Urania's Mirror or a View of the Heavens: Perseus, holding the head of Medusa, from a set of constellation cards, circa 1825. Repro ID: AST0049 ©National Maritime Museum, Greenwich, London In the constellation of Perseus there is a bright star. Over several hours, its brightness often varies. Arab astronomers thought that this star was the evil eye of the Gorgon (Medusa) winking at them. They named it Algol, the 'demon' star.

In the 18th century, John Goodricke of York, a young deaf-and-dumb astronomer, explained that Algol must be a binary star. This means two stars, orbiting one centre of gravity. One of the stars is fainter than the other. When the fainter star passes in front of the brighter star, it blocks the light coming from it and the star dims.