A 46° halo is a rare

atmospheric An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosphere ...

optical phenomenon Optical phenomena are any observable events that result from the interaction of light and matter. All optical phenomena coincide with quantum phenomena. Common optical phenomena are often due to the interaction of light from the Sun or Moon with ...

that consists of a

halo HALO, halo, halos or haloes may refer to: Most common meanings * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head * ''Halo'' (franchise), a sci-fi video game series (2001–2021) Arts and en ...

with an apparent radius of approximately 46° around 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 ...

. At solar elevations of 15–27°, 46° halos are often confused with the less rare and more colourful supralateral and infralateral arcs, which cross the parhelic circle at about 46° to the left and right of the sun.The 46° halo was first explained as being caused by refractions through ice crystals in 1679 by the French physicist Edmé Mariotte (1620–1684). See: Mariotte, ''Quatrieme Essay. De la Nature des Couleur'' (Paris, France: Estienne Michallet, 1681). Sun dogs as well as the 22° and 46° halos are explained in terms of refractions from ice crystals o
pages 466–524
The 46° halo is similar to, but much larger and fainter than, the more common

22° halo A 22° halo is an atmospheric optical phenomenon that consists of a halo with an apparent radius of approximately 22° around the Sun or Moon. Around the Sun, it may also be called a sun halo. Around the Moon, it is also known as a moon ring, st ...

. The 46° halo forms when sunlight enters randomly oriented

hexagon In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A regular hexagon is de ...

ice crystal Ice crystals are solid water (known as ice) in symmetrical shapes including hexagonal columns, hexagonal plates, and dendritic crystals. Ice crystals are responsible for various atmospheric optical displays and cloud formations. Formation ...

s through a

prism PRISM is a code name for a program under which the United States National Security Agency (NSA) collects internet communications from various U.S. internet companies. The program is also known by the SIGAD . PRISM collects stored internet ...

face and exits through a hexagonal base. The 90° inclination between the two faces of the crystals causes the colours of the 46° halo to be more widely dispersed than those of the 22° halo. In addition, as many rays are deflected at larger angles than the angle of minimum deviation, the outer edge of the halo is more diffuse. (including an illustration and an animation) To tell the difference between a 46° halo and the infralateral or supralateral arcs, one should carefully observe sun elevation and the fluctuating shapes and orientations of the arcs. The supralateral arc always touches the

circumzenithal arc The circumzenithal arc, also called the circumzenith arc (CZA), the upside-down rainbow, and the Bravais arc,"Mémoire sur les halos et les phénomènes optiques qui les accompagnent", J. de l' École Royale Polytechnique 31(18), 1-270, A. Brav ...

, while the 46° halo only achieves this when the sun is located 15–27° over the horizon, leaving a gap between the two at other elevations. In contrast, supralateral arcs cannot form when the Sun is over 32°, so a halo in the region of 46° is always a 46° halo at higher elevations. If the Sun is near 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 ...

, however, circumhorizontal or infralateral arcs are located 46° under the Sun and can be confused with the 46° halo.

References

External links

Atmospheric Optics - 46° Radius Halo
- including a HaloSim computer simulation and an Antarctica fish eye photo. {{DEFAULTSORT:46 Halo Atmospheric optical phenomena