The circles of this grid, known by their Arabic term almucantars
, are altitude lines above your local horizon.
Pesek, I.: 1995, Systematic deformations of the apparent almucantar
in homogenization astrolabe data for new solution of Earth orientation parameters from optical astrometry.
This instrument is a multi-wavelength, automatic sun/sky radiometer that measures atmospheric spectral transmission and sky radiance in the almucantar
(sky scanning at a constant zenithal angle) and principal plane (sky scanning at a constant azimuthal angle) using a combination of eight ion-assisted deposition interference filters: 1020, 870, 670, 500,440, 380, 340 for the aerosol channels and 940 nm for the water vapour of the total atmospheric column.
Besides the hemispheric image, all information (RGB values, cloud classification, and cloud cover) are extracted from almucantar
apertures at 30[degrees] and 45[degrees] elevation in order to be analyzed together with corresponding azimuth scans of HATPRO and MIRA-36.
She points out, for example, that "the projection of the semicircular ribs of the Trinity barrel vault is directly analogous to the astrolabist's method of projecting almucantars
(circles of celestial latitude) ranging above the observed horizon" (99).