Part of the experience during Monday’s total solar eclipse will involve looking down, not up.
If the right circumstances come together, streaks of light and shadow will writhe across the ground, a phenomenon sometimes called shadow snakes or shadow bands. Scientists aren’t sure what causes them and NASA is asking people to take video recordings and submit them for study.
In an online article, NASA reported on the first mention of shadow bands in literature, found in the written version of Icelandic oral histories from the ninth century, and in the observations of astronomer George Airy in 1842
“As the totality approached, a strange fluctuation of light was seen upon the walls and the ground, so striking that in some places children ran after it and tried to catch it with their hands,” Airy wrote.
Angela Speck, director of astronomy at the University of Missouri, said she thinks the phenomenon is caused by the jagged edge of the moon. As the disc of the moon and the sun come into conjunction, the sun peeks between the mountains of the moon, creating bright spots known as Bailey’s Beads. The same effect is visible on the surface of the earth, Speck said.
“The edge of the moon is not smooth,” she said. “As you are getting to totality, the levels of light changes. Where the sun is behind a mountain, there is less light, and as come through the gaps the light overlaps, creating an interference pattern. I think that is what you are actually seeing.”
The NASA article is not as certain. Shadow bands don’t appear with every eclipse and the phenomenon, when it does occur, is variable.
“Instead, the intensity, motion and direction of these bands seems to be related to the same phenomenon that makes stars twinkle,” the article states.
Turbulent air in the upper atmosphere refracts the beams of light, the article states, which causes the light to be focused and unfocused in a random, rapidly changing pattern.
“The movement of these atmospheric cells is random between each eclipse and each viewing location, so the appearance and movement of shadow bands cannot be predicted beforehand,” NASA stated.
Speck said refraction may play a part in the display but she doesn’t think it is a complete explanation.
To help NASA study the shadow bands, the space agency is asking individuals to set up a simple recording. It requires a piece of white paper or cardboard, 1 meter square, marked with a line pointing to the direction of the sun and another set of markings showing the directions north, south, east and west. Place a meter stick on the white surface to show the size of the bands.
Set up a smart phone, video camera or still camera to take a continuous set of images and put a time stamp on the photography. The bands will appear for about a minute before and a minute after the eclipse moves to totality, if they appear at all.
Getting the video or the images to NASA is easy, said Laurie Castillo, spokeswoman for the space agency.
“Post it on social media and tweet at NASA and we would be happy to take a look at it,” she said.
NASA has a number of other ideas for individuals to help gather data about the eclipse for future study. In addition to video of shadow bands, the agency is asking people to record temperature changes during the eclipse, she said. A phone application called the Global Observer is available to record the data, Castillo said.
“In some reports during eclipses, temperatures have dropped as much as 15 degrees,” she said.
— You can reach Rudi Keller at firstname.lastname@example.org.