningforuse
Teaching Methods
 Discussions
 Handson learning
 Modeling
Skills Summary
This activity targets the following skills:

Critical Thinking Skills
 Analyzing
 Understanding
Connections to National Standards, Principles, and Practices
NCTM Principles and Standards for School Mathematics
 Measurement (68) Standard 1: Understand measurable attributes of objects and the units, systems, and processes of measurement
National Science Education Standards
 (58) Standard D3: Earth in the solar system
What You’ll Need
Materials You Provide
 Paper
 Pencils
Required Technology
 Internet Access: Required
 Tech Setup: 1 computer per small group, Projector, Speakers
 PlugIns: Flash
Physical Space
 Classroom
 Gymnasium
 Parking lot
Grouping
 Largegroup instruction
Background Information
Our solar system includes the sun and eight planets that orbit around the sun. The smaller, inner planets include Mercury, Venus, Earth, and Mars. The inner planets are rocky and have diameters of less than 13,000 kilometers. The outer planets include Jupiter, Saturn, Uranus, and Neptune. The outer planets are called gas giants and have a diameter of greater than 48,000 kilometers. Pluto, which used to be considered the ninth planet, is now classified as a dwarf planet and part of the Kuiper Belt. The main asteroid belt lies between the orbits of Mars and Jupiter, separating the inner and outer planets. Relative size means how big the planets are when compared to each other and the sun. Relative distance means how far apart the planets are when compared to each other and the sun.
Prior Knowledge
 planets in our solar system
 order of planets
Recommended Prior Activities
 None
Vocabulary
area of the solar system between the orbits of Mars and Jupiter filled with asteroids.
(AU) (150 million kilometers/93 million miles) unit of distance equal to the average distance between the Earth and the sun.
width of a circle.
celestial body that is nearly spherical but does not meet other definitions for a planet.
length between two points relayed in nonphysical units such as time.
rough measurement using depth perception, as how something far from the viewer looks smaller.
copy of an object that is larger or smaller than the actual object, but maintaining the same proportions.
the sun and the planets, asteroids, comets, and other bodies that orbit around it.
Video
Websites
Tips & Modifications
Modification
If you do not have enough space to do the kinesthetic model, you can modify this activity to have students create a stringandbead solar system model in the classroom by converting astronomical units to a 10 centimeter/AU (4 inch) scale. Students will need a string 4.5 meters long. Have students tie beads in place to represent planetary distances.
Sun 0.0 AU=0 cm
Mercury 0.4 AU=4 cm
Venus 0.7 AU=7 cm
Earth 1.0 AU=10 cm
Mars 1.5 AU=15 cm
Asteroid Belt 2.8 AU=28 cm
Jupiter 5.2 AU=52 cm
Saturn 9.6 AU=96 cm
Uranus 19.2 AU=192 cm
Neptune 30.0 AU=300 cm
Tip
Make sure students understand that these are only models or visualizations of the relative distances between the planets and the sun. Planets all orbit the sun at different velocities, so they rarely form a straight line from the sun. Instead, they appear somewhere along their orbital paths.
Funder