In 1969 the Apollo 11 crew left a panel of mirrors on the Moon, a hundred little glass corners, each about an inch and a half across, so that people on Earth could fire lasers at them and time the trip there and back. The main thing those mirrors have told us since is that the Moon is leaving: it drifts about an inch and a half further away every year, roughly as fast as a fingernail grows. So each mirror is as wide as one year of the Moon going. Nobody planned that. It is the kind of coincidence that would be too neat in a story, which is why I’m glad it turned up in a measurement instead.