circle of illumination

circle of illumination

[′sər·kəl əv ə‚lü·mə′nā·shən]
(geology)
The edge of the sunlit hemisphere, which forms a circular boundary separating the earth into a light half and a dark half.
References in periodicals archive ?
The great circle on the earth's surface that is orthogonal to the sun's rays is known as the circle of illumination which is formed by the intersection of the earth with the xz plane.
833[degrees] beyond the circle of illumination (which is the great circle obtained by the intersection of the earth with the xz plane) and, thus, it is daybreak at T and sunset at S, since the earth turns counterclockwise when viewed from above the North Pole.