A total solar eclipse will be seen across the United States on August 21st, but the entire show won't be up in the sky.
Mysterious shadow bands will streak across the landscape seconds before and after totality. NASA says that scientists don't fully understand what they are.
Shadow bands are described as thin wavy lines that move in parallel over plain-colored surfaces.
NASA reports that there have been many attempts to explain the phenomenon over the years:
"Because shadow bands are unpredictable from eclipse to eclipse, there does not seem to be a firm connection with the relatively fixed circumstances of an eclipse. Instead, the intensity, motion and direction of these bands seem to be related to the same phenomenon that makes stars twinkle. In the upper atmosphere there are turbulent cells of air that act like lenses to focus and de-focus the sharp-edged light from the solar surface just before totality. 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."
The bands are rarely captured on camera. NASA says that you can help investigate them by using video cameras or rapid still photography to capture the fleeting images.
NASA says that this is a good method to record the bands:
"A large 1-meter square piece of white paper or poster board is essential. Use this as the screen and set up your camera to photograph or record continuous video of this screen as the crescent of the solar surface disappears at the start if the eclipse, and re-appears at the end of the eclipse. Place your digital camera in ‘sports photography’ movie mode so that when you depress the button your camera will take a continuous stream of still images. Make sure a meter stick is placed on the screen so that you can establish size. Also make sure that your pictures or video are time stamped so you can determine their speed, and changes in intensity and direction. Also on the screen, draw a line pointed in the direction of the eclipsing sun during totality, and a line directed North-South and East-West."