||Visual angle on the unit sphere
Let AOB be a triangle in the three dimensional Euclidean space R3. When we look at this triangle from various viewpoints, the angle AOB changes its appearance, and its 'visual size' is not constant. This visual angle is realized on the unit sphere space S2 centered at O. In this paper, we will investigate the contours of visual size in the unit sphere. Several geometric properties and a stochastic property are presented.