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Hands-on science (no lab required).

After reading "Meet Your New Neighbor" (p. 8), try this hands-on activity to learn how a planet's density, or measure of the amount of matter in a given volume, affects its shape.

Most planets aren't perfectly round. Rather, they are oblate, or slightly flat at their poles and bulging around their centers. This pumpkin shape is a result of the planet spinning on its axis, or the imaginary line around which an object rotates, and the planet's density.


Which material would you expect to form a more oblate shape: a "planet" made from lightweight construction paper or from a heavy manila folder?


ruler * scissors * 1 sheet of construction paper * 1 manila folder * one-hole paper punch * transparent tape * 4 pencils of equal length * pencil and paper


1 Measure and cut two 2.5- by 17.5-centimeter (1- by 7-inch)-long strips of construction paper.

2 Measure and draw a dot at the following distances on each strip: 1.25 cm (.5 in.), 8.75 cm (3.5 in.), and 16.25 cm (6.5 in.). Make sure the dots are centered.

3 On one strip, label the dots in order "A," "D," and "B." On the other strip, label the dots in order "B," "C," and "A."

4 Punch a hole through dots "A" and "B" on each strip.

5 Overlap the holes labeled "A" and use tape to form one long strip of paper. Don't cover the holes with the tape.

6 Overlap the holes labeled "B" and use tape to form a circle. Don't cover the holes with the tape.

7 Tape the eraser ends of two pencils together to form one long pencil.

8 Insert the pencil through hole "B," then through hole "A."

9 Use tape to attach the paper circle at hole "A" about 2.5 cm (1 in.) from the top of the pencil. Adjust the paper to form a circle.

10 Measure and record the polar distance, the distance between points "A" and "B," and the equatorial distance, between points "C" and "D."

11 With the paper circle at the top, roll the taped pencils back and forth between your palms five times. Measure and record the polar and equatorial distances again.

12 Readjust the paper to form a circle with the same size you measured in Step 10. Repeat Step 11 two more times.

13 Repeat Steps 1 to 12 using a paper circle made from a manila folder.


1. What happened to the polar and equatorial distances of the paper circles after rolling the pencil?

2. Which paper circle--the light paper or the heavy folder--changed its shape more after being rolled?

3. Would you expect a dense planet to have a circular or oblate shape? Explain.


1. The polar distance should have decreased while the equatorial distance should have increased.

2. The paper circle made from the lightweight paper should have changed its shape more than the circle made from the heavy manila folder.

3. A dense planet should have a more circular shape Like the paper circles, as a planet spins on its axis, an apparent outward force tugs away from the center, flattening out the planet Denser planets have a stronger gravity to counteract the outward tug, so they tend to be more circular.
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Title Annotation:planets density
Publication:Science World
Date:Nov 28, 2005
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