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Activity: Worlds to Scale

LEVEL
Ages: general audiences

RATIONALE
A graphic demonstration of the varied and enormous sizes of worlds in our Solar System.

LENGTH
1-2 hours, in advance, to prepare the model.
15-20 minutes for demonstration.

OBJECTIVES
Describe the relative sizes of planets, moons, and our Sun.

MATERIALS

• Several sheets of foamcore or cardboard
• Rope, at least 30 meters
• Optional: clear tape, salt & pepper
• Optional: paper or string to show distances of moons from their planets
• Ruler, compass, pencil, and string with pushpins, for drawing circles

PREPARATION

1. Using the radius for each world, set the compass or string length, and draw a cardboard circle. Cut it out and label it with the name of the world.
2. For asteroids, cut approximations of the three largest, then use salt to simulate most of the rest. Catch these between two pieces of clear tape (or one folded over). For comets, use a combination of salt and finely-ground pepper.
3. Measure the rope to the cirucumference of our Sun. (I use a bright yellow nylon rope.)
4. If desired, cut string or paper to show the relative distances of the larger moons from their planets. This can be interesting -- Earth's Moon is quite distant compared to the sizes, while Mars's moons (and Pluto's partner world) are fairly close together. It takes extra time, though, and can be a pain in the neck.

PROCEDURE

I usually start by taking an inflatable "Earth", and comparing it to three different size balls. The one that's about 1/4 the diameter of Earth is approximately the right size to represent our Moon. Then I wrap a string 9-and-a-half times around Earth, and spread it out to show the distance between Earth and Moon relative to their sizes. I use this to explain why I don't use the same scale to show the sizes and distances in our Solar System.

I start with Jupiter and go down the planets by size, handing them to people to hold. Then I ask them to line up in order from Sun (at one side of the room or space). We might argue a bit about whether to put Pluto before or after Neptune (it was closer to our Sun 1979-1999). Typically, someone comes up with the Asteroid Belt, so I give the taped cutouts and salt to hold between Mars and Jupiter. Then I point out that the "storehouses" of comets begin at about Neptune, and have someone hold the taped salt & pepper at the end. If I'm doing moons, I hand out the larger moons (anything larger than Pluto, basically) to the people with the planets they're orbiting. (This may seem like a waste of time, but I think it's important to show that there are moons that are worlds -- real places, bigger than some of the traditional planets.)

If I'm doing this with a class, there are usually people left over, so they get to stand in a circle and hold parts of the rope representing our Sun. Note that Sun is about 10 Jupiters across, and Jupiter is about 10 Earths across, so the sizes can really boggle the mind.

SPACING

*corrected from previous version; thanks to the teachers who noticed the error.

I include the asteroid belt (the edges are approximate). I also refer to the Pluto/Charon system, rather than the planet Pluto, because the two bodies are essentially a double planet. The Kuiper Belt of comets starts approximately at Neptune's orbit, and the Oort Cloud (of comets) goes to about 4000 times the average distance to Pluto.

Uranus is pronounced YOO-rah-nus. Charon can be pronounced several ways, but planetary scientists tend to pronounce it SHAR-on.

Credits:
Data from Beatty, Chaikin, The New Solar System 3rd Ed, 1990, Sky Pub.

Copyright © 1998, Elizabeth E. Roettger. Permission to reproduce the instructions is granted only when full copies, including this paragraph, are reproduced and distributed.

Last updated: 21 Feb 2001. URL: http://www.nthelp.com/eer/HOAworld.htm