The sky is dark at night —this is a fundamental observational fact of cosmology that can be observed by everyone. It 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 the light of the distant stars are much weaker than the sun. But how is this possible when there are so many stars that have been shining into space for so long? And how dark is the sky? 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’?

Tom Geballe of Gemini Observatory will answers these questions, as well as delving into historical and scientific attempts to understand this simple yet important observation