Arc of Vision

The least distance from the Sun at which a planet is visible when the Sun is below the horizon. Diurnal Arc. The arc through which the Sun passes from sunrise to sunset. At the Equinox this arc is 180°, or 12 hours of right ascension. With increased latitude (distance away from the Equator) and nearness to the Solstices it becomes larger or smaller. Nocturnal Arc. That portion of 360°, or 24 hours, which remains after subtracting the Diurnal Arc. Semi Arc. Diurnal and Nocturnal, Half of either the Nocturnal or Diurnal Arc, measured from the Midheaven or Immum Coeli to the horizon. Arc of Direction. The distance between a significator and the point where it forms an aspect with a promittor, measured in degrees and minutes of the Equator; distance from the place of a planet to the body of same, or to a point where an aspect will be formed thereto. It may be measured either in Sidereal Time according to Tables of Ascension, or in Right Ascension as computed by spherical trigonometry. In primary directions this Arc is translated into time in the proportion of one degree of arc to a year of time, or five minutes of arc to one month of time. (v. Directions.)