When watching celestial bodies rather than human ones, the dilemma remains. Sometimes, it’s best to stare directly. Other times, you get better results through averted vision — looking off to the side.
The strategy is due to the retina’s design. Images are focused through our pupils onto the fovea, the center spot, where the light meets 8 million cone-shaped cells crammed into a small area. This zone of so-called photopic vision works only in bright light.
Digression: The location of what we see is commonly taken to lie “out there” in front of us. But everything we see, the images themselves, actually occur in the occipital lobes of our brains. In a very real sense, there is no external world. You perceive only the inside of your brain, where everything visual actually takes place.
Too far out? Let’s continue with the second variety of vision, which kicks in when light dims. Away from the fovea lie 20 million rod-shaped cells, whose sin is colorblindness. Not one rod lurks in the middle 1° of vision; they’re most densely packed 15° to 20° from dead center. So, in low-light situations, when cones no longer operate, we suffer a 1° blind spot at the center of vision.
There’s also a second blind spot caused by the optic nerve. This one’s not centrally located, and we don’t usually notice it: If an object is hidden at the blind spot of one eye, it will be seen by the other.
Anyway, to most clearly observe using rods, you need to look about 15° away from the object of interest, while nonetheless keeping your attention on it.
Rods are also very slow-reacting, which is why it takes so long to acquire night sensitivity. Scotopic vision switches on whenever ambient light falls below 0.3 lamberts, which, alas, is precisely the realm vampires and astronomers most adore. More than 99 percent of the things we observe telescopically shine at these low light levels. And after enduring these deficiencies, scotopic vision still delivers only 20/200 acuity, 10 times less sharp than photopic vision. You’ve always sensed the truth of this. Haven’t you noticed that details on galaxies are smudgy, while the Moon can be wonderfully sharp? That’s where astrophotography comes to the rescue — by revealing the fine features and colors of faint objects that our rods simply can’t perceive, ever.
So, the real fun starts when we learn how best to put scotopic averted vision to work. These nights, Cancer is still in the west at nightfall. Its famous Beehive Cluster (M44) was noted through history as a blob, a nebula. Yet people with normal eyesight can gloriously split it into individual stars with just their naked eyes — but only by using averted vision. Wham! The blur breaks into distinct star points.
At that same hour, Lyra rises in the east. Look at Epsilon (ε) Lyrae, the nearest star to Vega. Try it with direct, then averted, vision. Here, direct vision works best because Epsilon is bright enough to stimulate some cones, allowing good sharpness resolution. If you’re blessed with 20/15 vision, Epsilon splits into two components that provide the best eyesight test in all the heavens.
Use averted vision under dark skies to increase the naked-eye Pleiades or to telescopically glimpse spiral arms on NGC 4565’s dust lane, but not to discern fine planetary detail such as Saturn’s Cassini Division. This tool requires some practice and care.
Who would have guessed that in dim light, the clearest scenes materialize only when we don’t look at them?