How and when to see the gegenschein, cousin of the zodiacal light and one of the greatest night sky naked-eye challenges.
Tell someone they can see the gegenschein at their dark-sky site and you'll probably be met with disbelief. But as with many celestial sights, it's easier to see than you'd think — if you know what to look for and when.
First off, how do you pronounce that word anyway? It's GAY-gen-shine, the German word for "counter-shine." The gegenschein is a faint, diffuse brightening along the ecliptic directly opposite or counter the Sun. At local midnight (1 a.m. Daylight-Saving Time), the counterglow appears as a round to oval patch of light about 8-10° across within the zodiac constellation crossing the southern meridian at that time. You can see it an hour or two earlier or later, but it's highest and easiest to spot during the midnight hour.
As with its brighter cousin the zodiacal light, we're seeing sunlight reflecting off dust ejected by comets and released during asteroid crackups. The greater part of it is concentrated in the plane of the solar system, the reason both phenomena are centered on the ecliptic, home to the planets, Moon, and Sun. Sunlight scattered forward off dust in the direction of the Sun creates the zodiacal light. Back-scattered light from dust directly opposite the Sun toward the asteroid belt gives us the gegenschein.
Since the gegenschein lies opposite the Sun, much like a full Moon or planet at opposition, sunlight strikes the dust particles square on. All the tiny shadows cast are hidden behind each and every grain so they don’t subtract from the belt’s brightness, creating a brighter spot in the sky. The Moon experiences a similar "bump" in brightness at the time of full phase. Astronomers call it the opposition effect. We also see the same phenomenon as a halo of light around our heads when looking at smooth or regularly-textured ground with the Sun at our back.
From the darkest sites you can actually "see" the path of the ecliptic as the hazy, zodiacal band, a much fainter extension of the both the zodiacal light and gegenschein that wraps all the way around the sky. Twenty miles north of my home in Duluth, Minnesota, the counterglow is plainly visible on moonless, transparent nights during the fall and spring. And that's with the southwestern sky aglow with city light pollution. On the best nights, I've been able to trace a 50° segment of the fainter zodiacal band. If I can see it with my old-guy eyes, you can, too.
To be successful, your eyes need to be fully dark adapted and the southern sky should be as free of light pollution as possible. For mid-northern observers, there are two peak viewing seasons: October–November and February–March. At these times, the gegenschein is relatively high in the sky and little hindered by atmospheric absorption. If you can see the weak glow of the Milky Way in Taurus, you should be able to make out the gegenschein, which is similarly dim.
Check the calendar and look up at the appropriate spot along the zodiac for a very diffuse, puffy smudge larger than you might imagine — nearly one outstretched fist (10°) across. Play your eye around the spot using the same averted vision technique you'd employ to eke out detail on a deep-sky object through the telescope. Trust your gut if you think you see it then look again in an hour. Has it moved westward with the stars? Yes? Congratulations!
Look again another night and then another until your familiarity with the counterglow's appearance becomes second nature.
Several nights ago, I estimated the gegenschein's diameter at nearly 10° with "wings" of fainter zodiacal band extending from either side toward the Pleiades and Aquarius like a giant ghostly Band-Aid.
You might think that December and January would make for best viewing, when the counterglow peaks in altitude, but the Milky Way gets in the way, making it extremely difficult to tell the two apart. The best viewing window this season continues through about October 20 then opens again from November 4–20. Remember that the gegenschein reaches peak altitude at 1 a.m. local daylight time (which ends in the U.S. and Canada at 2 a.m. November 1, 2015) or midnight local standard time.
Good luck in your gegenschein quest. I love the thought of the Sun far below my feet at midnight and knowing that Earth's shadow, which easily covers the full Moon at its opposition point, can't touch the enormity of the interplanetary dust cloud facing us at the midnight hour. Such a strangely inspiring sight.
I'd be remiss without one last mention of the zodiacal light, which, like the gegenschein, is on best display this month and next. And how can you pass up the sight of three bright planets — Venus, Mars, and Jupiter — nestled within its fuzzy glow? Look east for a large, rightward-leaning column of hazy light starting about 2 hours before sunrise now through about October 25.
No matter what your schedule, there's plenty of cosmic dust for everyone.