The human eye is a remarkable device. Most of what we know about the world and universe around us we owe to this 1-inch diameter gelatinous globe.
As much as the eye can show us around our daily world, it requires plenty of light to work properly.
It’s not made for viewing in low-light situations and, as you can imagine, it’s quite limited when it comes to astronomical observing.
For example, go out to a dark site some night and just try to see colors around you.
Sure, it’s easy to see colors of taillights or flashlights, or even distant city lights, but these are relatively bright.
Check out the colors of your car or someone’s shirt without shining a light on them, and you’ll see only shades of gray.
This is because the color receptors near the center of the retina — the “cones” — are triggered only by relatively bright light.
The shades of gray we see come courtesy of the sensors known as “rods”; these surround the cones and work pretty well under dim lighting.
We can improve the ability of our eyes to see in darkness by a process known as dark adaptation.
You’ll get a great demonstration of dark adaptation if you try to find a seat in a dark movie theater after entering from a bright sunlit street.
By the time you’re ready to head out to the refreshment stand or restroom, you’ll discover that navigating inside the theater is no longer a problem.
That’s because our pupils dilate in the darkness to allow in more light. This process takes time — at least 30 minutes — but it does occur eventually.
To have a chance to see the stars most effectively, we must allow dark adaptation to take place, and that requires at least 30 minutes without looking into a bright white light.
Astronomers always allow plenty of time for this process to occur before we begin our observations, and once we do, we protect our night vision by using only red light to find our way around or to use star maps or logbooks.
Even so, there’s another challenge our eyes face when peering at the night sky. After you’re dark-adapted, choose something faint in the sky and stare directly at it.
A good subject for this experiment is the Pleiades star cluster, midway up in the western sky after dark this week.
If you stare directly at it, you may be surprised to watch it disappear!
Why should this be? Because when you stare directly at a faint object, you’re focusing its light onto the center of the retina, where the color-sensing cones reside, and they don’t work with faint light.
So whatever you’re staring at will vanish.
Now suppose you avert your gaze a tiny bit up or down or to the side. You’ll be focusing its light onto the rods; they work pretty well under such conditions, and the Pleiades will pop sharply back into view.
This “averted vision” technique helps overcome some of the limitations of our amazing eyes.
It also works when peering through a telescope or binoculars, and even while walking or driving after dark.
