If the universe were uniformly populated with stars stretching to infinity in all directions, as casual observation suggests, then every line of sight from the Earth would end on the surface of a star and the sky would be uniformly ablaze with light. The fact that the sky is dark at night tells us that there is something wrong with this apparently reasonable deduction. This conflict between theory and observation is known as Olber's Paradox although in fact it was first stated by Kepler. Lord Kelvin argued in 1901 that the night is dark because the stars have a finite age. The visible universe has a finite size, which is determined by the speed of light and the age of the stars, and therefore only a fraction of the sky is covered by stars (Phil. Mag Ser 6, 1901,2,161-167).