An almucantar (also spelled almucantarat or almacantara) is a

circle A circle is a shape consisting of all points in a plane that are at a given distance from a given point, the centre. Equivalently, it is the curve traced out by a point that moves in a plane so that its distance from a given point is con ...

on the celestial sphere parallel to the horizon. Two stars that lie on the same almucantar have the same

altitude Altitude or height (also sometimes known as depth) is a distance measurement, usually in the vertical or "up" direction, between a reference datum and a point or object. The exact definition and reference datum varies according to the context ...

. The term was introduced into European astronomy by

monastic Monasticism (from Ancient Greek , , from , , 'alone'), also referred to as monachism, or monkhood, is a religion, religious way of life in which one renounces world (theology), worldly pursuits to devote oneself fully to spiritual work. Monastic ...

astronomer Hermann Contractus of Reichenau, Latinized from the

Arabic Arabic (, ' ; , ' or ) is a Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C. E.Watson; Walter ...

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. The term can also refer to the entire process of the solar disk crossing the horizon and its accompanying atmospheric effects. Terminology A ...

and sunset, 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 wit ...

. Usually made of

pear Pears are fruits produced and consumed around the world, growing on a tree and harvested in the Northern Hemisphere in late summer into October. The pear tree and shrub are a species of genus ''Pyrus'' , in the family Rosaceae, bearing the po ...

tree or

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

, 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 navigational instrument that was used to measure the altitude of a celestial body, in particular the Sun or Moon. When observing the Sun, users kept the Sun to their back (hence the name) and observed the shadow cast by the u ...

. 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 of aerosols. 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 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 un ...

, 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 The Optical Society three times a month. It was established in 1962 with John N. Howard as founding editor-in-chief. The journal covers all aspects of optics, photonics, imagin ...

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