The riddle of why is the sky is dark at night. This celebrated riddle originated in the sixteenth century. In 1823, Wilhelm Olbers presented it in the simplest terms: In an infinite universe, populated everywhere with stars, a line of sight in any direction when extended out into space must ultimately intercept the surface of a star. Hence stars should cover the entire sky. And if all stars are sunlike, the sky at every point should blaze as brightly as the disk of the Sun. Olbers has been credited incorrectly with the discovery of the riddle; he did, however, express it in this lucid form, and showed that the riddle still holds even when stars are irregularly distributed in clusters.