Central angle

Angle AOB is a central angle

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. Central angles are subtended by an arc between those two points, and the arc length is the central angle of a circle of radius one (measured in radians). The central angle is also known as the arc's angular distance. The arc length spanned by a central angle on a sphere is called spherical distance.

The size of a central angle Template:Math is Template:Math or Template:Math (radians). When defining or drawing a central angle, in addition to specifying the points Template:Mvar and Template:Mvar, one must specify whether the angle being defined is the convex angle (<180°) or the reflex angle (>180°). Equivalently, one must specify whether the movement from point Template:Mvar to point Template:Mvar is clockwise or counterclockwise.

Formulas

If the intersection points Template:Mvar and Template:Mvar of the legs of the angle with the circle form a diameter, then Template:Math is a straight angle. (In radians, Template:Math.)

Let Template:Math be the minor arc of the circle between points Template:Mvar and Template:Mvar, and let Template:Mvar be the radius of the circle.

Central angle. Convex. Is subtended by minor arc Template:Math

If the central angle Template:Math is subtended by Template:Math, then $$0^{\circ }<\mathrm{\Theta }<180^{\circ },\mathrm{\Theta }={\left(\frac{180L}{\pi R}\right)}^{\circ }=\frac{L}{R}.$$

Central angle. Reflex. Is not subtended by Template:Math

If the central angle Template:Math is not subtended by the minor arc Template:Math, then Template:Math is a reflex angle and $$180^{\circ }<\mathrm{\Theta }<360^{\circ },\mathrm{\Theta }={(360-\frac{180L}{\pi R})}^{\circ }=2\pi -\frac{L}{R}.$$

If a tangent at Template:Math and a tangent at Template:Math intersect at the exterior point Template:Math, then denoting the center as Template:Math, the angles Template:Math (convex) and Template:Math are supplementary (sum to 180°).

Central angle of a regular polygon

A regular polygon with Template:Math sides has a circumscribed circle upon which all its vertices lie, and the center of the circle is also the center of the polygon. The central angle of the regular polygon is formed at the center by the radii to two adjacent vertices. The measure of this angle is $2\pi /n.$

See also

• Chord (geometry)
• Inscribed angle
• Great-circle navigation

External links

• Template:Cite web interactive
• Template:Cite web interactive
• Inscribed and Central Angles in a Circle