An almucantar (also spelled almucantarat or almacantara) is a

circle A circle is a shape consisting of all point (geometry), points in a plane (mathematics), plane that are at a given distance from a given point, the Centre (geometry), centre. The distance between any point of the circle and the centre is cal ...

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, ...

parallel to 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 ...

. Two

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

s that lie on the same almucantar have the same

altitude Altitude is a distance measurement, usually in the vertical or "up" direction, between a reference datum (geodesy), datum and a point or object. The exact definition and reference datum varies according to the context (e.g., aviation, geometr ...

. The term was introduced into European astronomy by

monastic Monasticism (; ), also called monachism or monkhood, is a religious way of life in which one renounces worldly pursuits to devote oneself fully to spiritual activities. Monastic life plays an important role in many Christian churches, especially ...

astronomer Hermann Contractus of

Reichenau Reichenau may refer to: *Reichenau Island, a German island in Lake Constance **Reichenau Abbey, a former Benedictine monastery on the island **Reichenau, Baden-Württemberg, a municipality that encompasses the namesake island and five separate area ...

, Latinized from the

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

word ' ("the almucantar, sundial", plural: '), derived from ' ("arch, bridge")

Almucantar staff

An almucantar staff is an instrument chiefly used to determine the time of

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 ...

and

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 ...

, in order to find the amplitude and consequently the variations of the

compass A compass is a device that shows the cardinal directions used for navigation and geographic orientation. It commonly consists of a magnetized needle or other element, such as a compass card or compass rose, which can pivot to align itself with No ...

. Usually made of

pear Pears are fruits produced and consumed around the world, growing on a tree and harvested in late summer into mid-autumn. The pear tree and shrub are a species of genus ''Pyrus'' , in the Family (biology), family Rosaceae, bearing the Pome, po ...

tree or

boxwood ''Buxus'' is a genus of about seventy species in the family Buxaceae. Common names include box and boxwood. The boxes are native to western and southern Europe, southwest, southern and eastern Asia, Africa, Madagascar, northernmost So ...

, with an arch of 15° to 30°, May, William Edward, ''A History of Marine Navigation'', G. T. Foulis & Co. Ltd., Henley-on-Thames, Oxfordshire, 1973, it is an example of a

backstaff The backstaff is a celestial navigation, navigational instrument that was used to measure the Horizontal coordinate system#Altitude, altitude of a astronomical object, celestial body, in particular the Sun or Moon. When observing the Sun, users ...

. The sun casts that shadow of a vane (B in the adjacent image) on a ''horizon vane'' (A). The horizon vane has a slit or hole to allow the observer to see the horizon in the distance. The observer aligns the horizon and shadow so they show at the same point on the horizon vane and sets the ''sighting vane'' (C) to align his line of sight with the horizon. The altitude of the sun is the angle between the shadow vane and the sighting vane (B-A-C).

Solar almucantar

The almucantar plane that contains the Sun is used to characterize multiple

scattering In physics, scattering is a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including particles and radiat ...

aerosols An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be generated from natural or human causes. The term ''aerosol'' commonly refers to the mixture of particulates in air, and not to t ...

. Measurements are carried out rapidly at several angles at both sides of the Sun using a

spectroradiometer A spectroradiometer is a light measurement tool that is able to measure both the wavelength and amplitude of the light emitted from a light source. Spectrometers discriminate the wavelength based on the position the light hits at the detector array ...

or a

photometer A photometer is an instrument that measures the strength of electromagnetic radiation in the range from ultraviolet to infrared and including the visible spectrum. Most photometers convert light into an electric current using a photoresistor, ...

. There are several models to obtain aerosol properties from the solar almucantar. The most relevant were developed by Oleg Dubovik and used in the NASA

AERONET AERONET - the AERONET (AErosol RObotic NETwork) is a network of ground-based sun photometers which measure atmospheric aerosol properties. The measurement system is a solar-powered CIMEL Electronique 318A spectral radiometer that measures Sun ...

network and by Teruyuki Nakajima (named SkyRad.pack).SkyRad.pack

References

*Adelaide Observatory: Almucantar graphs of hour angles, Adelaide, R. E. E. Rogers, Govt. printer, 1927. *Chandler, Seth Carlo, (1846–1913): The almucantar, Cambridge, J. Wilson and Son, 1887. *Dubovik, O. and M. D. King, 2000: A flexible inversion algorithm for retrieval of aerosol optical properties from Sun and sky radiance measurements,"

Journal of Geophysical Research The ''Journal of Geophysical Research'' is a peer-reviewed scientific journal. It is the flagship journal of the American Geophysical Union. It contains original research on the physical, chemical, and biological processes that contribute to the u ...

, 105, 20 673-20 696
pdf version
*Nakajima T, Tonna G, Rao RZ, et al.:Use of sky brightness measurements from ground for remote sensing of particulate polydispersions,

Applied Optics ''Applied Optics'' is a peer-reviewed scientific journal published by Optica three times a month. It was established in 1962 with John N. Howard as founding editor-in-chief and Patricia Wakeling as managing editor. Wakeling worked on the journal ...

35 (15), 2672–2686, 1996

External links

"Compendium on Using the Device Known as the Almucantar Quarter"
is an Arabic manuscript from 1757 about the Almucantar Quarter {{1728 Astronomical coordinate systems Measuring instruments Astronomical instruments Navigational equipment Celestial navigation Historical scientific instruments

Almucantar staff

Solar almucantar

See also

References

External links