Sailors read a crescent with the whole dark disc faintly visible inside it as a warning of a coming storm, and a Scottish ballad hangs a shipwreck on it.

Sir Patrick Spens sees it and says he fears a deadly storm, and the crew drowns. The sight itself is earthshine — sunlight bounced off Earth onto the Moon's night side — and it shows best in very clean air, which is often the calm before a front. As omens go it has better physical grounding than most.