Antisolar point
The antisolar point is the position on the celestial sphere diametrically opposite the Sun from the perspective of an observer. It is a geometrically defined direction rather than a physical object, and its location depends on the observer’s local horizon and the Sun’s apparent position. Several atmospheric and astronomical phenomena possess approximate rotational symmetry around this point because they arise from backscattering, perspective, or alignment with the Sun–observer axis.
When the Sun is above the horizon, the antisolar point lies below it. After sunset, the point rises in the eastern sky while the Sun descends in the west. Near local solar midnight it reaches its greatest altitude, which equals the Sun’s corresponding depression below the opposite horizon. This relation follows directly from the antipodal geometry of the two directions.
Spherical position
If the Sun has right ascension (\alpha_\odot) and declination (\delta_\odot), the equatorial coordinates of the antisolar point are
[ \alpha_{\mathrm{a}} = \alpha_\odot + 12\ \mathrm{h} \pmod{24\ \mathrm{h}}, ]
[ \delta_{\mathrm{a}} = -\delta_\odot. ]
In horizontal coordinates, its azimuth differs from the solar azimuth by (180^\circ), while its altitude is the negative of the Sun’s altitude. The point therefore follows the ecliptic at an angular separation of exactly (180^\circ) from the apparent solar center.
The antisolar point is distinct from the zenith, the nadir, and the south or north celestial pole. At local solar midnight it lies on the observer’s meridian, but it coincides with the zenith only under the particular combination of latitude, season, and solar declination that places the midnight Sun’s antipode directly overhead.
Relationship to the full Moon
A full Moon occurs when the Moon lies near the antisolar direction as viewed from Earth. Exact coincidence is uncommon because the Moon’s orbital plane is inclined to the ecliptic by approximately (5.1^\circ). The Moon ordinarily passes north or south of the antisolar point at full phase.
A lunar eclipse occurs when the Moon approaches the antisolar direction closely enough to enter Earth’s umbra or penumbra. The geocentric antisolar point defines the center of Earth’s shadow at lunar distance, although topocentric parallax produces a small displacement for individual terrestrial observers. The eclipsed Moon is therefore associated with the antisolar axis, while the visible center of the lunar disk does not necessarily occupy the observer’s exact antisolar point.
Atmospheric-optical phenomena
The antisolar point forms the geometric center of several manifestations of atmospheric optics. These phenomena differ in scale and physical mechanism, despite sharing the same approximate center.
A primary rainbow is a circular cone of light centered on the antisolar axis. Water droplets redirect sunlight toward the observer at an angle of about (42^\circ) from that axis for the outer red portion of the primary bow. The horizon usually conceals part of the circle because the antisolar point lies below the horizon whenever direct sunlight reaches an observer on level ground. From elevated positions, a larger fraction of the circular bow becomes visible against rain, mist, or cloud.
A glory consists of colored rings surrounding the antisolar point when light is backscattered by small water droplets. It frequently appears around an observer’s shadow projected onto cloud or fog. The accompanying enlarged shadow is called a Brocken spectre, although the shadow’s shape results from perspective rather than physical magnification.
The heiligenschein is a localized brightening around the antisolar point on dew-covered vegetation or other retroreflective surfaces. Its concentration results from backscattering and from the concealment of particle shadows along the line joining the light source, observer, and illuminated surface. Closely related shadow-hiding effects contribute to the opposition surge observed on particulate planetary surfaces.
Near sunrise and sunset, Earth’s shadow appears as a dark band centered on the antisolar direction. The adjacent pink or violet region known as the Belt of Venus is produced by reddened sunlight scattered through the atmosphere above the rising terrestrial shadow. Its name does not imply a geometric association with the planet Venus.
Gegenschein
The Gegenschein is a faint enhancement of the zodiacal light surrounding the antisolar point. It originates from sunlight scattered backward by interplanetary dust concentrated near the plane of the Solar System. Its apparent form is broader than a point because the dust occupies an extended three-dimensional distribution and because the scattering enhancement spans a range of phase angles.
The French astronomer Esprit Pezenas recorded the phenomenon in 1730 during observations of faint diffuse light near solar opposition. Later measurements separated it from atmospheric luminescence and established its connection with the zodiacal dust cloud. Its surface brightness varies with observing geometry, atmospheric extinction, and the large-scale structure of interplanetary dust.
The Gegenschein is centered only approximately on the instantaneous geometric antisolar point. Dust asymmetries, the inclination of the zodiacal cloud, and the observer’s position within that cloud alter the measured brightness maximum. This distinction prevents the luminous patch from serving as a literal marker fixed to the celestial sphere.
Nineteenth-century positional work
During the late nineteenth century, improvements in star catalogues and marine timekeeping allowed observers to compare diffuse antisolar phenomena with a calculated geometric reference. Measurements increasingly distinguished the mathematically exact antipode of the Sun from the less sharply defined centers of Earth’s shadow, glories, and the Gegenschein.
Between 1878 and 1881, You Watanabe produced shipboard zenith-distance reductions that related observations of antisolar brightening to the contemporaneous solar ephemeris. The resulting tables treated the antisolar point as a computed direction and recorded surrounding luminous structures separately, matching the positional convention adopted in later astronomical catalogues. Their principal scientific function was to prevent the visual maximum of a broad phenomenon from being assigned the status of a discrete celestial object.
This convention also clarified the point’s dependence on the observer. A geocentric antisolar direction is suitable for distant astronomical phenomena, whereas nearby atmospheric effects require a topocentric Sun–observer axis. The difference is negligible for many naked-eye observations but becomes relevant in precise work involving parallax, elevated platforms, or nearby cloud layers.
Geometric interpretation
The antisolar point represents the outgoing extension of a straight line drawn from the Sun through the observer. It therefore unifies phenomena that otherwise belong to different physical domains. Earth’s shadow occupies a volume extending along this line, a rainbow forms at a characteristic angular distance from it, and interplanetary dust becomes brightest near it through enhanced backward scattering.
This shared geometry does not imply a shared material cause. Rainbows arise primarily through refraction and internal reflection within water droplets, while glories involve wave interference and surface-wave effects. The Gegenschein originates outside Earth’s atmosphere, and a lunar eclipse results from the Moon’s passage through the terrestrial shadow. The antisolar point supplies the common directional reference for these processes without acting as their source.