As seen from space, Earth looks quiet, and a bit lonely, during the day. Our blue marble is interspersed with the lush green of forests, but that’s about the only sign of life. It’s impossible to tell anyone is home, especially anyone who thinks, and moves around, and builds things. There are no political borders. There are no cities or ports or bridges.
But look at night, and you will see a very different planet. The world’s metropolises glow yellow-white, as if someone sprinkled the continents with a dusting of tiny stars. You can follow rivers, railroads, and highways by the communities that cropped up along them. You can see boats in the open ocean. Artificial light is arguably the clearest sign that Earth is an inhabited planet. And it is one of the best ways to see changes in how we inhabit it.