The gegenschein (German pronunciation: [ˈɡeːɡənʃaɪn] German for "counter shine") is a faint brightening of the night sky in the region of the antisolar point.
The gegenschein appears as a softly glowing oval region a few degrees wide and 10–15° in length, oriented along the plane of the ecliptic. The gegenschein is so faint that it is invisible with the presence of any moonlight or light pollution, or if it falls in the vicinity of the Milky Way.
Like the zodiacal light, the gegenschein is sunlight reflected by interplanetary dust. Most of this dust is orbiting the sun in about the ecliptic plane, with a possible concentration of particles at the L2 Earth-Sun Lagrangian point.
It is distinguished from zodiacal light by its high angle of reflection of the incident sunlight on the dust particles. It forms a slightly more luminous, oval glow directly opposite the Sun within the band of luminous zodiacal light. The intensity of the gegenschein is (relatively) enhanced because each dust particle is seen in full phase.
The gegenschein was first described by the French Jesuit astronomer and professor Esprit Pézenas (1692–1776) in 1730. Further observations were made by the German explorer Alexander von Humboldt during his South American journey from 1799 to 1803. It was also Humboldt who gave the phenomenon its German name Gegenschein.
The Danish astronomer Theodor Brorsen published the first thorough investigations of the gegenschein in 1854. He was also the first to observe that the Zodiacal light can embrace the complete sky, because under near-perfect conditions a feeble light bridge connecting the Zodiacal light and the gegenschein can be observed. Besides, Brorsen had already proposed the correct explanation for the gegenschein (interplanetary dust reflections).
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