Gegenschein

The Gegenschein is a faint, oval glow of sunlight scattered by interplanetary dust, visible opposite the Sun in a dark night sky. Its enhancement near the antisolar point is associated with backscattering by dust particles.

Zodiacal light: A diffuse glow produced when sunlight scatters from interplanetary dust in the inner Solar System. The Gegenschein is the faint antisolar counterpart to this broader dust-scattered glow.
Ecliptic: The apparent annual path of the Sun across the celestial sphere, close to the plane of Earth's orbit. The Gegenschein lies on the ecliptic, where interplanetary dust is concentrated.
Poggendorff phenomenon: A visual illusion in which a line crossing an intervening shape appears misaligned. This is unrelated to the Gegenschein despite the similar name.
Zodiacal cloud: The flattened distribution of interplanetary dust orbiting the Sun, extending through the inner Solar System. Its three-dimensional structure sets the broader dust background against which the Gegenschein appears.
Opposition surge: A sharp increase in brightness seen when a surface or particulate medium is viewed near the direction opposite illumination. Backscattering near exact opposition helps explain the Gegenschein's localized brightness.
Solar elongation: The angular distance between the Sun and another celestial object as seen from an observer. An elongation near 180 degrees places the glow opposite the Sun.
F zodiacal cloud: A proposed dust ring near Earth's orbit, associated with gravitational perturbations from Earth. Dust structures near Earth's orbit have been considered as possible contributors to antisolar brightness.
Dust grain: A small solid particle whose size, shape, and composition affect how it absorbs and scatters light. The glow's brightness and polarization depend on the properties of its scattering grains.
Interplanetary dust: Small solid particles orbiting the Sun, concentrated chiefly in the Solar System's inner regions. These particles scatter the sunlight that produces the Gegenschein.
Sky brightness: The amount of visible light arriving from a region of the sky, including natural and artificial sources. The Gegenschein's low surface brightness makes dark skies essential for seeing it.

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