Did you ever look up at the sky and wonder how far away the planets are?

Planetary astronomer Heidi Hammel uses the world's most powerful telescopes to explore the furthest reaches of our solar system. By studying other planets, she helps us learn more about Earth.

In this activity, you will be an "Astronomer for a Day." Calculating distances in space is one of the things an astronomer does in her work. Using toilet paper as a measuring tool, you will measure and model the distances between the Sun and the planets in our solar system.

 1 roll of toilet paper (500 sheets) 10 large pieces of paper or large index cards Rocks, stones, or pebbles to hold everything in place Sketch pad, notebook, or paper on clipboard Large poster-size piece of paper Markers 2 pencils Glitter, glue, and tape, or any decorating supplies Large work area: a big backyard, wide sidewalk, or park Camera (optional, but useful!) Photos of planets Planetary Distance Data Sheet Planet Journal A fellow scientist
 Big Sun, Little Earth Did you know that the Sun's diameter is more than 100 times bigger than Earth's diameter? If you made one sheet of toilet paper equal to 10 million miles, then the Sun takes up a little less than 1/10th of one sheet of toilet paper. Earth is 100 times smaller than that little strip of toilet paper. Try This for Size: Fold one sheet of toilet paper into tenths. That's the diameter of the Sun. Next, make a single dot with a pencil or pen on a separate sheet of toilet paper. This dot represents the diameter of Earth.

1. Invite a friend.

Call up a good friend and invite her over. (It's best to do this activity on a day when there's not a lot of wind.) Then get to work!

2. Print this activity.

Get everything on the "What You Need" list and spread it out on a table or desk.

4. Make planet signs.

Fold 10 pieces of paper or index cards in thirds to make a pyramid-shaped sign. Write the names of the nine planets and the Sun on the signs. Decorate them with photos, glitter, and markers. Gather your signs, instruction sheets, and other supplies and go outside. For safety reasons, bring an adult or get permission from the adult in charge.

5. Pick a spot for the Sun.

Choose an important spot to place your sign for the Sun, such as a big tree, your house, or a mailbox. Make sure that you pick a safe spot, where no cars or pedestrians will disturb or endanger you. Secure the sign with a stone or rock.

6. Test your spot by taking 40 big steps.

Test your choice by starting at the Sun and walking 40 yards or 40 big steps in one straight line. Make sure you don't have to cross streets or go through any big objects, such as trees, buses, or cars. If you do, find another spot and start again. You need one safe, straight line with nothing blocking the path to do your work.

7. Measure for Mercury.

Mercury is the planet closest to our Sun. Find the distance and matching number of toilet paper sheets on your Planetary Distance Data Sheet. One person should stand at the Sun, holding the toilet paper roll. The other person should pull the paper out until you have the number of toilet paper sheets you need. Then tear the paper off and fold down the end to make 3.6 sheetsâ€”your exact measurement. (If the paper breaks, don't worry. Just try again.) If there's some wind, you might want to place a stone on the paper to hold it in place.

8. Mark the spot for Mercury.

Place and secure your Mercury name card. Take a picture of both Mercury and the Sun (or sketch them), writing down the number of toilet paper sheets and miles. Check Mercury off on your Planetary Distance Data Sheet.

9. Find Venus.

One person stands at Mercury, while the other walks away in a straight line. Count aloud the number of toilet paper sheets you need to get from Mercury to Venus, and mark the spot with your Venus name card. Take a picture and note your progress.

10. Finish your solar system model.

Repeat this process for the rest of the planets: Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto. Continue to check off your work on your Planetary Distance Data Sheet and take pictures to record your progress.

11. You did it!

You have just constructed a model that shows the relative distances between the Sun and planets in our solar system. Nice work.

12. Analyze it.

In your Planet Journal, write down some of the things you learned on your Planetary Roll Call adventure.

13. Clean it up.

Make sure you pick up all your signs and toilet paper before you go back inside.