Seeing the 'Unseeable'


Last updated 6/2/2022 at 9:20am

One of the best tests of the averted vision stargazing technique for viewers in the Earth's Northern Hemisphere lies in the northern sky: the Little Dipper.

On a recent visit to my orthopedic surgeon's office, I was struck by her remarkable ability to point out even the smallest anomalies on my knee X-rays and MRIs. "How in the world did she ever spot those?" I remember thinking. The answer, of course, is that she has an expert knowledge of the subject and undoubtedly has developed a few useful tricks of the trade along the way.

The same is true for astronomers. People are often amazed by our ability to find things among a seemingly random maze of stars. Of course, astronomers have an expert knowledge of the sky as well as years of practice, but that's not the whole secret. To excel at stargazing, we must first understand how human vision works so we can maximize the power of our eyesight.

One of our "secrets" is to use a process we've all experienced: dark adaptation. Enter a movie theater on a sunny day and you know how tough it can be to find your way to a seat. After spending time in the darkened theater, however, seeing around the room is no challenge.That's because, in darkness, our pupils dilate to allow in more light. This process takes time – often more than 20 minutes – but it does eventually happen.

Step back onto the sunny street again, however, and the sudden shift from dark to light can be stunning. Yet within only seconds you're seeing normally again.

Astronomers always allow plenty of time for this process to occur before we begin our observations. Once dark adaptation is completed, we protect our night vision by using only red light to find our way around or to use star maps or logbooks.

Another one of astronomers' tricks is a technique called averted vision. The sensors at the center of the retina are known as "cones"; they see colors quite well, but only under bright conditions. Only the "rods" – the gray sensors surrounding the cones – can see faint light, but they do so at the expense of color. So, to see dim celestial smudges more clearly, we astronomers focus light onto our rods by glancing slightly off to the side of faint objects.

Try out these techniques in a very dark environment. Allow yourself to become fully dark-adapted by avoiding white light. Take with you a flashlight that is covered with red cellophane or get yourself a red LED flashlight. Then find some faint objects in the sky and begin averting your vision to see them.

One of the best tests of these techniques for viewers in the Earth's Northern Hemisphere lies in the northern sky: the Little Dipper. Choose one of its faintest stars and force yourself to stare directly at it. What happens? It vanishes. That's because you're trying to force the cones to work; gaze just slightly off to the side of the star and let the rods do the work, and you'll see the star just fine.

Unless you're an astronomer you probably weren't aware of the tricks used by us "night folk," but you are now. So, no more excuses; get out there and enjoy those feeble photons raining down upon us from afar!

Visit Dennis Mammana at

Rendered 06/28/2024 11:13