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Searching for obscurity

What makes us look up?
bob_berman_2009
During the next three months, Uranus shines at an unusual magnitude 5.7. If you’re in a city, that’s still too faint for the unaided eye. But here in my isolated hometown of Willow, New York, population 156, Uranus’ rare brightness provokes tail-wagging excitement.

If you can visit rural friends between now and Thanksgiving, you, too, can see Uranus with no optical aid. I hope that this past spring you already observed the asteroid Vesta with your naked eye, since it reached an easy magnitude of 5.5. And while you’re in the country, count the Pleiades. Six are obvious, but from dark locales, 11 are not difficult to find if you have good eyesight.

Who would get excited about such dim, barely there objects?

We do.

In truly unpolluted places, the dimmest test target is probably the Triangulum Galaxy (M33). Until I spotted it myself from Organ Pipe Cactus National Monument in southern Arizona, I doubted it was even possible. Then two autumns ago, I saw it from my own backyard. M33 is surprisingly big and perfectly round. But man, it’s faint. It’s part of a parade of dim but very cool targets.

This year Sirius A and B are separated by about 10", letting backyard telescopes finally see the Dog Star’s famous companion, “The Pup.” Glimpsing this white dwarf is hard because it’s 25,000 times dimmer than the primary. For the next decade, they are maximally separated in their 50-year orbit, spread apart by the same distance as the Sun from Uranus. I’ve enjoyed the much easier white dwarf 40 Eridanus B. But seeing the Earth-sized Pup, the most famous white dwarf, delivers a thrill. It’s the same thrill as seeing Uranus naked-eye. Or counting 11 Pleiads.

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Can you see Uranus without a telescope? If you live in — or travel to —  a rural area this fall, the answer may be yes. Amateur astronomers revel in challenges like spotting the magnitude 5.7 disk of our solar system’s seventh planet.

NASA/JPL-Caltech/Kevin M. Gill
Why? Is it the challenge? The test of night vision? You hardly need astronomy for that. After nightfall, many homes have scurrying cockroaches. Residents could theoretically tiptoe into a dark kitchen to assess how many they can count. Taking such an inventory would provide an easy appraisal of nocturnal vision acuity. Yet amateur astronomers probably prefer to count stars in the Pleiades, even though it inconveniently requires stepping into chilly air. This suggests we’re motivated by something other than testing eyesight.

A more basic assessment to see whether real darkness even exists in your environment involves merely checking for the presence of color. Human vision uses two separate physiological processes that switch on and off beyond our control. The first, which yields sharp, colorful images whenever the environment is bright, is called photopic vision. The second, which produces less distinct grayscale images in low-light conditions, is scotopic vision. At any given moment, it’s easy to tell which is operating. 

Observing any galaxy through a telescope automatically employs scotopic vision. Its maximum acuity is 20/200. That’s legally blind. Let’s repeat: Observing a galaxy through a telescope renders you legally blind. It’s no wonder that at the eyepiece, you cannot perceive the sharp galactic detail displayed in this magazine’s photographs. And since scotopic is grayscale, no one’s ever directly seen the Whirlpool’s campfire-yellow core or its cobalt spiral arms.

A luminosity below 0.7 lambert is where our vision switches to scotopic and thus becomes colorless. (A lambert is defined as the brightness of a light source that emits 1 lumen per square centimeter.) For comparison, the average flat-panel TV has a brightness of 100 lamberts. So the next time you’re observing from your backyard, check to see if there’s any color around you. If the ambient light pollution is sufficient to make grass look green and show your home’s exterior color, you automatically know your photopic vision is operating — which also means no Uranus. It’s simply not dark enough.

An easy alternate demo of your personal scotopic/photopic boundary involves checking star tints. There are no green stars, but the pastel blue of Rigel, Spica, and Sirius, or the orange of Aldebaran and Betelgeuse, are obvious because these are bright enough to reliably provoke photopic vision. But now look at the Great Square of Pegasus, nicely up at midnight. Scheat, upper right, is reddish, but at 2nd magnitude may or may not stimulate a photopic response. Markab, lower left, is blue, but at 3rd magnitude almost definitely won’t show its color. Now boost these stars to 0 magnitude by using binoculars. Bingo. Their colors dramatically pop. Binoculars let you determine where your scotopic vision kicks in.

Travel to where there’s no doubt. Drive a few hours until you reach “nowhere.” In early autumn, the reward is the year’s best faint, intricate celestial detail, such as the countless Dali-esque dust lanes in the Cygnus Milky Way. Such grayscale attractions are endless because the faint universe is a bottomless realm.
And if this majestic obscurity belongs to any group of people, it belongs to us.

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