Almucantar

An almucantar (also spelled almucantarat or almacantara) is a circle on the celestial sphere parallel to the horizon.

The term was introduced into European astronomy by monastic astronomer Hermann Contractus of Reichenau, Latinized from the Arabic word al-muqanṭarah ("the almucantar, sundial", plural: al-muqanṭarāt), derived from qanṭarah ("arch, bridge")[1][2] 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.

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

The most relevant were developed by Oleg Dubovik and used in the NASA AERONET network and by Teruyuki Nakajima (named SkyRad.pack).

James and John Knapton, et al. {{cite encyclopedia}}: Missing or empty |title= (help)

The celestial sphere with the zenith and almucantar marked in red, the horizon in green, and the path of a star or the Sun in blue.
This is a drawing of an almucantar staff. There are three vanes - the horizon vane (A), the shadow vane (B) and the sighting vane (C).