## radius- (MATH) 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.
• GEOMETRY - (ASTRON) An old instrument for measuring the angular distance between two celestial objects.
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