Earth Notes: The Ghostly Glow of Gegenschein

Dec 21, 2016

If you look toward the eastern horizon just before dawn on a clear moonless night, you should see a ghostly white glow shining up through the dark sky.  This is zodiacal light, caused when passing comets and colliding asteroids shed space dust. That dust scatters sunlight upwards well before sunrise.

Gegenschein in lower right of image, Milky Way arcs above it.
Credit Lowell Observatory

More challenging is the search for the gegenschein. From the German word for “counter-shine,” this is a much dimmer cousin of zodiacal light.

Gegenschein is also caused by space dust, but appears when sunlight strikes the dust particles square on when they are in Earth’s shadow, directly opposite the Sun.

The exceptional dark skies on the Colorado Plateau offer a good chance to see gegenschein, especially in October and November and February and March. In these peak months, the elusive glow is relatively high in the sky, away from the Milky Way.

Around midnight on a moonless night, find a place away from town. Let your eyes fully adapt to the dark, then look nearly overhead for a weak, diaphanous, oval cloud about the size of one outstretched fist across. An hour later, if that fuzzy glow has moved westward with the stars, you’ve found gegenschein!

At that moment, the Sun is directly below your feet, as you gaze up at an enormous cloud of interplanetary dust that easily dwarfs the Earth.