All circles are similar: a dilation from the center carries one onto another. A central angle equals its arc, an inscribed angle is half its arc, an angle in a semicircle is a right angle, a tangent is perpendicular to the radius at the point of contact, and a radius perpendicular to a chord bisects it. The opposite angles of a quadrilateral inscribed in a circle add to 180°. An arc is the same fraction of the circumference as its angle is of 360°, so arc length divided by radius depends only on the angle: that ratio is the radian measure. A sector is the same fraction of the area. This week the crew chalks the turntable circle and aims the spotlight.