Relative to the size of the Earth, our Moon is more massive than any other planet-moon combination in the solar system. For that reason, it has a big influence on life on Earth. So what would happen if the Moon were to vanish; or perhaps if we had multiple moons? That is the subject of a feature by Ethan Siegel, written for July’s Physics World, a special issue celebrating the 50th anniversary of the Apollo 11 Moon landing.