Key concepts
Light
Rotation
Planetary motion

Introduction
When was the last time you really examined the shadows around you? Most people don't pay much attention to shadows and how they change. That's because most people don't think shadows are all that important. But the movement of shadows in sunlight is evidence of something very important, something that it took people thousands of years to figure out—that the Earth spins like a top and orbits around the sun.

Background
Every morning the Earth spins or rotates so that the side you are on faces the sun. As the Earth spins you see the sun come up over the eastern horizon. As the Earth keeps spinning and your day progresses, the sun seems to move across the sky. When the Earth turns so that the side you are on faces away from the sun, you see the sun disappear below the western horizon. From where you are, on the surface of the Earth, it looks like the sun is moving—but it is really the movement of our own planet that makes the sun seem to move and the shadows change.

Materials

• A sunny day (for the most dramatic results, try early or late in the day)
• A yard, driveway, playground or picnic table
• A watch or clock
• Colored chalk of at least four different colors (if sketching on pavement)
• Crayons of at least four different colors (if sketching on paper)
• A big piece of paper (butcher paper or an unfolded paper grocery bag)
• At least four small rocks or other heavy objects to weigh down paper edges
• A shadow maker—an object with an interesting shape such as a bottle, toy truck or action figure

Procedure
• Find something with an interesting shape to serve as your shadow maker, such as a bottle, a doll, a toy truck or an action figure.
• Go outside and find a place where you can see your shadow. Put the big piece of paper on a driveway or sidewalk, or on a picnic table if there is one. Place rocks on the edges of the paper to prevent it from blowing away or moving.
• Place your shadow maker in the middle of the paper. Why do you think you would want to place the object in the center?
• Use a crayon or chalk to trace around the shadow maker's current shadow. Is the shape you trace the same size as the thing? Is it the same shape? What do you think will happen to the shadow’s size, shape and direction as the sun changes position (or more accurately, as we change position in relation to the sun)?
• Wait 20 minutes, and then trace the shadow again using a different color crayon or chalk. Has the shadow moved? Has its size or shape changed?
• Continue to trace the shadow every 20 minutes until you have captured the shadow's movement. How has the shadow changed? Why do you think that is?
• Extra: To continue experimenting with shadow art, try some of these changes: trace the shadow every 10 minutes, or every 30 minutes, or once an hour, or once every two hours. How do the spaces between the shadows compare?
• Extra: Try the activity at different times of the day. What do the shadow patterns look like in the morning? Around noon? Late in the day? Do you notice any patterns?

Observations and results
All day long, shadows are shrinking and growing and changing shape. You might discover that your sketches will not work very well around noontime. At noon, when the sun is high in the sky, shadows will just look like dark puddles around the bottoms of objects. Early in the morning and late in the day, shadows change size and shape much faster than you may expect. In the morning and in the afternoon, when the sun is lower in the sky, objects will have much longer shadows.

If you want, you can turn this into a game with friends or family. Mark the shadow of something tall, such as a utility pole, then have each person mark their guess as to where the shadow will fall with a different color of chalk. Wait 15 minutes and see whose guess was closest.

Shadows can also be useful in navigating. In North America, for example, shadows will point toward the north at around noon because at that time the sun is close to due south. Therefore you can find which direction is approximately north at noon if you have a shadow to watch.

More to explore
Making a Sun Clock, from the Exploratorium
Light and Shadows: The Sun Moves in the Sky, from Learn NC
The Reason for the Seasons, from Allentown School District Planetarium
Seasons and Shadows: Investigate How Shadows Shift throughout the Year, from the Exploratorium

Rad Shadow Sketch was developed by the Exploratorium and is featured on page 44 of Exploralab: 150 Ways to Investigate the Amazing Science All Around You. Created by the Exploratorium, Exploralab is a book that takes curious kid scientists ages 8–12 through 24 hours’ worth of household investigations, experiments and discoveries.

This activity brought to you in partnership with Exploratorium