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Arc
From Latin: arcus "a bow, arch,"
Definition: A portion of the circumference of a circle
Try this Drag one of the orange dots that define the endpoints of the blue arc. The arc will change accordingly.
(If there is no image below, see support page.)

An arc is a portion of the circumference of a circle. In the figure above, the arc is the blue part of the circle. Strictly speaking, an arc could be a portion of some other curved shape, such as an ellipse, but it almost always refers to a circle. To avoid all possible mistake, it is sometimes called a circular arc.

A straight line is drawn between the end points of the arc would be a chord of the circle.

If the arc length is exactly half the circle, this called a semicircular arc. See Semicircle definition.

Naming and identification
Arcs are named in a similar way to line segments.
  1. They can be labelled with a single letter. In the figure above, the blue arc can be called "arc a".

  2. They can be labelled by their endpoints, the blue arc above would be called "arc AB". or "arc BA", the order of the endpoints does not matter. As a shorthand this can be written as the letters AB with a curving line above them
    Example:     which is read "arc AB".
Attributes


Arc Length The length of the curved arc line. See Arc Length page for more.
Radius Radius of an arc The radius of the circle of which the arc is a part.
See Radius of an Arc for ways to calculate the arc radius when you know other properties of the arc.
Central Angle Central angle of an arc The angle subtended by the arc to the center of the circle of which it is a part. This angle is always twice the peripheral angle (see below). See Central Angle of an Arc for more.
Inscribed Angle Inscribed angle of an arc The angle subtended by the arc to any point on the circumference of the circle of which it is a part. This angle is always half the central angle (see above). See Inscribed Angle of an Arc for more.

Other circle topics

General

Angles in a circle

Arcs

(C) 2009 Copyright John Page