Night Watchman: Black on Black

In June's drab skies, Pluto providesa test of sky-watching expertise.

By Bob Berman|Monday, June 01, 1998
If you love the night sky, you’ve come to the wrong month. The solstice arrives at 10:03 a.m., eastern standard time, on the twenty-first, and these shortest nights are sandwiched by twilights that now linger at their yearly maximums. June offers just five hours of full darkness from places at New York’s latitude, and less than two hours from the northernmost states.

When night finally does fall, you get no reward for waiting. The sky is unusually drab. All the naked-eye planets are at or near their dimmest, at the distant half of their orbits, and don’t rise until after midnight. Mercury is the lone evening star—but only on paper. It’s buried deep in dusk’s twilight. The brilliant winter stars are gone; the Milky Way has not risen. No meteor showers sparkle the heavens.

But rather than surrender to the celestial blackout, consider a challenge: Try to observe Pluto, the planet most famous for its barely there visibility. Pluto is the only planet now reaching opposition (meaning it’s closest to Earth), arriving at its brightest, out-all-night position on May 28. There is debate (dangerously close to assuming the same profundity as the obnoxious old question: Is Australia a continent?) about whether Pluto really is a planet. Can a chunk of ice some 1,400 miles across (about half the size of our moon) properly be called a planet? My position: Who cares?

Whatever Pluto is, you’ll need at least an eight-inch telescope to find it at its daunting magnitude of 13.7; with a bigger scope the task is easier. (Although personally, I find it a challenge with our observatory’s 16-inch Cassegrainian telescope.) Because Pluto is more than a thousand times dimmer than the faintest naked-eye stars of summer, a detailed finder chart, good setting circles, and a lot of patience are prerequisites.

The good news is that our quarry now lies near a few naked-eye stars. None are major luminaries like Antares, some distance below. That’s okay. Pluto would be overwhelmed if it lurked adjacent to such brilliance. Look a degree below little Upsilon in the constellation Ophiuchus, just a degree east of its border with Scorpius, in the south at midnight. There, just 3 degrees to the upper right of medium-bright Zeta Ophiuchi—are a hundred tiny dots. Your prey is among them. Sketch the field, and if one dot has moved a few days later, you’ve found Pluto. Whether or not this is a thrill would make a good question on a personality test.

Too bad it’s so distant. Pluto shares similarities with Earth that go beyond the lack in both worlds of tasty supermarket tomatoes. The two are the only planets with a single large moon (Pluto’s is more than half its own size) and the only ones with atmospheres of mostly nitrogen.

Beyond this, Pluto is a mystery. Earth’s largest telescopes can do nothing with a world so small and distant, and even the Hubble Space Telescope can’t reveal more than vague, but intriguingly dramatic, surface splotches. After Saturn’s strange moon Iapetus—which looks like a global yin-yang pattern, with one hemisphere sprayed black and the other white—Pluto is the most contrasting body in the solar system. The explanation for Pluto’s highly differentiated features will have to await nasa’s proposed Fast Track to Pluto mission.

The view will get worse. Pluto arrived at its once-every-248-year close approach to Earth and sun—inside Neptune’s orbit—in 1989. Now the planet is slowly pulling back into the depths of night. Next January it crosses Neptune’s orbit to resume its role as farthest planet until the twenty-third century.

Better hurry.
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