Points of View: Shadows, Photons, Planets, and Life

Abstract

I combine here a potpourri of topics that have been of keen interest over my career. The first is gnomonics, the art and science of sundials: at its heart are the shadows that elegantly chart the motions of the Sun on a defined surface. More broadly, shadows have been amazingly productive in astronomy through the centuries. Examples range from the ancient Greeks (the distances of Moon and Sun) to the Scientific Revolution (Roemer's determination of the speed of light). They also include twentieth century science (solar eclipse observations relative to general relativity) and current twenty-first century research (transits of extrasolar planets). The ability to observe a shadow depends on one's point of view, and I have been fascinated with how our astronomical points of view have been reconstructed over time. For instance, our previous views of the Universe have been continually challenged as we have accessed new types of photons, i.e., the opening of the electromagnetic spectrum that began with radio astronomy seventy years ago. Notions of what a planet is and how it works have been revolutionized by our capability to relocate to other planets and to look at planet Earth from the outside, as well as, over the past decade, to chart new worlds in orbit about other stars. This change is part of a long historical sequence in which we have gone from the seven planets of the ancient Greeks, to the six planets of Copernicus, to the nine planets of the mid-twentieth century to the similar to 150 of today. Finally, we are in the midst of attaining an entirely new vantage point from which to surveil the phenomenon of life. Although still confined to one example, this astrobiologist believes that the signs augur well that life too, will eventually lose its uniqueness, just as has already happened to our Sun, our Earth, and our Galaxy.