Nocturnal luminosity is one of the few reliable sources of information about the country. It implies that North Korea’s economy is poorer, more volatile and more vulnerable to weather than formerly thought.

Night lights are a strong proxy for economic activity. A new paper by the IMF finds that they explain 44% of the variation in countries’ gdp per person—as close a tie as that between a person’s height and hand size. In places where records are poor or manipulated, night lights offer an alternative measure of output...