圆心角, 指顶点在圆心上的角. 因为顶点在圆心上, 所以角的两边与圆的半径共直线.

圆周角为圆心角的½.vv

A central angle is an angle whose apex (vertex) is the center O of a circle and whose legs (sides) are radii intersecting the circle in two distinct points A and B. The central angle is subtended by an arc between those two points, and the arc length is the central angle (measured in radians) times the radius. The central angle is also known as the arc's angular distance.