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The antisolar point is the abstract point on the celestial sphere directly opposite the Sun from an observer's perspective. This means that the antisolar point lies above the horizon when the Sun is below it, and vice versa. On a sunny day, the antisolar point can be easily found; it is located within the
South Pole Halos – Anthelic View – Atmospheric Optics
, accessed 13 September 2013
The antisolar point is the abstract point on the celestial sphere directly opposite the Sun from an observer's perspective. This means that the antisolar point lies above the horizon when the Sun is below it, and vice versa. On a sunny day, the antisolar point can be easily found; it is located within the shadow
A shadow is a dark area where light from a light source is blocked by an opaque object. It occupies all of the three-dimensional volume behind an object with light in front of it. The cross section of a shadow is a two-dimensional silhouette ...
of the observer's head. Like the zenith
The zenith (, ) is an imaginary point directly "above" a particular location, on the celestial sphere. "Above" means in the vertical direction ( plumb line) opposite to the gravity direction at that location ( nadir). The zenith is the "high ...
and nadir, the antisolar point is not fixed in three-dimensional space, but is defined relative to the observer. Each observer has an antisolar point that moves as the observer changes position.
The antisolar point forms the geometric center of several optical phenomena, including subhorizon haloes, rainbow
A rainbow is a meteorological phenomenon that is caused by reflection, refraction and dispersion of light in water droplets resulting in a spectrum of light appearing in the sky. It takes the form of a multicoloured circular arc. Rainbows ...
s, glories, the Brocken spectre, and heiligenschein. Occasionally, around sunset
Sunset, also known as sundown, is the daily disappearance of the Sun below the horizon due to Earth's rotation. As viewed from everywhere on Earth (except the North and South poles), the equinox Sun sets due west at the moment of both the spr ...
or sunrise
Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning. The term can also refer to the entire process of the solar disk crossing the horizon and its accompanying atmospheric effects.
Terminology ...
, anticrepuscular rays appear to converge toward the antisolar point near the horizon. However, this is an optical illusion caused by perspective; in reality, the "rays" (i.e. bands of shadow) run near-parallel to each other.
Also around the antisolar point, the gegenschein is often visible in a moonless night sky
The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.
Natural light sources in a night sky inc ...
away from city lights, arising from the backscatter of sunlight by interplanetary dust. In astronomy
Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...
, the full Moon
The full moon is the lunar phase when the Moon appears fully illuminated from Earth's perspective. This occurs when Earth is located between the Sun and the Moon (when the ecliptic longitudes of the Sun and Moon differ by 180°). This mean ...
or a planet
A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a ...
in opposition lies near the antisolar point. During a total lunar eclipse
A lunar eclipse occurs when the Moon moves into the Earth's shadow. Such alignment occurs during an eclipse season, approximately every six months, during the full moon phase, when the Moon's orbital plane is closest to the plane of the Eart ...
, the full Moon enters the umbra
The umbra, penumbra and antumbra are three distinct parts of a shadow, created by any light source after impinging on an opaque object. Assuming no diffraction, for a collimated beam (such as a point source) of light, only the umbra is cast. ...
of Earth's shadow
Earth's shadow (or Earth shadow) is the shadow that Earth itself casts through its atmosphere and into outer space, toward the antisolar point. During the twilight period (both early dusk and late dawn), the shadow's visible fringe – somet ...
, which the planet casts onto its atmosphere, into space, and toward the antisolar point.
Anthelic point
The anthelic point is often used as a synonym for the antisolar point, but the two should be differentiated. While the antisolar point is directly opposite the sun, always below the horizon when the sun is up, the anthelic point is opposite but at the same elevation as the sun, and is therefore located on the parhelic circle. There are several halo phenomena that are centered on or converge on the anthelic point, such as the anthelion, Wegener arcs, Tricker arcs and the parhelic circle itself.Les CowleySouth Pole Halos – Anthelic View – Atmospheric Optics
, accessed 13 September 2013
See also
* Heiligenschein * Opposition surge * Subparhelic circle *Sylvanshine Sylvanshine is an optical phenomenon in which dew-covered foliage with wax-coated leaves retroreflect beams of light, as from a vehicle's headlights. This effect sometimes makes trees appear snow-covered at night during the summer. The phenomenon w ...
References
{{Reflist, 30em Spherical astronomy