The universe is BIG an we are small! Scale of the Solar System

I love teaching about space! Space seems almost inherently engaging to students and they love to learn about planets and stars and think about aliens. One of the challenges of space-related lessons is that the sizes of universe objects are so mind-boggling-ly large that students (and adults) have trouble wrapping their brains around the vastness involved…as well as how teeny tiny we are as humans!

In this lesson we build a foundation for grasping the idea of a giant universe. This lesson can be a basis for the Next Generation Science Standard:

MS ESS1-3 Analyze and interpret data to determine scale properties of objects in the solar system.

and is related to, but not directly correlated with Texas Science Standards

8.8(A) describe components of the universe, including stars, nebulae, and galaxies, and use models such as the Hertzsprung-Russell diagram for classification.

Materials Required: (per group)

  • piece of toilet paper 100 squares long (or about 4.5 meters of adding machine tape)
  • labels for the names of the planets (in order, because I find students often do not know or remember the order despite my very educated mother’s just serving us nachos)
  • assorted spherical objects with a wide variety of sizes (grains of sand, beads, balloons, beach balls, golf balls, etc.)

I like to start this lesson with a story (as usual) about how, when I was little, I wanted to create a model of the solar system on my ceiling fan so I could watch the planets going around and around the sun (lamp) in the middle.

But, being the science person that I am, I didn’t want to just put stickers on the ceiling fan, I wanted to have the distances between the planets and the sizes of the planets be accurate. I used the scale of 10,000 km in real outer space would equal 1 mm on my model. I did some calculations and I found that my plans wouldn’t really work because the smallest I could make the model and still have things be reasonable sizes would be about 4.5 meters long and my ceiling fan definitely did NOT have blades that were 4.5 meters long. And that is not even including the distance to our sadly departed planet Pluto.

Then I challenge the students to complete my childhood dream of having an accurate scale model of the solar system. Each group gets a piece of toilet paper 100 squares long (or you can do some measurement practice and have the students measure out the distance themselves using adding machine tape), labels for the planets, and assorted round objects. Tell the students that they need to select which object of their assorted objects should be used to represent which planet (and the sun) and where each object should go along the toilet paper (or tape). Let them ponder and try things out…

Now this lesson with the scale is actually slightly deceiving because, even using the scale 10,000 km = 1 mm we still can’t really accurately model the sizes of the planets using the same scale. The radius of the sun is 696,000 kilometers, which means the diameter of the sun is about 1,392,000 kilometers. On our model with the scale of 10,000 km = 1 mm, the sun would be 139.2 mm which is 13.9 2 cm. The sun is really really big in comparison to the planets, so the planets would be essentially invisible using that scale.

The sun is REALLY big compared to Earth and the other planets!

Students share out their predictions of what objects would be used to represent the planets and where each object would go. I use the scale for the planet object sizes of 10,000 km = 10 cm….so ONE HUNDRED times bigger than the planets would actually be. Using that scale….

  • The sun would be about the size of a grapefruit in the center
  • Mercury would be about the size of a pencil lead (0.5 mm) – 1 square of toilet paper from the sun (10 cm = 1 square of TP)
  • Venus would have a diameter about equal to the thickness of a dime – 1.8 squares of toilet paper from the sun
  • Earth would also have a diameter about equal to the thickness of a dime – 2.5 squares of toilet paper from the sun
  • Mars would be about the size of a larger pencil lead (0.7 mm) – 3.8 squares of toilet paper from the sun
  • Jupiter would be the size of a large marble (14.3 mm diameter) – 13.2 squares of toilet paper from the sun
  • Saturn would be the size of a small marble (12 mm diameter) – 24.2 squares of toilet paper from the sun
  • Uranus would be represented by ball bearings (5.2 mm) – 76 squares of toilet paper from the sun .
  • Neptune would also be a ball bearing (5.8 mm) – 100 squares of toilet paper from the sun.

Here is a link to a full chart with the size conversions for the solar system distances using one scale (using both the metric measurements and the toilet paper model) and planets using the 100 times larger scale.

I used to use this activity as an introduction to a lesson about light years, but the light years objective has (thankfully) since been removed from the TEKS. Still, bringing in the idea of how long it would take to travel between different objects in space can be useful for helping to reinforce just how BIG space is. Here is a reference chart. This chart is great to use to have students predict first and then “reveal” the answer to, again, show how our ideas of space need some work.

Finally, we wrap up the lesson by looking at the amazing “Scale of the Universe” app (**The app uses flash, so it won’t work on some platforms, you can also access youtube videos as a work around**). I usually start at human size and then zoom out to the “observable universe.” Students always want to see the other way (zoom in) as well, but we have to wait for that until we talk about atoms.

Minds blown! Enjoy.

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