May. 21, 2015

Map Sun Trails

by Lawrence Hall of Science

Click to enlarge images
In this activity from Lawrence Hall of Science, you'll use photosensitive paper to record how the sun moves across the sky. 
Be sure to share what you've learned about the sun during Science Friday's Science Club with the hashtag #ExplainTheSun.
Age Level: 6 and up 
Preparation: 5 minutes 
Activity: 5 minutes
Cleanup: 5 minutes
  • Sunprint paper
  • Box—the open top should be larger than the piece of photosensitive paper
  • Scissors
  • Pan of water 
Step 1 
Use the scissors to make a small hole (approximately 0.5 cm diameter) in the bottom of the box. 
Step 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. 
Step 3 
Leave it out for as long as possible, until the sun goes down. 
Step 4 
Rinse the paper in the water, and let it develop. 
What's Going On? 
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.
Going Further
  • 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? 
Since 1975, the Sunprint Kit has provided fun, learning and creative stimulation for the curious of all ages. Originally developed as a teaching tool by educators at the Lawrence Hall of Science on the UC Berkeley campus, all proceeds from the sale of Sunprint Kits continue to aid our mission to inspire and foster the learning of science and mathematics for all. 
About Lawrence Hall of Science

The Lawrence Hall of Science, UC Berkeley's public science center, has been providing parents, kids, and educators with opportunities to engage with science since 1968. Learn more at

The views expressed are those of the author and are not necessarily those of Science Friday.

Science Friday® is produced by the Science Friday Initiative, a 501(c)(3) nonprofit organization.

Science Friday® and SciFri® are registered service marks of Science Friday, Inc. Site design by Pentagram; engineering by Mediapolis.