NASA astronaut Donald Roy Pettit photographed cities on Earth from the International Space Station for more than two decades, creating a visual archive about the evolution of the planet's nighttime lighting. Throughout his four missions to the ISS, Pettit documented cities from around the world, collecting images that allow observation of a significant transformation in urban lights.
The main unusual conclusion Pettit reached is that cities are changing color. The characteristic yellow and orange glow, which for years defined the appearance of cities seen from space, is gradually disappearing. This change is due to the replacement of traditional sodium vapor lamps by other lighting systems.
Sodium vapor lamps were for a long time responsible for the typical yellowish and orange tone of city nighttime lighting. Pettit compiled his images to demonstrate this transition, which becomes especially visible when observed from space. This transformation represents a subtle but significant change in how humanity illuminates its urban centers at night.
