A central angle of a circle is an angle with its vertex at the center of the circle. An arc is an unbroken part of the circle. There are major and minor arcs. The measure of a minor arc is the measure of its central angle. The measure of a major arc is 360 minus the measure of the minor arc. If two points are the endpoints of a diameter, then the two arcs are called semicircles. The measure of a semicircle is always 180 degrees.