Rarely do we think about light and the process commonly known as seeing. You arise, let us say, quite early on a Saturday morning, cup of coffee in hand. You peer out the front window and, incredibly, you see a figure across the street wearing only his pajamas casually raking leaves. Shocked, you run to the door, open it, and tell him that you see him.

More precisely put, detectors on your face (eyes) have picked up visible light composed of packets of energy called photons coming from the scantily clad neighbor. Those photons, strictly speaking, are not that person, simply electromagnetic energy radiating away. Moreover, the energy packets have taken time, albeit an astonishingly short time, to travel from his location to your eyes. You may, by now, be thinking that this author is editorially picking nits. Not so, particularly when the subject is astronomy.

Photons, in terms of physics, possess no rest mass (massless). Trust me on this or look it up. In this context, light is, indeed, light (weightless). Moreover, there are no stationary photons; all travel at the speed of light, 186,282 miles per second in a vacuum. Coupling these facts with vast astronomical distances, an astute observer comes to an inescapable conclusion: The stars above appear as they were some time ago.

The moon, for example, is only a bit more than one light second away. Nearby star Sirius is 8.6 light years off, while the star cluster M13 is much farther at about 25,000 light years. Telescopes receiving light from M13 in the constellation Hercules (visible for some by unaided eye from dark sky locations) are detecting the object as it was 25,000 years in the past. Typically, stars survive for billions of years, and so almost all of them currently visible in M13 are still producing light. Imaged galaxies that are billions of light years away are another matter. Component stars may not all currently exist. Some may have exploded to create exotic objects such as black holes or neutron stars.

Notice that when light year distance is expressed in units above one or more, light travel time and light year distance are equal.

— Kent Fairfield is a volunteer with Pine Mountain Observatory and a lifelong amateur astronomer. He can be reached at kent.fairfield@gmail.com. Other PMO volunteers also contributed to this article.