SEMIDIAMETER

, or Radius, of a circle or sphere, is a line drawn from the centre to the circumference. And in any curve that has diameters and a centre, it is the radius, or half diameter, or a line drawn from the centre to some point in the curve.

The distances, diameters, &c, of the heavenly bodies, are usually estimated by astronomers in Semidiameters of the earth; the number of which terrestrial Semidiameters, contained in that of each of those planets, is as below.