Why is the sky dark at night? It sounds like a child’s question, and it is, but it bothered Kepler in 1610 and the German astronomer Wilhelm Olbers in the 1820s. If the stars go on forever, every line of sight should end on one, and the whole sky should blaze like the surface of the sun.

The first answer that holds up came not from an astronomer but from Edgar Allan Poe, in Eureka, a long prose poem about the universe that he first gave as a lecture in New York in February 1848. The dark gaps between the stars, he suggested, are where the light hasn’t arrived yet: the background is so far off that no ray from it has had time to reach us. Lord Kelvin worked the same idea through properly in 1901, and cosmologists now agree it is the heart of the answer. The universe has an age, so when we look out we are looking back, and the past runs out.

Poe thought Eureka was the most important thing he would ever write. Most readers took it for the odd late work of a man best known for a raven. He was right about the sky anyway. Darkness isn’t always emptiness. Sometimes it’s light that is still on its way.