Most of the cloud images you see on TV come from high-altitude weather satellites that are parked about 22,000 miles above the equator.

There's also a group of lower-flying satellites that are part of a Department of Defense program. These orbit about 500 miles up, and were originally designed to help nighttime aircraft navigation by picking up moonlight reflecting off clouds. But the Air Force discovered that on evenings with little moonlight, the satellites could detect the lights from cities. Over many clear nights, this data was pieced together to produce a global image of city lights.

With the lights as markers, it's easy to see the outlines of the continents. The brightest lights clearly locate the most urbanized areas, information that's been used even by global warming researchers to determine what temperature data has been contaminated by urban effects. Tomorrow night, I'll return to this stunning image, and highlight a few specific areas.