\Ra"di*us vec"tor\
1. (Math.) A straight line (or the length of such line)
   connecting any point, as of a curve, with a fixed point,
   or pole, round which the straight line turns, and to which
   it serves to refer the successive points of a curve, in a
   system of polar co["o]rdinates. See {Co["o]rdinate}, n.

2. (Astron.) An ideal straight line joining the center of an
   attracting body with that of a body describing an orbit
   around it, as a line joining the sun and a planet or
   comet, or a planet and its satellite.