, also spelled almucantarat
, is a circle
on the celestial sphere
parallel to the horizon
. Two stars
that lie the same almucantar have the same altitude
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°, 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.
The almucantar plane that contains the Sun is used to characterize multiple scattering
. 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
- 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 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