The sky is dark at night – this is a fundamental observational fact of cosmology that can be observed by everyone. It also is fundamental to our existence, to our physiology, and to our cultures. The obvious answer to the question “why is it dark at night?” is that the sun is shining on the other side of the earth and that the light of the distant stars is much weaker than the sun. But how can this be so when there are so very many stars? And how dark is the sky? And is it dark only to eyes like ours that are sensitive to visible light, or is it also dark to infrared, ultraviolet, x-ray, and radio “eyes”? Understanding in more detail why the sky is dark gives us important information about the nature of our universe. Geballe will discuss these issues as well as the historical scientific attempts to understand this simple and important observation.