The creative minds of the astronauts aboard the International Space Station (ISS) are always on the alert for a great photograph of Earth, and this one doesn’t disappoint. Taken onSept. 29, 2011, with a Nikon D3S digital camera and a 20mm lens, the image shows how the artificial light from human settlements appears with a characteristic yellow tinge. The green light of the aurora borealis also shines brightly, seeming to reflect off Earth’s surface in Canada. A small white patch of light is almost certainly lightning from a storm on the East Coast (image top right). Part of the ISS appears across the top of the image.
The photograph highlights the Chicago metropolitan area as the largest cluster of lights, next to the dark patch of Lake Michigan. The other largest metropolitan areas include St. Louis, Minneapolis–St. Paul, and the Omaha–Council Bluffs region on the Nebraska–Iowa border. The northeastern seaboard lies just beyond the Appalachian Mountains, a dark winding zone without major cities.
Light clusters give an immediate sense of relative city size. Demographers have used night-time satellite imagery to make estimates of city populations—especially in the developing world, where growth can be rapid.
The sense of scale changes significantly in oblique views. Des Moines is 200 kilometers from Omaha and 375 kilometers from Minneapolis, yet the distances appear roughly the same in this view. In addition to the major metropolitan areas, the rectangular north-south-east-west layout of townships is clearly visible in the rural, lower left of the image. This pattern instantly gives the sense of north orientation (toward the top left corner) and is a distinctive characteristic of theUnited Statesthat helps astronauts quickly know which continent they are flying over at night.
In contrast to the regular township pattern, interstate highways converge on St. Louis, Chicago and other large cities—much like wheel spokes around a central hub.
Rivers—major visual features in daylight—become almost invisible at night. The course of the Mississippi River appears as a slightly meandering zone from Minneapolis through St. Louis and continues out of the lower right corner.