Almucantar

Almucantar

An Almucantar, also spelled almucantarat or almacantara, is a circle on the celestial sphere parallel to the horizon. Two stars that lie the same almucantar have the same altitude.

Almucantar staff

An almucantar staff is an instrument chiefly used to determine the time of sunrise and sunset, in order to find the amplitude and consequently the variations of the compass. Usually made of pear tree or boxwood, 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, ISBN 0 85429 143 1] it is an example of a backstaff.

The sun casts that shadow of a vane (B in the image at the right) 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.

olar almucantar

The almucantar plane that contains the Sun is used to characterize multiple scattering of aerosols. Measurements are carried out rapidly at several angle at both sides of the Sun using a spectroradiometer or a photometer. 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).

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, 105, 20 673-20 696 [http://aeronet.gsfc.nasa.gov/PDF/Dubovik_and_King_(2000).pdf 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 35 (15), 2672-2686, 1996


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  • Almucantar — hat Seth C. Chandler (in Cambridge, Mass., U.S.) ein von ihm 1879 konstruiertes, zur Beobachtung gleicher Sternhöhen bestimmtes Instrument genannt. Auf einer zentralen vertikalen Säule, um die der ganze Oberteil des Instruments drehbar ist, ruht… …   Lexikon der gesamten Technik

  • Almucantar — Al mu*can tar, n. [F. almucantarat, almicantarat, ultimately fr. Ar. al muqantar[=a]t, pl., fr. qantara to bend, arch.] (Astron.) A small circle of the sphere parallel to the horizon; a circle or parallel of altitude. Two stars which have the… …   The Collaborative International Dictionary of English

  • almucantar — /al myooh kan teuhr/, n. Astron. a circle on the celestial sphere parallel to the horizon; the locus of all points of a given altitude. Also called parallel of altitude. [1350 1400; prob. < ML almucantarath < Ar al muqantarat the almucantars,… …   Universalium

  • almucantar — noun /ælməˈkæntə/ a small circle on the celestial sphere, parallel to the horizon, that is used in astronomy and navigation to show altitude of a star or any other heavenly body …   Wiktionary

  • almucantar — [ˌalmə kantə] noun Astronomy 1》 a circle on the celestial sphere parallel to the horizon. 2》 a telescope mounted on a float resting on mercury, used to determine stellar altitude and azimuth. Origin ME: from med. L. almucantarath or obs. Fr.… …   English new terms dictionary

  • almucantar — al·mu·can·tar …   English syllables

  • almucantar — /ælmjəˈkæntə/ (say almyuh kantuh) noun 1. a circle of altitude, parallel to the horizontal plane. 2. an instrument for measuring altitudes and azimuths. Also, almacantar. {Middle French almucantarat, from Arabic qanṭarah bridge} …  

  • almucantar —   n. Astronomy, circle of celestial sphere parallel with horizon; parallel of altitude; telescope that, in rotating, sweeps out curves of this kind …   Dictionary of difficult words

  • almucantar — n. (also almacantar) Astron. a line of constant altitude above the horizon. Etymology: ME f. med.L almucantarath or F almucantara etc., f. Arab. almukantarat sundial f. kantara arch …   Useful english dictionary

  • almucantar staff — noun : an ancient instrument having an arc of 15 degrees and used at sea to take observations of the sun s amplitude and to find the variation of the compass …   Useful english dictionary

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