Scaling the Solar System

Modified version of a typical solar system scale model activity (often called Solar System on a String) in order to emphasize one of the ways that scientists use models. See this example of such an activity.

Author: Lisé Whitfield

Overview: In this activity your class will construct a scale model of the solar system using a basketball to represent the sun. They will calculate the appropriate diameter of the scale model planets and their distances from the sun. They will then make models of the planets out of clay and, as a class, set up the model over a distance of approximately 1.3km.

If you do not have 1.3 km to walk with your students, there are two alternatives:

• Measure the longest distance they can use on the school grounds and use this to set their scale for planetary distances. You will then need to use a different scale for the planets' sizes.
• Use a large picture of the school and area from Google maps satellite view and mark the points where each planet would go.

In contrast to similar activities, this lesson includes some discussion about the use of models in science specifically how models are used to visualize and understand things we cannot see. When we can accurately visualize or model the solar system, we are better able to understand how it works, make predictions about the impacts of various changes in the solar system, and ask questions about it.

Lesson concepts: As a result of this lesson, students will know:

• Scientists use models to:
— study particular aspects of a complex system
— build a basic understanding of a system
— narrow one's focus and/or eliminate possibilities
— gain a more intuitive understanding of a system
— figure out what questions to ask about a complex system or problem
— help us visualize systems that are too large or too small to see
Although models are often used during the "Testing Ideas" part of the Science Flow Chart, they can also be used for "Exploration and Discovery" — to make observations, get inspiration, and determine what questions to ask.
• The names, sizes, and distances from the sun of all nine planets in our solar system

Materials:

• Copy of the diameter and distance from the sun for each planet and the sun. See the data table at UCAR's Windows to the Universe website for the information. You can choose, based on the level of your students, to use either Earths or kms for the distance units. It is often easier for students to visualize in units of Earths. You will also need to explain what an AU is. You can make this into a worksheet for each student or simply write it up on the board.
• A basketball (or other similar sized ball)
• Modeling clay (at least 9 different colors, but you'll need only a very small amount of each color)
• Rulers (1 per student pair)
• Toothpicks (9)
• Calculators (1 per student pair)

Time: One to two periods

Grouping: Whole class and pairs

Teacher background on the Solar System:
Our solar system consists of a star, our sun, and 8 planets. Pluto was considered a planet until 2006 when the International Astronomical Union reclassified it as a "dwarf planet". For the purposes of this activity, we've included Pluto, but you can refer to it as a dwarf planet. The planets are (in order, from the Sun, outward): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. A new mnemonic used to remember the planets in order is, "My Very Educated Mother Just Served Us Nachos."

The solar system consists of eight planets orbiting around one star: the Sun. Neptune, the farthest planet from the Sun, orbits at approximately 30 astronomical units (AU) from the Sun. An astronomical unit is a unit of length used by astronomers. One astronomical unit equals the average distance from Earth to the Sun — about 93 million miles (150 million km). The solar system also includes the Kuiper Belt, a comet-rich area that begins near Neptune's orbit and stretches far beyond it, to about 50 AU from the Sun. Part of Pluto's elliptical orbit extends far into the Kuiper Belt. Beyond Pluto's orbit is another region of icy objects in our solar system, called the Oort Cloud, which extends approximately 50,000 AU from the Sun. In addition, there is an asteroid belt that lies in the zone between the orbits of Mars and Jupiter.

For more information on the solar system, see the Windows on the Universe site from the National Center of Atmospheric Research. This site gives all information at three different student learning levels and in both Spanish and English.

Student prerequisites: Students should be familiar with the names of all the planets in the solar system and the order in which they appear.

Procedure:

1. Ask students to describe an airplane, a space shuttle, or a bulldozer in detail. Allow them to struggle a bit, then show them a model of it and let them see how much easier the model is to visualize. Another alternative is to ask them "Why do we build model rockets in class?" to generate discussion of models.
2. Then ask the students to name the planets on our solar system. Write the names up on the board in the correct order from the sun. Ask them how big the planets are and how far away they are. After some struggle with this, offer up the solution of building a model to help.
3. Show them the basketball and ask them how big each of the planets would be if the sun was the size of the basketball. Ask for a few volunteers to come up to the board and draw and label each of the planets compared to the basketball. Make sure there is enough room on the board so that you can leave these drawings up until the activity is completed.
4. When this drawing of the solar system is complete, ask students how far apart each of these planets would be if the sun were a basketball. Would they fit in this room? Would they fit in the school? Write up some of their guesses about the size of the solar system on the board and make sure you can keep the answers there until the activity is completed.
5. Segue to the activity by asking students how they can figure out if their guesses are correct. Remind them of the model idea if they don't mention it ? "Let's make a model that accurately represents the size and distance from the sun of all the planets. This model will give us a better idea of the sizes and distance in our solar system."
6. If your students are more advanced and have done scaling before you can go directly to step 10. For students who need a review or have never done scaling, explain the concept. You may want to use the analogy of a map. Show them the scale on a map and explain that the scale shows the relationship between distances shown on the map and actual distances. For instance every cm on a map might represent 100 miles.
7. The class needs to figure out what their scaling factor will be for the solar system if the basketball is going to represent the sun. Explain that you will need to measure the basketball and compare it to the real diameter of the sun. Then you can determine how many km each cm of the basketball represents. Ask for a couple of volunteers to measure the basketball. Obviously the ball is a sphere making an accurate measurement somewhat difficult, but get a good estimate and write this number on the board.
8. Show students the measurements in the table given in the "Materials" section. Do the scaling factor calculation by dividing the actual diameter of the sun by the diameter of the basketball. Explain that the answer represents the number of km in every cm of your model. Write this number up on the board and underline or draw a box around it. Students will need to use it for the rest of the activity.
9. Explain that we will use this scaling factor to figure out the sizes and distance from the sun of all the planets in our model. Demonstrate how to use the scaling factor to determine the diameter and distance from the sun of a hypothetical planet (so that you leave the real planets for the students). For example, you could make up a planet that has a diameter of 9500 km and is located 300,000,000 km from the sun. To find the model diameter, divide the actual diameter by the scaling factor. Likewise, to find the model distance from the sun divide the actual distance by the scaling factor. Be sure to use proper units in your example calculations since you want students to have an understanding of the units as well.
10. Assign one planet to each pair of students. Depending on your class size, you may need to have more than two students assigned to each planet or assign more than one planet to a group. Adjust appropriately for your circumstances. If you have varying abilities in your classroom, it would be wise to assign the larger planets to struggling students and the smallest planets to very patient and careful students. Making an appropriately sized model of Pluto, for instance, takes some dedication! Communicate clearly that you expect each group to determine the model diameter and distance from the sun of their assigned planet. As they work, circulate around the classroom to check answers and help struggling students.
11. Pass out a different color of clay, a toothpick, and ruler to each group. Instruct them to make a model of their planet with the correct diameter out of the clay. If possible, put it on the end of the toothpick so it doesn't get lost. As mentioned earlier, the larger planets will be relatively easy to make models of (Jupiter will likely be golf-ball sized), but planets like Pluto and Mercury will be extremely challenging as they are about the size of a grain of sand, unless you have access to very precise calipers or rulers. Just encourage students to do their best. After all, the main objective is to get a sense of the relative sizes. It doesn't have to be perfect!
12. When everyone has finished making their planets out of clay, first call up the Earth group and have them compare Earth to the sun. Then call up the other groups starting with Mercury and have them hold their planet next to the basketball sun and compare to Earth.
13. When all the planets are represented, have the whole class take a look at the model. Refer back to their drawings on the board from Step 2. Ask the class, "What can we learn from this model?" Students may offer answers like "The sun is really big!" or "Earth is small," etc. Take this opportunity to discuss the value of models in science. Refer back to the list in the lesson concepts section. Sometimes scientists use models to study very particular things about a complex system, but models can also be used to build a basic understanding of a system, to narrow one's focus, to eliminate possibilities, to gain a more intuitive understanding of a system, and to figure out what questions to ask about a complex system or problem. In this case, looking at a model of the solar system like this allows us to get a better sense of its size. This may enable us to come up with valuable questions about the solar system or the planets in it. Although models are often used during the "Testing Ideas" part of the Science Flow Chart, they can also be used for "Exploration and Discovery" — to make observations, get inspiration, and determine what questions to ask.

NOTE: If you are unable to walk the distances of this model solar system or to use Google maps to place them on a map, you can stop here and simply make these clay models into a mobile or other permanent display.

This is good stopping place for the day if your periods are shorter. The next part will take about an hour, depending on the pace of your group.

1. After this brief discussion, announce that you need to complete the model by placing the planets at the correct distances from the sun. Ask students to give you the model distances they calculated. They will be in AU, which will be difficult for students to imagine. Take Mercury as an example and convert it into something the students will have a better feel for, like feet or meters. Ask students how far away this distance is. In the classroom? Outside? If outside, where? How far from the classroom?
2. Have the students count how many paces it takes for them to walk a distance of 3 or 10 meters on a marked practice area. You can then tell them the multiplying factor to reach Mercury and do this for each planet. For example, if Mercury is going to be 25 meters away and they measured their pace for 3 meters, they will need to multiply their pace by 8 to find how many paces to take. Then have them take a few more steps (1/3 of their measured paces for 3 meters) when they get to 24 meters.
3. The total distance of the model will be roughly 1.3km, so you need a wide, open space you can walk this distance and leave the clay planets. Have the student count their paces from the start (where you've left the basketball sun) to each planet. This will keep them occupied during the long walk. At Mercury's distance you'll leave the planet on the toothpick and continue to the next planet. Continue from there, leaving each clay planet at its appropriate distance.
4. When you reach the end, have students look back and try to see the sun and each planet. Sometimes I've had a parent volunteer or teaching assistant stay at each place and then hold up the sun and each planet so that everyone can get a good look at the model. They will not be able to see most of the planets. You can add in that if we included the next closest star to our solar system, Alpha Centauri, it would be 8000km away!
5. Return to the classroom, take a second look at the distances from Step 4. Ask students again what they learned from this model. Hopefully they will have a better idea of the scale of things in our solar system and will be excited about continuing to learn about astronomy. They may ask more questions — hopefully! Point out again that models are useful in this way in science. Sometimes scientists construct scale models to:
— study particular aspects of a complex system
— build a basic understanding of a system
— narrow one's focus and/or eliminate possibilities
— gain a more intuitive understanding of a system
— figure out what questions to ask about a complex system or problem
— help us visualize systems that are too large or too small to see
Although models are often used during the "Testing Ideas" part of the Science Flow Chart, they can also be used for "Exploration and Discovery" — to make observations, get inspiration, and determine what questions to ask.

Extensions: Ask students to reflect on this activity and the use of models in science:

• How would they describe the solar system to a friend who is not in this class?
• How did building the model of the solar system change the way they visualize the solar system?
• What additions or changes would they make to the model if they could do it again?
• What other models have they seen or used?

An Understanding Science lesson
© 2010 The University of California Museum of Paleontology, Berkeley, and The Regents of the University of California