A phenomenon sometimes seen briefly several minutes before and
after totality as rapidly shimmering, irregular bands of shadow on the ground
and walls. (A white surface helps make them more visible since shadow bands
have low contrast.)
Bands may be a few centimeters (1/2 to 2 inches) apart, up to a meter (a few feet) apart and travel a few meters per second (about 10 feet per second). The cause of shadow bands is probably the refraction or distortion of light from the thin solar crescent by the Earth's atmosphere. Even veteran eclipse observers do not always see them. They are also difficult to photograph. (Use fast film and a fast lens with short exposure times—1/250 second or less.) Most intense bands seem to occur over dry, warm areas rather than from coastal and shipboard areas. (Water surfaces may help to thermally stabilize air layers that would otherwise cause the bands.)
Refer to Figure 3.3. Shadow bands vary considerably in both width and separation, but range most frequently between 2 and 5 cm (0.75 and 2 in) in width and are separated from one another by 5 to 25 cm (2 to 10 in).
Their direction of motion across the ground seems to depend upon where an observer is located along the eclipse path and whether the bands are observed before or after totality. Their velocities vary most often between 1.5 and 3 m (5 and 10 ft) per second.
In short :
- The flying shadows consist of undulating, moving bands or shades of light and shadow of some centimetres to some decimetres in width. The intervals between light and dark are alike and holes of some meters can come between the pattern.
- The bands are perpendicular to the edge of the moon's shadow further away from totality, and parallel with the edge of the shadow near totality. The band orientation runs parallel with the sun's crescent. The band orientation is the same near the central line before and after totality. Near the extremities of the totality zone, the band orientation has shifted up to 90 degrees.
- The turbulence, which is mainly the cause of the shadow bands, is under 2 kilometres altitude. Turbulence in the tropopause would have no impact on the shadow bands 2 to 3 seconds away from totality.
- Although they sometimes seem stationary, they can reach a speed of some meters per second. One shadow band can appear and disappear so fast that it moves individually over more than one meter before it dissolves.
- Shadow bands are visible from some seconds to various minutes before and after totality. During partial and annular solar eclipses, this could even be more. The bands themselves have the biggest contrast when there is space between them, and the least when the sun's crescent is at it's smallest.
- The contrast between light and shadow is bigger when wavelengths are shorter. So the shadow bands must be seen the best in blue light and the worst in red light. The band gap on scale (band spacing) is the square root of the wavelength near totality.
- Longer solar eclipses would have shadow bands with more contrast and linearity.
- Because the turbulence and the winds are not predictable, the explicit shadow bands can not be predicted. But with the meteorology they can be better understood.
Shadow Bands - Sicily 1870