The Universe in the Classroom

Classroom Activity: A Grapefruit Saturn

Lynda Filip
John Percy

University of Toronto



It is crucial that students first understand that light is an entity that travels through space outward from its source. Many 10- and 11-year-old students think of light as a source (light bulb, stars), an effect (bright patches on the ground) or a state (brightness). These are all associated with light, but they are not the light itself.

To determine what your students think about the behavior of light, ask these questions before the activity:

1. What is light?

2. Where is light in this room?

3. How does light from the Sun get to Saturn?

It also helps for students to draw a picture of how light from a lamp causes a shadow.


Grade Level

This activity is designed for students in grades 7 to 9, but can be simplified for grades 5 or 6. Younger students can eyeball the distances, rather than calculate them.


Students will construct a scale model of Saturn; use it to explain a Voyager image of Saturn's shadow on its rings; and, by trial and error, discover that light moves through space in straight lines.


Part 1: Pre-Activity Announcement

During the previous class, announce, mysteriously, that everyone will need to bring in a drawing compass, pencil, ruler, calculator, flashlight, and...grapefruit. The grapefruit should have a circumference of about 11 1/4 inches (28.5 centimeters). When they go shopping, students can measure circumference by taking an 11 1/4-inch piece of string to the store and wrapping the string around the grapefruits on display. Students can cut the strings in class and take them home. (From now on, we will use metric units, as astronomers do.)

Part 2: Discussion of Saturn

Hand out photocopies of the image that the Voyager 1 space probe took after its flyby of Saturn. It shows the planet at an angle impossible to see from Earth. Have your students write a brief, but detailed, description of what they see. Draw a picture of Saturn on the board or overhead and ask a few students to share their observations while you label the following: lit side, dark side, 'A', 'B', and 'C' rings, and so-called Cassini division. If the students correctly identify the ring's shadow on the planet and the planet's shadow on the rings, label these as well. If not, leave these features as a mystery that students will solve by constructing their own Saturns.

You can also show a video clip from the opening of Star Trek: Voyager, in which the starship glides along the icy rings of a planet, or a NASA animation of the Voyager flyby of Saturn.

Part 3: Constructing the Model

1. Break into groups of 4 or 5. Have two materials-managers from each group pick up handouts, transparencies, scissors, paint, brushes, and toothpicks for the whole group. Have each student tape his or her transparency to a blank piece of paper in order to make their pencil markings more visible.

2. For older students: Go over the handout. Explain that they will have to calculate the widths of the rings of their grapefruit Saturn so that their models will be proportioned correctly.The proportionality equation is:

radius of Saturn radius of grapefruit

width of ring width of model ring

So, the width of the model ring is:

radius of grapefruit x width of ring

radius of Saturn

The radius of Saturn is 60,330 kilometers. The radius of the 28.5-centimeter grapefruit is 4.5 centimeters. For larger or smaller grapefruits, use the formula:

radius = circumference ÷ 2 ÷ 3.1416.

For younger students: Write the precalculated widths of Saturn's rings on the board (see chart below) and have students copy the numbers onto their handouts. These figures apply to the 28.5-centimeter grapefruit:

Feature Scaled width (cm)
'A' ring 1.1
Cassini division 0.3
'B' ring 1.9
'C' ring 1.3
Space between Saturn and 'C' ring 1.0

3. Students must now figure out the sizes of the circles they will draw with their compasses. For example:

4. Advise your students to place their compass points in the very center of the transparency and proceed to draw the five concentric circles (see handout). They should then cut away the transparency on the outside and inside.

5. Next, students paint their rings. They should place each ring on a scrap piece of paper. The 'C' and 'A' rings need one thin coat of paint, and the 'B' ring a very thick coat of paint. Sprinkle glitter on the rings before the paint dries. If you are using glitter paint or glitter glue, let the first coat dry for 20 minutes and then add another.

6. Finally, they can assemble their models. Stick four toothpicks equidistant around the "equator" of the grapefruit. Then place the ring system on the toothpicks. To prevent the ring from falling off, use glue, staples, or small pieces of folded plastic to hold it in place.

Part 4: Casting Shadows

This part of the activity may be done in the next class period.

1. Draw students' attention to the Voyager image of Saturn. Find out what they think is going on by asking:

2. Turn off all the lights in the classroom and cover the windows. Break into groups of 2 to 4. Each group should have one flashlight, which represents the Sun. Have students explore different flashlight and grapefruit positions, recording what happens. Challenge them to recreate the shadow in the Voyager image by tilting their models and positioning them relative to the flashlight.

3. Students should find that:
(i) Only a slight tilt of the face of Saturn towards the Sun will produce a shadow as in the Voyager image. What does this tell them about Saturn? (Answer: It is tilted in its orbit around the Sun.)
(ii) As they raise or lower the flashlight relative to the grapefruit, the shadow of Saturn on its rings changes. When the flashlight is high, the shadow is a semicircle falling on the rings. As the flashlight is lowered, the shadow stretches out until it no longer appears curved, but straight -- as in the Voyager image. Have your students hold a piece of paper perpendicular to the planet as an extension of the ring system. This will catch the top part of the shadow, showing it is actually still curved.
(iii) When the shadow of the planet on the rings is straight, as in the Voyager image, it falls on the equator.

4. Your students may enjoy making shadows of Saturn on the walls. Point out that the shadows of Saturn on its rings and on the classroom wall are caused by the same property of light -- the fact it travels in straight lines.

5. Have the groups discuss and answer the following question: What causes the shadows? Suggest that the groups make a series of drawings that show the shadow for different flashlight heights. The drawings should also show the light rays from the flashlight.

Light usually travels in perfectly straight lines. For this reason, when sunlight reaches Saturn, it doesn't curve -- it either hits the planet or misses. If you look behind the planet, the area where light is blocked by the planet is dark (the shadow).

6. After a few groups have shared their answers, ask: What does the shape of the shadows tell you about the path of light? (It is a straight line.)


1. How long does it take for sunlight to reach Saturn? Light travels at 300,000 kilometers per second and Saturn is 1,427,000,000 kilometers from the Sun. (Answer: 1.3 hours.)

2. To find Saturn in the night sky, students can consult a star chart in the newspaper or magazines such as Astronomy, Sky & Telescope, or Mercury. The planet is often, but not always, visible at night.

3. Students can write a report about the Voyager missions to Saturn in the early 1980s. They can compare the trajectory of Voyager 1 with that of Voyager 2 and select the most important discoveries. They can also read up on the Cassini spacecraft that was launched on Oct. 15, 1997, and is due to arrive at Saturn in 2004. The November Astronomy and September/October Mercury had cover stories on Cassini.

LYNDA FILIP is an education graduate of the University of Toronto. She is now teaching in Chapel Hill, N.C.
JOHN R. PERCY is an astronomy professor at the University of Toronto. His email address is

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