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Europe at night, satellite image. City lights (yellow) show areas of dense population. There are few lights in North Africa (bottom) and the Arctic (top) since they are more sparsely populated. This image was produced by combining data from the Defense Meteorological Satellite Program (DMSP) and NOAA's POES satellites. (KEYSTONE/SCIENCE PHOTO LIBRARY/PLANETARY VISIONS LTD/SCIENCE PHOTO LIBRARY)