# Accurate Model of The Solar System

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## Objective

Students create an accurate scale model of our solar system.

#### Big Idea

The actual sizes of the planets and the distances between them is impossible to represent in a textbook.

## Context and Supplies

Most people have never seen an accurate model of the solar system. One reason for this is it cannot be represented on a single piece of paper or on a screen. The planets are extremely small when compared to the distance between them and for that reason is it not easily modeled.

The purpose of this lesson is to change the mental model that most students have about the size of the planets and our place in the solar system. This is done using actual diameters and distances from the sun and students will apply mathematical calculations to create an accurate model of the sun and planets in our solar system. This activity involves CCSS Math Practice 4: Model with mathematics and NGSS Science Practice 2: Developing and using models, Science Practice 4: Analyzing and interpreting data and Science Practice 5: Using mathematics and computational thinking. This lesson builds off the previous lesson, Going Full Circle, only the objects in orbit are the planets, not human-made satellites. This involves application of NGSS performance standard HS-ESS1-4: Use mathematical or computational representations to predict the motion of orbiting objects in the solar system.

To do this activity, I have a large orange beach ball (38 cm diameter) that represents the sun. I also have a collection of smaller spheres such as a golf ball, a fooze ball, marbles of various sizes, pins with a ball on the end and a small stopper that has a grain of sand sized dot on it. These represent the planets of our solar system relative to the beach ball. Before the lesson, I measure each sphere to make sure they accurately represent all the planets relative to the sun model. For my own reference, I have the Solar System Calculator MS-Excel document which has the scaled sizes of the planet models relative to the sun model. The cell in yellow is where I put the diameter of the ball which represents the sun. The diameters and distances (in the blue cells) for each planet are then calculated automatically. The other astronomical data in the table I leave, should I want to someday expand this lesson.

## Students Scale Down the Planets

25 minutes

Before class begins, I have the various spheres at the front of the class, ordered from small to large, and the first slide of the Scale Power Point displayed on the whiteboard. I tell students that today they will make an accurate scale model of our solar system using the various spheres in the front of the room, with the biggest one representing our sun.

If this is an honors class, I know that most of the students are able to determine how to make a scale and perform the conversions without much guidance. However, if this is a college prep class, the students need more structure and guidance. For that class, I go through the scaling exercise on the power point. The students take notes and perform the calculations in their notes as I go through the presentation. I ask the students if they have ever seen a model of our solar system. I ask what it looked like and have them describe it. Usually it is like the solar system pictures on the first slide on the Scale Power Point. Then, I hand out the Solar System to Scale worksheet. Students complete their own sheet, but they are allowed collaborate. Then I call up groups of 5 to the front of the room where I have the model spheres in size order, each with a letter label (e.g. the smallest sphere is labeled A, the second smallest B and so on). I have the students predict which sphere represents each of the 8 planets by putting the letter on the planet name on the worksheet.  I tell the students that not all spheres have to be used and some spheres can be used more than once. In general, students guess that the planets are much larger than they really are. I have a soccer ball which most students believe is Jupiter.

Students work through the worksheet as I circulate the class. The first thing students must do is determine the scale. The diameter of the actual sun is 1,390,000 km. The size of our scaled down sun (beach ball) is 38 cm. So the scale is 38 cm/1,3900,000 km. If students multiply the rest of the distances by this number, they have the scaled down diameters and distances for each of the planets. The orbital distances are scaled down using the same scale (38 cm/1,390,000 km).

While I circulate, I use Solar System to Scale-Solutions to make sure students have the correct scale and that their calculations are what is expected. If not, I help them correct their mistakes. If a student finishes early, they are to work on answering the rest of the questions on the back of the sheet. It takes the students no more than 15 minutes to complete the scaled distance calculations. At that point, we move on to create our scale model.

## Making Our Solar System

25 minutes

After all students have completed the scaled diameter and distance calculations, it is time to build our model. I ask for volunteers for each of the bodies in our solar system starting with our sun. As students volunteer, I hand them the correctly sized, scaled down sphere and they stand in the front of the room. For the planets, when a student volunteers , I ask them what their calculated value was for the planet. They shout it out and I ask the rest of the class if they agree.

Once our solar system volunteers are lined up in the front of the room, in the same order as the planets in our solar system, I instruct each planet to memorize the scaled distance of their planet from the sun. At that point we move to a larger space. Ideally, I take my class outside to the front of the school (pictured below) so that we can go all the way to Jupiter (which is 200 meters from the sun!). If the weather is bad, then we stay inside in the hallway, which can accommodate the scaled down distance form the sun to Mars (59 meters).

All students start at one end and Pace Out the Planets. Then each of the planets take 1-meter steps out to their orbital distances, starting with Mercury, then Venus, Earth, Mars and Jupiter. I don't let Saturn go as that student would have to leave the high school grounds and Uranus would take a student to the next town!

After the students have seen our accurate scale model of the solar system, we return to reflect on what they learned. For students who want to share, I have them tell the class how their view of the solar system has changed. Usually, students comment about how amazing it is how far away and small the planets are relative to the sun and that the vast majority of our solar system is empty space. They put their reflection on the Solar System to Scale worksheet which I collect as students exit. 