Why is the sky dark? What’s happening between the stars?

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 apparent answer to the question “why is it dark at night?” is that (1) the sun is shining on the other side of the earth and (2) the light of the distant stars is much weaker than the sun.  But how can the second answer be so when there are so very many stars and they have been shining for so long? 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”?  I will discuss these questions as well as the historical scientific attempts to understand this simple and important observation.

Tom Geballe is an Emeritus Astronomer at the Gemini Observatory, in Hilo Hawaii. He received his bachelor (1967) and doctoral (1974) degrees in physics at the University of California, Berkeley, the latter under Nobel prize winner Prof. Charles Townes. Following postdoctoral appointments in physics at U. C. Berkeley and at Leiden University, and a Carnegie Observatory Fellowship in Pasadena, he joined the United Kingdom Infrared Telescope (UKIRT) in Hawaii in 1981 as a staff astronomer and then became Astronomer-in-Charge, Associate Director, and Head of Operations. His research interests include the Galactic center, interstellar gas and dust, star formation, novae, supernovae, unusual evolved stars, brown dwarfs, and the surfaces, atmospheres, and aurorae of solar system planets and moons.