Watch now: The Early Years of the Guardianship, with Richard Hollinger

jpegPIA02991

Global city lights. The Eastern U.S., Europe, and Japan are brightly lit by their cities, while the interiors of Africa, Asia, Australia, and South America remain (for now) dark and lightly populated. (Image by Craig Mayhew and Robert Simmon, NASA GSFC. Based on data from the Defense Meteorological Satellite Program).

Up Next...