# radius

In mathematics, the radius is the distance from the
point of intersection of the axes of symmetry of a closed curve. More specifically, the distance from the center of a circle to its circumference, from the center of a sphere to its surface, or from the center of a regular polygon to any one of its vertices.
All radii of a circle or sphere are equal; and it is generally profitable
to consider only the longest and shortest radii (semi-major and semi-minor
axes) of an ellipse. The **radius
of curvature**, *r*, at any point of a curve is *r* = 1/*κ*, where *κ* is the curvature.

In astronomy, a radius is an old instrument for measuring the angular distance between two celestial objects.