gegenschein (n) [gā΄ gэn shīn΄]
a diffuse, faint elliptical patch of light that appears in the night sky almost directly opposite the sun, thought to be sunlight reflected from dust in space. Also called counterglow.
In daylight, a phenomenon similar to the gegenschein — called the glory — can be seen from an airplane in clouds opposite the sun.
From the German gegen — against < Middle High German < Old High German gegin
+ German Schein — light < Middle High German schīn < Old High German scīn < scīnan (akin to the Old English scīnan) — to shine < Indo-European base skai– — to glimmer, be clear. From whence shimmer and the Greek skia — shadow.
“Of the four possible explanations of the gegenschein which have been proposed, not one so far has explained satisfactorily the meagre observational data concerning this phenomenon. The Glyden–Moulton hypothesis, which suggests a concentration of meteoric material at a libration point of the Sun–Earth system, seems incapable of explaining all the luminosity of the gegenschein. The zodiacal light hypothesis, which claims that the glow opposite the Sun results from a phase-function for the interplanetary dust layer which produces such a brightening, fails to explain the fact that the gegenschein usually lies approximately 3° west of the anti-solar point. The gaseous tail hypothesis, in explaining the light as the result of excitation of Earth-escaped gases by the interplanetary plasma, does not account for the fact that the colour of the gegenschein is slightly redder than that of the Sun. Finally, the dust tail hypothesis, although explaining the facts in a qualitative way, at least, requires a large and continuous source of dust particles in Earth’s neighbourhood.”
– John C. Brandt & Paul W. Hodge, “Lunar Dust and the Gegenschein,” Nature, December 9, 1961.
(Photo credit: From NASA’s Astronomy Picture of the Day, December 26, 2006.)
