By the inscribed angle theorem, an inscribed angle subtending a given chord is exactly half the central angle subtending the same chord, provided the inscribed angle's vertex lies on the major arc (the arc not containing the central angle's enclosed sector) relative to that chord. Here, OA and OB are perpendicular radii (central angle AOB = 90 degrees), and C lies on the remaining three-quarters of the circle (the major arc AB), so angle ACB is half of the 90 degree central angle.