This global view of Jupiter's moon, Io, was obtained during the tenth orbit of Jupiter by NASA's Galileo spacecraft on 19 September 1997 at a range of more than 500,000 km (310,000 miles). Io (which is slightly larger than Earth's moon) is the most volcanically active body in the solar system. Colors are enhanced. Credit: NASA/JPL/University of Arizona
The night sky is a dark place, with only the moon and stars to cast any light. But throughout the long winters high in the northern hemisphere there are nights that are illuminated with spectacular displays of green and blue light dancing across the skies. We are talking, of course, about the Northern Lights. The Aurora borealis, to give them their proper name.