Map Sun Trails

In this activity from Lawrence Hall of Science, you’ll use photosensitive paper to record how the sun moves across the sky. You can order sunprint paper online at www.sunprints.org

Be sure to share what you’ve learned about the sun during Science Friday’s Science Club with the hashtag #ExplainTheSun.

1. Use the scissors to make a small hole (approximately 0.5 cm diameter) in the bottom of the box.
2. As early in the morning as possible, place the Sunprint paper outside in the sun with the box positioned top-down over it, so the sun shines through the hole onto the paper.
3. Leave it out for as long as possible, until the sun goes down.
4. Rinse the paper in the water, and let it develop.

The Sunprint paper is coated with blue molecules that are are sensitive to ultraviolet (UV) light. When you expose the paper to UV light, two types of crucial molecules in the paper interact, forming a new molecule that is colorless. Their interaction is initiated by specific wavelengths of ultraviolet light. When the blue molecules are converted to the new colorless ones, the white of the paper base begins to show through. Areas of the paper that are not exposed to UV light still contain the original blue molecules, so they remain blue.

Two exciting things happen to the paper while you rinse it underwater. First, the blue molecules are water soluble, so the water carries them away, leaving only the white paper base in those areas. Second, the colorless compound that formed when sunlight hit the paper is not water soluble, so it cannot wash away in the water bath. It is sensitive to the water in another way, however. Just as the sun’s light stimulated a chemical change, the water stimulates another one: The water causes an oxidation reaction that turns the colorless compound into the deep blue of a finished Sunprint.

• What would happen if there were two holes, three holes, or lots of holes?
• What happens if the clouds blow across the sun during exposure? Are there any areas on your print where it looks like the sky was cloudier than at other times? How can you tell?
• Do you think the results are the same in the winter and in the summer?
• What might make them different?
• What happens if you leave the Sunprint out for a longer time, or a shorter one?

This activity is featured in DIY Sun Science, an iPhone/iPad app that allows you to investigate the Sun at home, at school, or anywhere you go! Download the free DIY Sun Science app at lawrencehallofscience.org/diysunscience