The portion of a celestial body's daily travel above the horizon between its rising and its setting is called its diurnal arc; halving this arc from horizon to midheaven or meridien gives its diurnal semi-arc. The portion below the earth is the nocturnal arc, so half of it becomes the nocturnal semi-arc. The Sun's nocturnal or diurnal semi-arc when in 0° Aries or 0° Libra, is six hours or 90° all over the Earth. At other seasons the one is greater or less than the other, according to the time of the year and the latitude of the place. The greatest discrepancy occurs where the N. or S. latitude is high, and when the Sun is in 0° Cancer or 0° Capricorn. The semi-arc is usually measured in degrees of RA passing over the Meridian; although it can be expressed in terms of time. on source: Encyclopedia of Astrology

Term used in Primary Directions to denote the arc from a Significator to the Horizon or to the Meridian.