The antisolar point is the abstract point on the

celestial sphere In astronomy and navigation, the celestial sphere is an abstract sphere that has an arbitrarily large radius and is concentric to Earth. All objects in the sky can be conceived as being projected upon the inner surface of the celestial sphere, ...

directly opposite the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

from an observer's perspective. This means that the antisolar point lies above the

horizon The horizon is the apparent curve that separates the surface of a celestial body from its sky when viewed from the perspective of an observer on or near the surface of the relevant body. This curve divides all viewing directions based on whethe ...

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 on a surface where light from a light source is blocked by an object. In contrast, shade occupies the three-dimensional volume behind an object with light in front of it. The cross-section of a shadow is a two-dimensio ...

of the observer's head. Like the

zenith The zenith (, ) is the imaginary point on the celestial sphere directly "above" a particular location. "Above" means in the vertical direction (Vertical and horizontal, plumb line) opposite to the gravity direction at that location (nadir). The z ...

and

nadir The nadir is the direction pointing directly ''below'' a particular location; that is, it is one of two vertical directions at a specified location, orthogonal to a horizontal flat surface. The direction opposite of the nadir is the zenith. Et ...

, the antisolar point is not fixed in

three-dimensional space In geometry, a three-dimensional space (3D space, 3-space or, rarely, tri-dimensional space) is a mathematical space in which three values ('' coordinates'') are required to determine the position of a point. Most commonly, it is the three- ...

, 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 In mathematics and physics, the centroid, also known as geometric center or center of figure, of a plane figure or solid figure is the arithmetic mean position of all the points in the figure. The same definition extends to any object in n-dime ...

of several

optical phenomena Optical phenomena are any observable events that result from the interaction of light and matter. All optics, optical phenomena coincide with Quantum mechanics, quantum phenomena. Common optical phenomena are often due to the interaction of ligh ...

, including subhorizon haloes,

rainbow A rainbow is an optical phenomenon caused by refraction, internal reflection and dispersion of light in water droplets resulting in a continuous spectrum of light appearing in the sky. The rainbow takes the form of a multicoloured circular ...

s, glories, the Brocken spectre, and heiligenschein. Occasionally, around

sunset Sunset (or sundown) is the disappearance of the Sun at the end of the Sun path, below the horizon of the Earth (or any other astronomical object in the Solar System) due to its Earth's rotation, rotation. As viewed from everywhere on Earth, it ...

sunrise Sunrise (or sunup) is the moment when the upper rim of the Sun appears on the horizon in the morning, at the start of the Sun path. The term can also refer to the entire process of the solar disk crossing the horizon. Terminology Although the S ...

, anticrepuscular rays appear to converge toward the antisolar point near the horizon. However, this is an

optical illusion In visual perception, an optical illusion (also called a visual illusion) is an illusion caused by the visual system and characterized by a visual perception, percept that arguably appears to differ from reality. Illusions come in a wide varie ...

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 Gegenschein (; ; ) or counterglow is a faintly bright spot in the night sky centered at the antisolar point. The backscatter of sunlight by interplanetary dust causes this optical phenomenon, being a zodiacal light and part of its zodiacal light ...

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 include moonlig ...

away from

city lights ''City Lights'' is a 1931 American synchronized sound film, sound romance film, romantic comedy drama, comedy-drama film written, produced, directed by, and starring Charlie Chaplin. While the film has no audible dialog, it was released with a ...

, arising from the

backscatter In physics, backscatter (or backscattering) is the reflection of waves, particles, or signals back to the direction from which they came. It is usually a diffuse reflection due to scattering, as opposed to specular reflection as from a mirror, ...

sunlight Sunlight is the portion of the electromagnetic radiation which is emitted by the Sun (i.e. solar radiation) and received by the Earth, in particular the visible spectrum, visible light perceptible to the human eye as well as invisible infrare ...

by interplanetary dust. In

astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...

, the full Moon or a

planet A planet is a large, Hydrostatic equilibrium, rounded Astronomical object, astronomical body that is generally required to be in orbit around a star, stellar remnant, or brown dwarf, and is not one itself. The Solar System has eight planets b ...

in opposition lies near the antisolar point. During a total

lunar eclipse A lunar eclipse is an astronomical event that occurs when the Moon moves into the Earth's shadow, causing the Moon to be darkened. Such an alignment occurs during an eclipse season, approximately every six months, during the full moon phase, ...

, the full Moon enters the umbra 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 – someti ...

, 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 Cowley
South Pole Halos – Anthelic View – Atmospheric Optics
, accessed 13 September 2013

References

{{Reflist, 30em Spherical astronomy

Anthelic point

See also

References