A total solar eclipse swept across parts of Greenland, Iceland, Portugal and Spain on Wednesday, producing a rare optical phenomenon known as "shadow snakes," or shadow bands, that scientists are still working to fully explain.
The eclipse occurred when the Moon passed between the Earth and the Sun, completely blocking sunlight along a narrow path of the planet. During the event, observers reported the shadow bands, a phenomenon skywatchers have described for centuries but that remains difficult to study.
The bands appear as long, dark lines separated by pale gaps that ripple across the ground or the sides of buildings shortly before and after totality, the period when the Moon fully covers the Sun and the sky darkens. The human eye can easily pick out the shifting lines, but their subtle contrast makes them notoriously hard to photograph or film.
Where the eclipse was visible
During Wednesday's eclipse, scientists were able to witness the phenomenon as the path of totality crossed the far north of Russia, eastern Greenland, western Iceland, the northern half of Spain and northeastern Portugal.
Spotting the "snakes" requires a smooth, light-colored surface. On grass or asphalt, they cannot be seen at all.
Scientists cautioned observers to watch closely so as not to miss them. The bands tend to appear just before totality, the most thrilling moment of the eclipse, when viewers are wearing solar filters and watching the last thin crescent of the Sun disappear.
Total eclipses will not last forever
According to NASA, tidal forces are gradually pushing the Moon farther from Earth, with its orbit widening by about 4 centimeters per century.
Over billions of years, this drift means the Moon will eventually be too far away to completely block the Sun, ending total solar eclipses. Annular eclipses, in which the Moon does not fully cover the Sun and a "ring of fire" appears around it, would still be possible.
