Gegenschein

The gegenschein appears as a diffuse patch or oval within the faint zodiacal band, centered close to the antisolar point—the position in the sky opposite the Sun. It is not a discrete celestial body. The glow is subtle and is most readily detected under very dark skies with good atmospheric transparency; light pollution and natural sky brightness can obscure it.

Sunlight scattered by interplanetary dust produces the broader zodiacal light. At the antisolar point, dust is viewed in a near-backscattering geometry, where light returns approximately toward its source. Observations show a narrow enhancement around this direction, often described as an opposition effect. Research has considered coherent backscattering and shadow-hiding among possible contributors to the enhancement; the exact contribution depends on dust properties and viewing geometry.

The apparent shape and brightness of the gegenschein vary over the year as Earth moves along its orbit and the line of sight through the dust cloud changes. A high-resolution study using the WIZARD wide-field camera analyzed optical observations made from 2003 to 2006. It reported that the peak brightness remained at the antisolar direction despite seasonal changes in the glow's morphology, and compared the measurements with models of interplanetary dust. Such observations help constrain the spatial distribution and scattering behavior of dust in the zodiacal cloud.

The gegenschein is one component of the wider zodiacal light, not a separate source of illumination. The zodiacal light is most conspicuous as bright cones extending from the Sun after dusk or before dawn, while the gegenschein lies in the much fainter portion of the zodiacal band opposite the Sun. Both arise from sunlight interacting with interplanetary dust, but the counterglow's special geometry produces its local brightness enhancement.