Semi-Arc

That portion of a celestial body's apparent daily travel, during which it remains above the horizon, from its rising to its setting, is called its diurnal arc; hence half of the arc, from horizon to midheaven, is its Diurnal Semi-Arc. The other half, most of which is under the earth, is its nocturnal arc, and half of it becomes the Nocturnal Semi-Arc. The Sun's semi-arc, diurnal or nocturnal, 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 R. A. passing over the Meridian; although it can be expressed in terms of time.