What are noctilucent clouds? “Good question. They hover near the edge of space, glowing electric blue. Some scientists think the clouds are seeded by space dust and fed by rocket exhaust. Others suspect they’re a telltale sign of global warming. Whatever causes these mysterious clouds, they are lovely, and summer is a good time to look for them. Check our gallery of recent sightings.” NASA

These are a relatively recent phenomenon, first seen in 1885 about two years after the Krakatoa eruption, which filled the atmosphere with volcanic ash worldwide., making sunsets so spectacular that evening skywatching became a popular pastime.