In Euclidean geometry, an arc is a closed segment of a differentiable curve in the two-dimensional plane; for example, a circular arc is a segment of a circle. If the arc segment occupies a great circle (or great ellipse), it is considered a great-arc segment.

The length of a circular arc of a circle with radius r and subtending an angle θ (measured in radians) with the circle centre, equals θr. For an angle α measured in degrees, the size in radians is given by (α/180°) × π, and so the arc length equals then (α/180°)πr.

In calculating the arc length of a polar function, small circular arcs are often used as a basis for approximation.

See also

• Arc length
• Other meanings of arc

Curves | Łuk (matematyka)