The Ghostly Glow of Gegenschein


Gegenschein from New Mexico (14mm 5DII)

The band of light at right is the familiar Milky Way. But what’s that faint stream of light to the left? It’s called the Gegenschein.

My last blog showed an image of the Zodiacal Light in the west. In fact, that glow extends up and continues all the way across the sky as a very faint stream of light at the threshold of vision called the Zodiacal Band. But at the place in the sky 180° opposite the Sun the Band intensifies to become a diffuse patch of light. It’s easy to see with the naked eye from a dark site if you know to look for it, and where it is likely to be. Last week, it was sitting below the constellation of Leo, in an area of sky that is pretty blank. So any faint glow stands out.

As with Zodiacal Light, what you are seeing is sunlight reflected off comet and asteroidal dust, but for the Gegenschein (German for “counterglow”) the light is coming from dust opposite the Sun that is scattering light back toward us and the Sun, mirror-like. The back-scattering effect makes the dust appear a little brighter at the point opposite the Sun.

Spring is a good time to spot the Gegenschein (true for the northern or southern hemisphere) as it then lies in a sparse area of sky away from the Milky Way. But you need a moonless sky and a dark site free of manmade light pollution. It and the fainter Zodiacal Band are among the sky’s most subtle sights. Again, Atmospheric Optics has more details.

A bonus with this shot are some short streaks of light below and to the right of the Gegenschein. I think those are from geostationary satellites flaring at the anti-solar point as they, too, acted as mirrors briefly catching the sunlight. Being geostationary – likely communications satellites you aimed fixed dishes at – they stayed still while the stars and tracking camera moved, and so created streaks.

– Alan, March 20, 2013 / © 2013 Alan Dyer

 

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