CIRCLES of Altitude or almucantars, are circles parallel to the horizon, having their common pole in the zenith, and diminishing as they approach the zenith.
Diurnal Circles are immovable circles, supposed to be described by the several stars, and other points of the heavens, in their diurnal rotation round the earth, or rather, in the rotation of the earth round its axis. The diurnal circles are all unequal; the equator is the largest.
Horary Circles, in Dialling, are the lines which show the hours on dials; though these be not drawn circular, but nearly straight.