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.

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!

Print this page, the Planetary Distance Data Sheet, and your planet photo page (http://pds.jpl.nasa.gov/planets/). Examine them carefully with your friend.

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

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.

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.

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.

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 sheets—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.

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.

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.

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.

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

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

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

If you were to map out the solar system in actual miles, you would have to travel around Earth's equator 1,445 times just to travel the distance between the Sun and Mercury (the closest planet). Fortunately, in this data sheet, one sheet of toilet paper represents 10 MILLION miles! This sheet allows you to see how spread out the solar system is on a more manageable scale—all with a roll of toilet paper!