There are lots of seriously vision-impaired people among us, and now we can save them from future eye exams. The late summer
sky offers a series of great vision tests. You can probe sharpness, faint-vision sensitivity, and even color blindness.
The night sky easily discloses the most common forms of color blindness. One of my friends is red/green colorblind. To him, the striking emerald tint of Uranus is totally absent.
A quick way to spot really bad eyesight is when the person sees the Seven Sisters (Pleiades) as a blur — no individual stars at all. This happens when eyesight is worse than 20/50. If you play quiz-show host and ask most normal-eyed people how many of the Seven Sisters they see, a majority may say “seven” because of the name, but six is almost always the honest number. “Seven” is suspicious because the eighth is the same magnitude as the seventh.
The classic test is Alcor, the companion to Mizar — the second star in the Big Dipper’s handle. This “horse and rider,” now in the northwest, was the standard eye exam for centuries. Supposedly, the desert Arabs believed only excellent eyes could detect it. But something is screwy with that: Alcor is an easy magnitude 4, and its 700″ separation from Mizar gives it the apparent width of an aircraft carrier. Maybe the ancients were all myopic and drove their camels around in circles. But since those desert-dwellers named most of the stars, a more logical explanation is that Alcor has brightened throughout the last few centuries.
A real test is Epsilon (ε) Lyrae, the nearest little star to Vega, almost straight up at nightfall. If you have 20/20 vision, it may appear single. With slightly sharper-than-average eyesight, you’ll split it in two with the naked eye. My experience is that in a typical group, about one in ten can see it. Teens do better than adults.
Epsilon Lyrae’s matching components are separated by 208″, or 3.5′. Call it 4′ and you’ve got human vision’s usual limit. On the Moon, that figure corresponds to craters about 250 miles wide. Any 7x binoculars or finder scope will let people with normal vision see all craters more than 35 miles wide, like Copernicus (57 miles) and Tycho (54).
What does it take to observe the duplicity in Albireo, everyone’s favorite double, high overhead this month? Its unequally bright components are 34″ apart and, as we’ve seen, we have to make them appear separated by 208″ if they’re to match Epsilon Lyrae. That means six times the power. A standard 7x binocular thus reveals Albireo’s components to anyone with close to normal vision. Try it.
Only astigmatism remains a problem when using binoculars or a telescope because focusing corrects any myopia or hyperopia automatically.
Faint objects are another story, but you’ve got to be in a dark setting to explore this issue at all. Even a decent sky shows all four stars of the Little Dipper’s bowl and the Milky Way, which is at its best this month. An even darker location reveals its mottled detail and makes the Andromeda Galaxy bright and clear. Let’s call that a “good” sky.
Upgrade to a “very good” location and the Hercules globular cluster remains visible, not just winking in and out. Here is where variations in vision show up: Some people won’t see it at all because their eyes are too insensitive to faint light.
When you experience “superb” conditions, the constellations virtually vanish because there are so many background stars. In such skies, the winter Milky Way (now in the pre-dawn east) is scarcely inferior to the summer version and dominates the heavens. Orion’s “belt” is embedded clearly in a loose star cluster. The Pinwheel Galaxy (M33) looks like a faint Moon-size blob southeast of Andromeda.
Relatively few astronomers have seen M33 with their naked eyes. Observing the Pinwheel Galaxy, therefore, is a major accomplishment, a rewarding goal without using equipment on a moonless September night in an inky-black region. It means your retinas’ faint-light sensitivity is A-OK.
Mental health probably can be tested, too, by seeing what patterns you discern in the constellations’ random inkblot designs. But let’s not go there.